It is believed that, around 510 million years ago, land masses, including North America and what is now Siberia, were laid along the equator. Below the equator, South America, India, Australia, Antarctica, and Africa were all melded into a single supercontinent, Gondwana.
Throughout geological history, the continents have moved and fragmented as new oceans opened, and coalesced with other continents as old oceans closed. It is not precisely known how the continents were distributed in the earliest Precambrian era. However, around 510 million years ago, the large continent -- the supercontinent -- known as Gondwana was situated over the southern polar region while smaller areas of land existed around the equator and in the Northern Hemisphere.
In paleogeography, Gondwana, originally Gondwanaland, was the southernmost of two supercontinents (the other being Laurasia) that were part of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago (Mya). Gondwana is believed to have sutured between 570 and 510 Mya, thus joining East Gondwana to West Gondwana. It separated from Laurasia 200-180 Mya (the mid Mesozoic era) during the breakup of Pangaea, drifting further south after the split.
Gondwana included most of the landmasses in today's Southern Hemisphere, including Antarctica, South America, Africa, Madagascar and the Australian continent, as well as the Arabian Peninsula and the Indian subcontinent, which have now moved entirely into the Northern Hemisphere.
The continent of Gondwana was named by Austrian scientist, Eduard Suess, after the Gondwana region of central northern India (from Sanskrit gondavana -- "forest of the Gonds"), from which the Gondwana sedimentary sequences (Permian-Triassic) are also described.
The adjective Gondwanan is in common use in biogeography when referring to patterns of distribution of living organisms, typically when the organisms are restricted to two or more of the now-discontinuous regions that were once part of Gondwana, including the Antarctic flora. For example, the Proteaceae, a family of plants known only from southern South America, South Africa and Australia, are considered to have a "Gondwanan distribution". This pattern is often considered to indicate an archaic, or relict, lineage.
Thursday, December 20, 2012
Wednesday, December 19, 2012
510 Million B.C.T. - The First Fish Appeared
Around 510 Million B.C.T., the first fish appeared.
The first fish were the ostracoderms, which appeared in the Cambrian, about 510 million years ago, and became extinct near the end of the Devonian, about 377 million years ago. Ostracoderms were jawless fishes found mainly in fresh water. They were covered with a bony armor or scales and were often less than 30 cm (1 ft) long. The ostracoderms are placed in the class Agnatha along with the living jawless fishes, the lampreys and hagfishes, which are believed to be descended from the ostracoderms, as are all jawed fishes, or gnathostomes. Paired fins, or limbs, first evolved within this group.
The first fish were the ostracoderms, which appeared in the Cambrian, about 510 million years ago, and became extinct near the end of the Devonian, about 377 million years ago. Ostracoderms were jawless fishes found mainly in fresh water. They were covered with a bony armor or scales and were often less than 30 cm (1 ft) long. The ostracoderms are placed in the class Agnatha along with the living jawless fishes, the lampreys and hagfishes, which are believed to be descended from the ostracoderms, as are all jawed fishes, or gnathostomes. Paired fins, or limbs, first evolved within this group.
Tuesday, December 18, 2012
525 Million B.C.T. - The First Vertebrates Appeared
Around 525 Million B.C.T., the first vertebrates, -- the first creatures with backbones --, appeared.
Vertebrates, also called Craniata, are animals of the subphylum Vertebrata, the predominant subphylum of the phylum Chordata. They have backbones, from which they derive their name. The vertebrates are also characterized by a muscular system consisting pimarily of bilaterally paired masses and a central nervous system partly enclosed within the backbone.
The subphylum is one of the best known of all groups of animals. Its members include the classes Agnatha, Chondrichthyes, and Osteichthyes (all fishes); Amphibia (amphibians); Reptilia (reptiles); Aves (birds); and Mammalia (mammals).
Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw the rise in organism diversity. The earliest known vertebrate is believed to be the Myllokunmingia. Another early vertebrate is Haikouichthys ercaicunensis. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail. All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.
Vertebrates, also called Craniata, are animals of the subphylum Vertebrata, the predominant subphylum of the phylum Chordata. They have backbones, from which they derive their name. The vertebrates are also characterized by a muscular system consisting pimarily of bilaterally paired masses and a central nervous system partly enclosed within the backbone.
The subphylum is one of the best known of all groups of animals. Its members include the classes Agnatha, Chondrichthyes, and Osteichthyes (all fishes); Amphibia (amphibians); Reptilia (reptiles); Aves (birds); and Mammalia (mammals).
Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw the rise in organism diversity. The earliest known vertebrate is believed to be the Myllokunmingia. Another early vertebrate is Haikouichthys ercaicunensis. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail. All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.
Friday, December 14, 2012
530 Million B.C.T. - The Eye Began to Evolve
Around 530 Million B.C.T., at the beginning of the Cambrian Explosion, the eye began to evolve. It is as though God having created the Universe wanted the sentient products of the Creation to behold and marvel at all that had been done.
The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of species. Certain components of the eye, such as the visual pigments, appear to have a common ancestry – that is, they evolved once, before the animals radiated. However, complex, image-forming eyes evolved some 50 to 100 times – using many of the same proteins and genetic toolkits in their construction.
Complex eyes appear to have first evolved within a few million years, in the rapid burst of evolution known as the Cambrian explosion. There is no evidence of eyes before the Cambrian, but a wide range of diversity is evident in the Middle Cambrian Burgess shale, and the slightly older Emu Bay Shale. Eyes show a wide range of adaptations to meet the requirements of the organisms which bear them. Eyes vary in their acuity, the range of wavelengths they can detect, their sensitivity in low light levels, their ability to detect motion or resolve objects, and whether they can discriminate colors.
The complex structure of the eye has been used as evidence to support the theory that they have been designed by the Creator, as it has been said to be unlikely to have evolved via natural selection. In 1802, the philosopher William Paley called the eye a miracle of "design". Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection at first glance seemed "absurd in the highest possible degree". However, he went on to explain that despite the difficulty in imagining it, this was perfectly feasible:
He suggested a gradation from "an optic nerve merely coated with pigment, and without any other mechanism" to "a moderately high stage of perfection", giving examples of extant intermediate grades of evolution. Darwin's suggestions were soon shown to be correct and current research is investigating the genetic mechanisms responsible for eye development and evolution.
The first fossils of eyes that have been found to date are from the lower Cambrian period (about 530 million years ago). This period saw a burst of apparently rapid evolution, dubbed the "Cambrian explosion". One of the many hypotheses for "causes" of this diversification, the "Light Switch" theory of Andrew Parker, holds that the evolution of eyes initiated an arms race that led to a rapid spate of evolution. Earlier than this, organisms may have had use for light sensitivity, but not for fast locomotion and navigation by vision.
Since the fossil record, particularly of the Early Cambrian, is so poor, it is difficult to estimate the rate of eye evolution. Simple modelling, invoking small mutations exposed to natural selection, demonstrates that a primitive optical sense organ based upon efficient photopigments could evolve into a complex human-like eye in approximately 400,000 years.
Whether one considers the eye to have evolved once or multiple times depends somewhat on the definition of an eye. Much of the genetic machinery employed in eye development is common to all eyed organisms, which may suggest that their ancestor utilized some form of light-sensitive machinery – even if it lacked a dedicated optical organ. However, even photoreceptor cells may have evolved more than once from molecularly similar chemoreceptors, and photosensitive cells probably existed long before the Cambrian explosion. Higher-level similarities – such as the use of the protein crystallin in the independently derived cephalopod and vertebrate lenses – reflect the co-option of a protein from a more fundamental role to a new function within the eye.
Shared traits common to all light-sensitive organs include the family of photo-receptive proteins called opsins. All seven sub-families of opsin were already present in the last common ancestor of animals. In addition, the genetic toolkit for positioning eyes is common to all animals: the PAX6 gene controls where the eye develops in organisms ranging from mice to humans to fruit flies. These high-level genes are, by implication, much older than many of the structures that they are today seen to control. They must originally have served a different purpose, before being co-opted for a new role in eye development.
Sensory organs probably evolved before the brain did. There is no need for an information-processing organ -- there is no need for a brain -- before there is information to process.
The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of species. Certain components of the eye, such as the visual pigments, appear to have a common ancestry – that is, they evolved once, before the animals radiated. However, complex, image-forming eyes evolved some 50 to 100 times – using many of the same proteins and genetic toolkits in their construction.
Complex eyes appear to have first evolved within a few million years, in the rapid burst of evolution known as the Cambrian explosion. There is no evidence of eyes before the Cambrian, but a wide range of diversity is evident in the Middle Cambrian Burgess shale, and the slightly older Emu Bay Shale. Eyes show a wide range of adaptations to meet the requirements of the organisms which bear them. Eyes vary in their acuity, the range of wavelengths they can detect, their sensitivity in low light levels, their ability to detect motion or resolve objects, and whether they can discriminate colors.
The complex structure of the eye has been used as evidence to support the theory that they have been designed by the Creator, as it has been said to be unlikely to have evolved via natural selection. In 1802, the philosopher William Paley called the eye a miracle of "design". Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection at first glance seemed "absurd in the highest possible degree". However, he went on to explain that despite the difficulty in imagining it, this was perfectly feasible:
...if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
He suggested a gradation from "an optic nerve merely coated with pigment, and without any other mechanism" to "a moderately high stage of perfection", giving examples of extant intermediate grades of evolution. Darwin's suggestions were soon shown to be correct and current research is investigating the genetic mechanisms responsible for eye development and evolution.
The first fossils of eyes that have been found to date are from the lower Cambrian period (about 530 million years ago). This period saw a burst of apparently rapid evolution, dubbed the "Cambrian explosion". One of the many hypotheses for "causes" of this diversification, the "Light Switch" theory of Andrew Parker, holds that the evolution of eyes initiated an arms race that led to a rapid spate of evolution. Earlier than this, organisms may have had use for light sensitivity, but not for fast locomotion and navigation by vision.
Since the fossil record, particularly of the Early Cambrian, is so poor, it is difficult to estimate the rate of eye evolution. Simple modelling, invoking small mutations exposed to natural selection, demonstrates that a primitive optical sense organ based upon efficient photopigments could evolve into a complex human-like eye in approximately 400,000 years.
Whether one considers the eye to have evolved once or multiple times depends somewhat on the definition of an eye. Much of the genetic machinery employed in eye development is common to all eyed organisms, which may suggest that their ancestor utilized some form of light-sensitive machinery – even if it lacked a dedicated optical organ. However, even photoreceptor cells may have evolved more than once from molecularly similar chemoreceptors, and photosensitive cells probably existed long before the Cambrian explosion. Higher-level similarities – such as the use of the protein crystallin in the independently derived cephalopod and vertebrate lenses – reflect the co-option of a protein from a more fundamental role to a new function within the eye.
Shared traits common to all light-sensitive organs include the family of photo-receptive proteins called opsins. All seven sub-families of opsin were already present in the last common ancestor of animals. In addition, the genetic toolkit for positioning eyes is common to all animals: the PAX6 gene controls where the eye develops in organisms ranging from mice to humans to fruit flies. These high-level genes are, by implication, much older than many of the structures that they are today seen to control. They must originally have served a different purpose, before being co-opted for a new role in eye development.
Sensory organs probably evolved before the brain did. There is no need for an information-processing organ -- there is no need for a brain -- before there is information to process.
Wednesday, December 12, 2012
530 Million B.C.T. - The Cambrian Explosion
Around 530 Million B.C.T., the Cambrian Explosion began.
The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 530 million years ago, of most major animal phyla, as demonstrated in the fossil record, accompanied by major diversification of organisms including animals, phytoplankton, and calcimicrobes. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species) and the diversity of life began to resemble that of today.
The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s, and in 1859 Charles Darwin discussed it as one of the main objections that could be made against his theory of evolution by natural selection. The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.
The Cambrian Explosion -- Earth's evolutionary equivalent to the "Big Bang" -- began some 530 million years ago. Within the span of a mere 5 million years, the ancestors of almost all animals suddenly appeared on the Earth. There have been a number of theories advanced concerning the reasons for this "creative" explosion. Some have said that an increase in the Earth's oxygen supply fueled the outburst. Others say that a decrease in carbon dioxide in the Earth's atmosphere led to mass "creation."
One of the more intriguing theories for the species explosion is that the evolution of eyes sparked the Cambrian "Big Bang." According to this theory, before the eye came along, there were just simple animals -- essentially just worms and jellyfish. However, once the evolution of the eye came along, massive natural selection pressures began to exert themselves forcing these simple life forms to alter themselves for purposes of protection and reproduction. These alterations led to life forms learning how to swim, burrow, hide, have armored body parts or reflect warning colors. And it was these adaptations that eventually led to our diversity of life.
At least, that is the theory.
The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 530 million years ago, of most major animal phyla, as demonstrated in the fossil record, accompanied by major diversification of organisms including animals, phytoplankton, and calcimicrobes. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species) and the diversity of life began to resemble that of today.
The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s, and in 1859 Charles Darwin discussed it as one of the main objections that could be made against his theory of evolution by natural selection. The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.
The Cambrian Explosion -- Earth's evolutionary equivalent to the "Big Bang" -- began some 530 million years ago. Within the span of a mere 5 million years, the ancestors of almost all animals suddenly appeared on the Earth. There have been a number of theories advanced concerning the reasons for this "creative" explosion. Some have said that an increase in the Earth's oxygen supply fueled the outburst. Others say that a decrease in carbon dioxide in the Earth's atmosphere led to mass "creation."
One of the more intriguing theories for the species explosion is that the evolution of eyes sparked the Cambrian "Big Bang." According to this theory, before the eye came along, there were just simple animals -- essentially just worms and jellyfish. However, once the evolution of the eye came along, massive natural selection pressures began to exert themselves forcing these simple life forms to alter themselves for purposes of protection and reproduction. These alterations led to life forms learning how to swim, burrow, hide, have armored body parts or reflect warning colors. And it was these adaptations that eventually led to our diversity of life.
At least, that is the theory.
Monday, December 10, 2012
542 Million B.C.T. - The Phanerozoic Eon Began
Around 542 Million B.C.T., the Phanerozoic Eon began.
The Phanerozoic Eon is the current geologic eon in the geologic timescale, and the one during which abundant animal life came to exist. The Phanerozoic Eon covers roughly the last 542 million years and goes back to the time when diverse hard-shelled animals first appeared. The name "Phanerozoic" derives from the ancient Greek words phaneros and zoic, meaning visible life, since it was once believed that life began in the Cambrian, the first period of the Phanerozoic Eon -- the first period of our current eon. The time before the Phanerozoic, the time called the Precambrian supereon is now divided into the Hadean, Archaean and Proterozoic eons.
The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla; the evolution of these phyla into diverse forms; the development of complex plants; the evolution of fish; the emergence of terrestrial animals; and the development of modern faunas. During this time span, tectonic forces caused the continents to move and eventually collect into a single landmass known as Pangaea, which then separated into the current continental landmasses.
The Phanerozoic Eon is the current geologic eon in the geologic timescale, and the one during which abundant animal life came to exist. The Phanerozoic Eon covers roughly the last 542 million years and goes back to the time when diverse hard-shelled animals first appeared. The name "Phanerozoic" derives from the ancient Greek words phaneros and zoic, meaning visible life, since it was once believed that life began in the Cambrian, the first period of the Phanerozoic Eon -- the first period of our current eon. The time before the Phanerozoic, the time called the Precambrian supereon is now divided into the Hadean, Archaean and Proterozoic eons.
The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla; the evolution of these phyla into diverse forms; the development of complex plants; the evolution of fish; the emergence of terrestrial animals; and the development of modern faunas. During this time span, tectonic forces caused the continents to move and eventually collect into a single landmass known as Pangaea, which then separated into the current continental landmasses.
Friday, December 7, 2012
700 Million B.C.T. - Iceball Earth
Around 700 million years ago, the Earth became an iceball.
During the 1960s, geologists discovered rocks about 700 million years old all over the world bore the signature of rough treatment from glaciers. The Soviet scientists, M. I. Budyko, proposed one possible cause: runaway global cooling. According to the theory, bright, white polar ice sheets reflect more of the sun's heat and light back into space than do darker land masses or open water. So as the ice sheets grow during an Ice Age, they exert a feedback effect that further cools the world. The bigger they get, the more cooling they cause and so the more the ice sheets grow. Budyko's theoretical models of the Earth's climate suggested that this feedback could pass a point of no return, leaving the planet to freeze over.
On the resulting Iceball Earth, ice was everywhere: even the oceans were frozen. Except for a few organisms clinging on or around volcanoes, no life could survive. The temperature around the world was an average minus 40 degrees centigrade. It was extremely cold.
How then could Iceball Earth have shaken off its Arctic glaze and return to being the liquid blue planet that we know today? The answer may lie in the volcanoes. Volcanoes thrusting through the ice would have continued to disgorge gases, mostly carbon dioxide. Carbon dioxide is a greenhouse gas that causes global warming. Today, volcanic carbon dioxide is kept in check by natural processes such as chemical weathering of rocks, which removes the gas from the atmosphere in the form of carbonate minerals. However, on Iceball Earth, weathering would be suppressed because there would be no rain to wash the carbon dioxide from the skies, and no exposed rocks to react with it. So volcanic carbon dioxide would accumulate gradually in the atmospher and warm the planet.
At some point, there would be enough of it to break the reign of ice, and the seas would thaw. Calculations suggest that a huge amount of carbon dioxide is needed to do this: about 350 times the amount in today's atmosphere. So once melting began, temperatures would soar and the planet would gravitate toward becoming a hothouse.
During the 1960s, geologists discovered rocks about 700 million years old all over the world bore the signature of rough treatment from glaciers. The Soviet scientists, M. I. Budyko, proposed one possible cause: runaway global cooling. According to the theory, bright, white polar ice sheets reflect more of the sun's heat and light back into space than do darker land masses or open water. So as the ice sheets grow during an Ice Age, they exert a feedback effect that further cools the world. The bigger they get, the more cooling they cause and so the more the ice sheets grow. Budyko's theoretical models of the Earth's climate suggested that this feedback could pass a point of no return, leaving the planet to freeze over.
On the resulting Iceball Earth, ice was everywhere: even the oceans were frozen. Except for a few organisms clinging on or around volcanoes, no life could survive. The temperature around the world was an average minus 40 degrees centigrade. It was extremely cold.
How then could Iceball Earth have shaken off its Arctic glaze and return to being the liquid blue planet that we know today? The answer may lie in the volcanoes. Volcanoes thrusting through the ice would have continued to disgorge gases, mostly carbon dioxide. Carbon dioxide is a greenhouse gas that causes global warming. Today, volcanic carbon dioxide is kept in check by natural processes such as chemical weathering of rocks, which removes the gas from the atmosphere in the form of carbonate minerals. However, on Iceball Earth, weathering would be suppressed because there would be no rain to wash the carbon dioxide from the skies, and no exposed rocks to react with it. So volcanic carbon dioxide would accumulate gradually in the atmospher and warm the planet.
At some point, there would be enough of it to break the reign of ice, and the seas would thaw. Calculations suggest that a huge amount of carbon dioxide is needed to do this: about 350 times the amount in today's atmosphere. So once melting began, temperatures would soar and the planet would gravitate toward becoming a hothouse.
Wednesday, December 5, 2012
1.1 Billion B.C.T. - The Supercontinent Rodinia Was Formed
Rodinia (from the Russian rodit, meaning "to give birth") is the name of a supercontinent, a continent which contained most or all of the Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era. It formed over one billion years ago by accretion and collision of fragments produced by the breakup of the older supercontinent, Columbia, which was assembled by global-scale 2.0-1.8 B.C.T. collisional events. Rodinia has entered popular consciousness as one of the two great supercontinents of earth history, the other being Pangaea.
Rodinia broke up in the Neoproterozoic and its continental fragments were re-assembled to form Pangaea 300-250 million years ago. In contrast with Pangaea, little is known yet about the exact configuration and geodynamic history of Rodinia. Paleomagnetic evidence provides some clues to the paleolatitude of individual pieces of the Earth's crust, but not to their longitude, which geologists have pieced together by comparing similar geologic features, often now widely dispersed.
The extreme cooling of the global climate around 700 million years ago (the so called Snowball Earth of the Cryogenian period) and the rapid evolution of primitive life during the subsequent Ediacaran and Cambrian periods are often thought to have been triggered by the breaking up of Rodinia.
Unlike later supercontinents, Rodinia itself was entirely barren. It existed before life colonized dry land, and, since it predated the formation of the ozone layer, it was too exposed to ultraviolet sunlight for any organism to inhabit it. Nevertheless, its existence did significantly influence the marine life of its time.
In the Cryogenian period, the Earth experienced large glaciations, and temperatures were at least as cool as today. Substantial areas of Rodinia may have been covered by glaciers or the southern polar ice cap.
Low temperatures may have been exaggerated during the early stages of continental rifting. Geothermal heating peaks in crust about to be rifted; and since warmer rocks are less dense, the crustal rocks rise up relative to their surroundings. This rising creates areas of higher altitude, where the air is cooler and ice is less likely to melt with changes in season, and it may explain the evidence of abundant glaciation in the Ediacaran period.
The eventual rifting of the continents created new oceans, and seafloor spreading, which produces warmer, less-dense oceanic lithosphere. Due to its lower density, hot oceanic lithosphere will not lie as deep as old, cool oceanic lithosphere. In periods with relatively large areas of new lithosphere, the ocean floors come up, causing the eustatic sea level to rise. The result was a greater number of shallower seas.
The increased evaporation from the larger water area of the oceans may have increased rainfall, which, in turn, increased the weathering of exposed rock. It has been shown that in conjunction with quick-weathering of volcanic rock, this increased rainfall may have reduced greenhouse gas levels to below the threshold required to trigger the period of extreme glaciation known as Snowball Earth.
Increased volcanic activity also introduced into the marine environment biologically active nutrients, which may have played an important role in the development of the earliest animals.
Rodinia broke up in the Neoproterozoic and its continental fragments were re-assembled to form Pangaea 300-250 million years ago. In contrast with Pangaea, little is known yet about the exact configuration and geodynamic history of Rodinia. Paleomagnetic evidence provides some clues to the paleolatitude of individual pieces of the Earth's crust, but not to their longitude, which geologists have pieced together by comparing similar geologic features, often now widely dispersed.
The extreme cooling of the global climate around 700 million years ago (the so called Snowball Earth of the Cryogenian period) and the rapid evolution of primitive life during the subsequent Ediacaran and Cambrian periods are often thought to have been triggered by the breaking up of Rodinia.
Unlike later supercontinents, Rodinia itself was entirely barren. It existed before life colonized dry land, and, since it predated the formation of the ozone layer, it was too exposed to ultraviolet sunlight for any organism to inhabit it. Nevertheless, its existence did significantly influence the marine life of its time.
In the Cryogenian period, the Earth experienced large glaciations, and temperatures were at least as cool as today. Substantial areas of Rodinia may have been covered by glaciers or the southern polar ice cap.
Low temperatures may have been exaggerated during the early stages of continental rifting. Geothermal heating peaks in crust about to be rifted; and since warmer rocks are less dense, the crustal rocks rise up relative to their surroundings. This rising creates areas of higher altitude, where the air is cooler and ice is less likely to melt with changes in season, and it may explain the evidence of abundant glaciation in the Ediacaran period.
The eventual rifting of the continents created new oceans, and seafloor spreading, which produces warmer, less-dense oceanic lithosphere. Due to its lower density, hot oceanic lithosphere will not lie as deep as old, cool oceanic lithosphere. In periods with relatively large areas of new lithosphere, the ocean floors come up, causing the eustatic sea level to rise. The result was a greater number of shallower seas.
The increased evaporation from the larger water area of the oceans may have increased rainfall, which, in turn, increased the weathering of exposed rock. It has been shown that in conjunction with quick-weathering of volcanic rock, this increased rainfall may have reduced greenhouse gas levels to below the threshold required to trigger the period of extreme glaciation known as Snowball Earth.
Increased volcanic activity also introduced into the marine environment biologically active nutrients, which may have played an important role in the development of the earliest animals.
Tuesday, December 4, 2012
1.2 Billion B.C.T. - Sexual Reproduction Evolved
Around 1.2 Billion B.C.T., the evolution of sexual reproduction began.
Sexual reproduction first appeared by 1.2 billion years ago in the Proterozoic Eon. All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species. Many protists reproduce sexually, as do the multicellular plants, animals, and fungi. There are a few species which have secondarily lost this feature, such as Bdelloidea and some parthenocarpic plants.
Organisms need to replicate their genetic material in an efficient and reliable manner. The necessity to repair genetic damage is one of the leading theories explaining the origin of sexual reproduction. Diploid individuals can repair a damaged section of their DNA via homologous recombination, since there are two copies of the gene in the cell and one copy is presumed to be undamaged. A mutation in an haploid individual, on the other hand, is more likely to become resident, as the DNA repair machinery has no way of knowing what the original undamaged sequence was. The most primitive form of sex may have been one organism with damaged DNA replicating an undamaged strand from a similar organism in order to repair itself.
Another theory is that sexual reproduction originated from selfish parasitic genetic elements that exchange genetic material (that is: copies of their own genome) for their transmission and propagation. In some organisms, sexual reproduction has been shown to enhance the spread of parasitic genetic elements (e.g.: yeast, filamentous fungi). Bacterial conjugation, a form of genetic exchange that some sources describe as sex, is not a form of reproduction, but rather an example of horizontal gene transfer. However, it does support the selfish genetic element theory, as it is propagated through such a "selfish gene", the F-plasmid. Similarly, it has been proposed that sexual reproduction evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids.
A third theory is that sex evolved as a form of cannibalism. One primitive organism ate another one, but rather than completely digesting it, some of the 'eaten' organism's DNA was incorporated into the 'eater' organism.
Sex may also be derived from prokaryotic processes. A comprehensive 'origin of sex as vaccination' theory proposes that eukaryan sex-as-syngamy (fusion sex) arose from prokaryan unilateral sex-as-infection when infected hosts began swapping nuclearized genomes containing co-evolved, vertically transmitted symbionts that provided protection against horizontal superinfection by more virulent symbionts. Sex-as-meiosis (fission sex) then evolved as a host strategy to un-couple (and thereby emasculate) the acquired symbiont genomes.
Sexual reproduction first appeared by 1.2 billion years ago in the Proterozoic Eon. All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species. Many protists reproduce sexually, as do the multicellular plants, animals, and fungi. There are a few species which have secondarily lost this feature, such as Bdelloidea and some parthenocarpic plants.
Organisms need to replicate their genetic material in an efficient and reliable manner. The necessity to repair genetic damage is one of the leading theories explaining the origin of sexual reproduction. Diploid individuals can repair a damaged section of their DNA via homologous recombination, since there are two copies of the gene in the cell and one copy is presumed to be undamaged. A mutation in an haploid individual, on the other hand, is more likely to become resident, as the DNA repair machinery has no way of knowing what the original undamaged sequence was. The most primitive form of sex may have been one organism with damaged DNA replicating an undamaged strand from a similar organism in order to repair itself.
Another theory is that sexual reproduction originated from selfish parasitic genetic elements that exchange genetic material (that is: copies of their own genome) for their transmission and propagation. In some organisms, sexual reproduction has been shown to enhance the spread of parasitic genetic elements (e.g.: yeast, filamentous fungi). Bacterial conjugation, a form of genetic exchange that some sources describe as sex, is not a form of reproduction, but rather an example of horizontal gene transfer. However, it does support the selfish genetic element theory, as it is propagated through such a "selfish gene", the F-plasmid. Similarly, it has been proposed that sexual reproduction evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids.
A third theory is that sex evolved as a form of cannibalism. One primitive organism ate another one, but rather than completely digesting it, some of the 'eaten' organism's DNA was incorporated into the 'eater' organism.
Sex may also be derived from prokaryotic processes. A comprehensive 'origin of sex as vaccination' theory proposes that eukaryan sex-as-syngamy (fusion sex) arose from prokaryan unilateral sex-as-infection when infected hosts began swapping nuclearized genomes containing co-evolved, vertically transmitted symbionts that provided protection against horizontal superinfection by more virulent symbionts. Sex-as-meiosis (fission sex) then evolved as a host strategy to un-couple (and thereby emasculate) the acquired symbiont genomes.
Thursday, November 29, 2012
1.8 Billion B.C.T. - The Columbia Supercontinent Began to Be Formed
Around 1.8 Billion B.C.T., the Columbia supercontinent was formed.
Columbia, also known as Nuna and Hudsonland, was one of Earth's ancient supercontinents. It is thought to have existed approximately 1.8 to 1.5 billion years ago (1.8-1.5 Ga) in the Paleoproterozoic Era. The assembly of the supercontinent Columbia (Nuna) was completed by global-scale collisional events during 2.1–1.8 Billion B.C.T. It consisted of the proto-cratons that made up the former continents of Laurentia, Baltica, Ukrainian Shield, Amazonian Shield, Australia, and possibly Siberia, North China, and Kalaharia as well. The evidence of Columbia's existence is based upon geological and paleomagnetic data.
Columbia is estimated to have been about 12,900 kilometers (about 8,000 miles) from North to South, and about 4,800 kilometers (about 3,000 miles) across at its broadest part. The east coast of India was attached to western North America, with southern Australia against western Canada. Most of South America spun so that the western edge of modern-day Brazil lined up with eastern North America, forming a continental margin that extended into the southern edge of Scandinavia.
Columbia was assembled along global-scale 2.0 to 1.8 Ga collisional orogens and contained almost all of Earth’s continental blocks. The cratonic blocks in South America and West Africa were welded by the 2.1-2.0 Ga Transamazonian and Eburnean Orogens; the Kaapvaal and Zimbabwe cratons in southern Africa were collided along the 2.0 Ga Limpopo Belt; the cratonic blocks of Laurentia were sutured along the 1.9–1.8 Ga Trans-Hudson, Penokean, Taltson–Thelon, Wopmay, Ungava, Torngat, and Nagssugtoqidain Orogens; the Kola, Karelia, Volgo-Uralia, and Sarmatia cratons in Baltica (Eastern Europe) were joined by the 1.9–1.8 Ga Kola–Karelia, Svecofennian, Volhyn-Central Russian, and Pachelma Orogens; the Anabar and Aldan Cratons in Siberia were connected by the 1.9–1.8 Ga Akitkan and Central Aldan Orogens; the East Antarctica and an unknown continental block were joined by the Transantarctic Mountains Orogen; the South and North Indian Blocks were amalgamated along the Central Indian Tectonic Zone; and the Eastern and Western Blocks of the North China Craton were welded together by the 1.85 Ga Trans-North China Orogen.
Following its final assembly at 1.8 Ga, the supercontinent Columbia underwent long-lived (1.8–1.3 Ga), subduction-related growth via accretion at key continental margins, forming a 1.8-1.3 Ga great magmatic accretionary belt along the present-day southern margin of North America, Greenland, and Baltica. It includes the 1.8-1.7 Ga Yavapai, Central Plains and Makkovikian Belts, 1.7-1.6 Ga Mazatzal and Labradorian Belts, 1.5-1.3 Ga St. Francois and Spavinaw Belts, and 1.3-1.2 Ga Elzevirian Belt in North America; the 1.8-1.7 Ga Ketilidian Belt in Greenland; and the 1.8-1.7 Transscandinavian Igneous Belt, 1.7-1.6 Ga Kongsberggian-Gothian Belt, and 1.5-1.3 Ga Southwest Sweden Granitoid Belt in Baltica. Other cratonic blocks also underwent marginal outgrowth at about the same time. In South America, a 1.8-1.3 Ga accretionary zone occurs along the western margin of the Amazonia Craton, represented by the Rio Negro, Juruena, and Rondonian Belts. In Australia, 1.8-1.5 Ga accretionary magmatic belts, including the Arunta, Mount Isa, Georgetown, Coen, and Broken Hill Belts, occur surrounding the southern and eastern margins of the North Australia Craton and the eastern margin of the Gawler Craton. In China, a 1.8-1.4 Ga accretionary magmatic zone, called the Xiong’er belt (Group), extends along the southern margin of the North China Craton.
Columbia began to fragment about 1.6 billion years ago, associated with continental rifting along the western margin of Laurentia (Belt-Purcell Supergroup), eastern India (Mahanadi and the Godavari), southern margin of Baltica (Telemark Supergroup), southeastern margin of Siberia (Riphean aulacogens), northwestern margin of South Africa (Kalahari Copper Belt), and northern margin of the North China Block (Zhaertai-Bayan Obo Belt).
Columbia, also known as Nuna and Hudsonland, was one of Earth's ancient supercontinents. It is thought to have existed approximately 1.8 to 1.5 billion years ago (1.8-1.5 Ga) in the Paleoproterozoic Era. The assembly of the supercontinent Columbia (Nuna) was completed by global-scale collisional events during 2.1–1.8 Billion B.C.T. It consisted of the proto-cratons that made up the former continents of Laurentia, Baltica, Ukrainian Shield, Amazonian Shield, Australia, and possibly Siberia, North China, and Kalaharia as well. The evidence of Columbia's existence is based upon geological and paleomagnetic data.
Columbia is estimated to have been about 12,900 kilometers (about 8,000 miles) from North to South, and about 4,800 kilometers (about 3,000 miles) across at its broadest part. The east coast of India was attached to western North America, with southern Australia against western Canada. Most of South America spun so that the western edge of modern-day Brazil lined up with eastern North America, forming a continental margin that extended into the southern edge of Scandinavia.
Columbia was assembled along global-scale 2.0 to 1.8 Ga collisional orogens and contained almost all of Earth’s continental blocks. The cratonic blocks in South America and West Africa were welded by the 2.1-2.0 Ga Transamazonian and Eburnean Orogens; the Kaapvaal and Zimbabwe cratons in southern Africa were collided along the 2.0 Ga Limpopo Belt; the cratonic blocks of Laurentia were sutured along the 1.9–1.8 Ga Trans-Hudson, Penokean, Taltson–Thelon, Wopmay, Ungava, Torngat, and Nagssugtoqidain Orogens; the Kola, Karelia, Volgo-Uralia, and Sarmatia cratons in Baltica (Eastern Europe) were joined by the 1.9–1.8 Ga Kola–Karelia, Svecofennian, Volhyn-Central Russian, and Pachelma Orogens; the Anabar and Aldan Cratons in Siberia were connected by the 1.9–1.8 Ga Akitkan and Central Aldan Orogens; the East Antarctica and an unknown continental block were joined by the Transantarctic Mountains Orogen; the South and North Indian Blocks were amalgamated along the Central Indian Tectonic Zone; and the Eastern and Western Blocks of the North China Craton were welded together by the 1.85 Ga Trans-North China Orogen.
Following its final assembly at 1.8 Ga, the supercontinent Columbia underwent long-lived (1.8–1.3 Ga), subduction-related growth via accretion at key continental margins, forming a 1.8-1.3 Ga great magmatic accretionary belt along the present-day southern margin of North America, Greenland, and Baltica. It includes the 1.8-1.7 Ga Yavapai, Central Plains and Makkovikian Belts, 1.7-1.6 Ga Mazatzal and Labradorian Belts, 1.5-1.3 Ga St. Francois and Spavinaw Belts, and 1.3-1.2 Ga Elzevirian Belt in North America; the 1.8-1.7 Ga Ketilidian Belt in Greenland; and the 1.8-1.7 Transscandinavian Igneous Belt, 1.7-1.6 Ga Kongsberggian-Gothian Belt, and 1.5-1.3 Ga Southwest Sweden Granitoid Belt in Baltica. Other cratonic blocks also underwent marginal outgrowth at about the same time. In South America, a 1.8-1.3 Ga accretionary zone occurs along the western margin of the Amazonia Craton, represented by the Rio Negro, Juruena, and Rondonian Belts. In Australia, 1.8-1.5 Ga accretionary magmatic belts, including the Arunta, Mount Isa, Georgetown, Coen, and Broken Hill Belts, occur surrounding the southern and eastern margins of the North Australia Craton and the eastern margin of the Gawler Craton. In China, a 1.8-1.4 Ga accretionary magmatic zone, called the Xiong’er belt (Group), extends along the southern margin of the North China Craton.
Columbia began to fragment about 1.6 billion years ago, associated with continental rifting along the western margin of Laurentia (Belt-Purcell Supergroup), eastern India (Mahanadi and the Godavari), southern margin of Baltica (Telemark Supergroup), southeastern margin of Siberia (Riphean aulacogens), northwestern margin of South Africa (Kalahari Copper Belt), and northern margin of the North China Block (Zhaertai-Bayan Obo Belt).
Wednesday, November 28, 2012
2.1 Billion B.C.T. - Eukaryotes Arose
By 2.1 Billion B.C.T., eukaryotes (cells with nuclei) were present on this Earth.
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the Greek eu ("good") and karyon ("nut" or "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and the Golgi apparatus. All large complex organisms are eukaryotes, including animals, plants and fungi. The group also includes many unicellular organisms.
Cell division in eukaryotes is different from that in organisms without a nucleus (Prokaryote). It involves separating the duplicated chromosomes, through movements directed by microtubules. There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (a cell having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.
Eukaryotes appear to be monophyletic, and thus make up one of the three domains of life. The two other domains, bacteria and archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things; even in a human body there are 10 times more microbes than human cells. However, due to their much larger size their collective worldwide biomass is estimated at about equal to that of prokaryotes.
The origin of the eukaryotic cell is considered a milestone in the evolution of life, since they include all complex cells and almost all multicellular organisms. The timing of this series of events is hard to determine. Some acritarchs are known from at least 1.65 billion years ago, and the possible algae Grypania has been found as far back as 2.1 billion years ago.
Organized living structures have been found in black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, dated at 2.1 billion years old. Eukaryotic life could have evolved at that time. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of a red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.
Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present in these rocks dated at 2.7 billion years old.
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the Greek eu ("good") and karyon ("nut" or "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and the Golgi apparatus. All large complex organisms are eukaryotes, including animals, plants and fungi. The group also includes many unicellular organisms.
Cell division in eukaryotes is different from that in organisms without a nucleus (Prokaryote). It involves separating the duplicated chromosomes, through movements directed by microtubules. There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (a cell having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.
Eukaryotes appear to be monophyletic, and thus make up one of the three domains of life. The two other domains, bacteria and archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things; even in a human body there are 10 times more microbes than human cells. However, due to their much larger size their collective worldwide biomass is estimated at about equal to that of prokaryotes.
The origin of the eukaryotic cell is considered a milestone in the evolution of life, since they include all complex cells and almost all multicellular organisms. The timing of this series of events is hard to determine. Some acritarchs are known from at least 1.65 billion years ago, and the possible algae Grypania has been found as far back as 2.1 billion years ago.
Organized living structures have been found in black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, dated at 2.1 billion years old. Eukaryotic life could have evolved at that time. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of a red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.
Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present in these rocks dated at 2.7 billion years old.
Tuesday, November 27, 2012
2.4 Billion B.C.T. - The Great Oxygenation Event Begins
Around 2.4 Billion B.C.T., rising levels of oxygen in the atmosphere provided an environment in which eukaryotes (cells with nuclei) would come to flourish.
The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe or the Oxygen Crisis or the Great Oxidation, was the biologically induced appearance of free oxygen (O2) in the Earth's atmosphere. Geological, isotopic, and chemical evidence suggest this major environmental change happened around 2.4 billion years ago.
Around 2.4 Billion B.C.T., cyanobacteria (see 3.5 Billion B.C.T. - Cyanobacteria Appeared) began producing oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE the excess free oxygen started to accumulate in the atmosphere.
Free oxygen is toxic to anaerobic organisms and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Additionally the free oxygen reacted with the atmospheric methane, a greenhouse gas, triggering the Huronian glaciation, possibly the longest snowball Earth episode. Free oxygen has been an important constituent of the atmosphere ever since.
The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe or the Oxygen Crisis or the Great Oxidation, was the biologically induced appearance of free oxygen (O2) in the Earth's atmosphere. Geological, isotopic, and chemical evidence suggest this major environmental change happened around 2.4 billion years ago.
Around 2.4 Billion B.C.T., cyanobacteria (see 3.5 Billion B.C.T. - Cyanobacteria Appeared) began producing oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE the excess free oxygen started to accumulate in the atmosphere.
Free oxygen is toxic to anaerobic organisms and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Additionally the free oxygen reacted with the atmospheric methane, a greenhouse gas, triggering the Huronian glaciation, possibly the longest snowball Earth episode. Free oxygen has been an important constituent of the atmosphere ever since.
Wednesday, November 21, 2012
2.5 Billion B.C.T. - The Proterozoic Eon Begins
Around 2.5. Billion B.C.T., the Proterozoic Eon began.
The Proterozoic Eon is a geological eon representing the time just before the proliferation of complex life on Earth. The name Proterozoic comes from Greek and means "earlier life". The Proterozoic Eon extended from 2.5 billion years ago to 542 million years ago, and is the most recent part of the informally named "Precambrian" time. It is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.
The well-identified events of this eon were the transition to an oxygenated atmosphere during the Mesoproterozoic; several glaciations, including the hypothesized Snowball Earth during the Cryogenian period in the late Neoproterozoic; and the Ediacaran Period (635 to 542 million years ago) which is characterized by the evolution of abundant soft-bodied multicellular organisms.
One of the most important events of the Proterozoic Eon was the gathering up of oxygen (the Great Oxygenation Event) in the Earth's atmosphere. Though oxygen was undoubtedly released by photosynthesis well back in the time of the Archean Eon, it could not build up to any significant degree until chemical sinks — unoxidized sulfur and iron — had been filled. Up until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level. Banded iron formations, which provide most of the world's iron ore, were also a prominent chemical sink; most accumulation ceased after 1.9 billion years ago, either due to an increase in oxygen or a more thorough mixing of the oceanic water column.
Red beds, which are colored by hematite, indicate an increase in atmospheric oxygen after 2 billion years ago. The oxygen buildup was probably due to two factors: a filling of the chemical sinks, and an increase in carbon burial, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere.
Throughout the history of the Earth, there have been times when the continental mass came together to form a supercontinent, followed by the break-up of the supercontinent and new continents moving apart again. This repetition of tectonic events is called a Wilson cycle. It is at least clear that, about 1.0 billion years ago to 830 million years ago, most continental mass was united in the supercontinent Rodinia. Rodinia was not the first supercontinent; it formed at about 1.0 billion years ago by accretion and collision of fragments produced by breakup of the older supercontinent, called Nuna or Columbia, which was assembled by global-scale 2.0–1.8 B.C.T. collisional events. This means that plate tectonic processes similar to today's must have been active during the Proterozoic Eon.
After the break-up of Rodinia about 800 million years ago, it is possible the continents joined again around 550 million years ago. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia. The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South-America, Antarctica and Australia. It is extremely likely, however, that the aggregation of continental masses was not completed, since a continent called Laurentia (roughly equivalent to current-day North America) had already started breaking off around 610 million years ago. It is at least certain that by the end of the Proterozoic Eon, most of the continental mass lay united in a position around the south pole.
The fossil evidence for the first advanced single-celled organisms, eukaryotes and multi-cellular life, -- the Francevillian Group Fossils --, roughly coincides with the start of the accumulation of free oxygen. This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria. It was also during the Proterozoic that the first symbiotic relationships between mitochondria (for nearly all eukaryotes) and chloroplasts (for plants and some protists only) and their hosts evolved.
The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago.
Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian period when the first fossils of animals including trilobites and archeocyathids appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 542 million years ago.
The Proterozoic Eon is a geological eon representing the time just before the proliferation of complex life on Earth. The name Proterozoic comes from Greek and means "earlier life". The Proterozoic Eon extended from 2.5 billion years ago to 542 million years ago, and is the most recent part of the informally named "Precambrian" time. It is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.
The well-identified events of this eon were the transition to an oxygenated atmosphere during the Mesoproterozoic; several glaciations, including the hypothesized Snowball Earth during the Cryogenian period in the late Neoproterozoic; and the Ediacaran Period (635 to 542 million years ago) which is characterized by the evolution of abundant soft-bodied multicellular organisms.
One of the most important events of the Proterozoic Eon was the gathering up of oxygen (the Great Oxygenation Event) in the Earth's atmosphere. Though oxygen was undoubtedly released by photosynthesis well back in the time of the Archean Eon, it could not build up to any significant degree until chemical sinks — unoxidized sulfur and iron — had been filled. Up until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level. Banded iron formations, which provide most of the world's iron ore, were also a prominent chemical sink; most accumulation ceased after 1.9 billion years ago, either due to an increase in oxygen or a more thorough mixing of the oceanic water column.
Red beds, which are colored by hematite, indicate an increase in atmospheric oxygen after 2 billion years ago. The oxygen buildup was probably due to two factors: a filling of the chemical sinks, and an increase in carbon burial, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere.
Throughout the history of the Earth, there have been times when the continental mass came together to form a supercontinent, followed by the break-up of the supercontinent and new continents moving apart again. This repetition of tectonic events is called a Wilson cycle. It is at least clear that, about 1.0 billion years ago to 830 million years ago, most continental mass was united in the supercontinent Rodinia. Rodinia was not the first supercontinent; it formed at about 1.0 billion years ago by accretion and collision of fragments produced by breakup of the older supercontinent, called Nuna or Columbia, which was assembled by global-scale 2.0–1.8 B.C.T. collisional events. This means that plate tectonic processes similar to today's must have been active during the Proterozoic Eon.
After the break-up of Rodinia about 800 million years ago, it is possible the continents joined again around 550 million years ago. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia. The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South-America, Antarctica and Australia. It is extremely likely, however, that the aggregation of continental masses was not completed, since a continent called Laurentia (roughly equivalent to current-day North America) had already started breaking off around 610 million years ago. It is at least certain that by the end of the Proterozoic Eon, most of the continental mass lay united in a position around the south pole.
The fossil evidence for the first advanced single-celled organisms, eukaryotes and multi-cellular life, -- the Francevillian Group Fossils --, roughly coincides with the start of the accumulation of free oxygen. This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria. It was also during the Proterozoic that the first symbiotic relationships between mitochondria (for nearly all eukaryotes) and chloroplasts (for plants and some protists only) and their hosts evolved.
The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago.
Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian period when the first fossils of animals including trilobites and archeocyathids appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 542 million years ago.
Tuesday, November 20, 2012
3.5 Billion B.C.T. - Cyanobacteria Appeared
It is believed that cyanobacteria may have appeared on the Earth as early as 3.5 billion years ago.
Cyanobacteria, also known as blue-green bacteria, blue-green algae, and Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria (Greek: kyanós = blue).
The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to endosymbiotic theory, chloroplasts in plants and eukaryotic algae have evolved from cyanobacterial ancestors via endosymbiosis.
Stromatolites of fossilized oxygen-producing cyanobacteria have been found from 2.8 billion years ago, possibly as old as 3.5 billion years ago.
The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria. The geologic record indicates that this transforming event took place early in our planet's history, at least 2.45-2.32 billion years ago, and probably much earlier. Geo-biological interpretation of Archean Eon sedimentary rocks remains a challenge; available evidence indicates that life existed 3.5 billion years ago, but the question of when oxygenic photosynthesis evolved continues to engender debate and research.
A clear paleontological window on cyanobacterial evolution opened about 2.0 billion years ago, revealing an already diverse biota of blue-greens. Cyanobacteria remained principal primary producers throughout the Proterozoic (2.5 Billion B.C.T. - 543 Million B.C.T.), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation.
The most common cyanobacterial structures in the fossil record are the mound-producing stromatolites and related oncolites. Indeed, these fossil colonies are so common that paleobiology, micropaleontology and paleobotany cite the Pre-Cambrian and Cambrian period as an "age of stromatolites" and an "age of algae."
Green algae joined the blue-greens as major primary producers on continental shelves near the end of the Proterozoic, but only with the Mesozoic era (251 Million B.C.T. - 65 Million B.C.T.) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form.
Today, the blue-green bacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and—in modified form—as the plastids of marine algae.
Cyanobacteria, also known as blue-green bacteria, blue-green algae, and Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria (Greek: kyanós = blue).
The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to endosymbiotic theory, chloroplasts in plants and eukaryotic algae have evolved from cyanobacterial ancestors via endosymbiosis.
Stromatolites of fossilized oxygen-producing cyanobacteria have been found from 2.8 billion years ago, possibly as old as 3.5 billion years ago.
The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria. The geologic record indicates that this transforming event took place early in our planet's history, at least 2.45-2.32 billion years ago, and probably much earlier. Geo-biological interpretation of Archean Eon sedimentary rocks remains a challenge; available evidence indicates that life existed 3.5 billion years ago, but the question of when oxygenic photosynthesis evolved continues to engender debate and research.
A clear paleontological window on cyanobacterial evolution opened about 2.0 billion years ago, revealing an already diverse biota of blue-greens. Cyanobacteria remained principal primary producers throughout the Proterozoic (2.5 Billion B.C.T. - 543 Million B.C.T.), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation.
The most common cyanobacterial structures in the fossil record are the mound-producing stromatolites and related oncolites. Indeed, these fossil colonies are so common that paleobiology, micropaleontology and paleobotany cite the Pre-Cambrian and Cambrian period as an "age of stromatolites" and an "age of algae."
Green algae joined the blue-greens as major primary producers on continental shelves near the end of the Proterozoic, but only with the Mesozoic era (251 Million B.C.T. - 65 Million B.C.T.) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form.
Today, the blue-green bacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and—in modified form—as the plastids of marine algae.
Monday, November 19, 2012
3.5. Billion B.C.T. - Photosynthesis Begins
It is believed that photosynthesis began around 3.5 billion years ago.
Photosynthesis (from the Greek photo, "light," and synthesis, "putting together", "composition") is a process used by plants and other organisms to convert the light energy captured from the sun into chemical energy that can be used to fuel the organism's activities. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product.
Photosynthesis is vital for all aerobic life on Earth. In addition to maintaining normal levels of oxygen in the atmosphere, photosynthesis is the source of energy for nearly all life on earth, either directly, through primary production, or indirectly, as the ultimate source of the energy in their food, the exceptions being chemoautotrophs that live in rocks or around deep sea hydrothermal vents. The average rate of energy capture by photosynthesis globally is immense, approximately 130 terawatts, which is about six times larger than the power consumption of human civilization. As well as energy, photosynthesis is also the source of the carbon in all the organic compounds within organisms' bodies. In all, photosynthetic organisms convert around 100–115 thousand million metric tons (i.e., 100–115 petagrams) of carbon into biomass per year.
Although photosynthesis can happen in different ways in different species, some features are always the same. For example, the process always begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls. In plants, these proteins are held inside organelles called chloroplasts, while in bacteria they are embedded in the plasma membrane. Some of the light energy gathered by chlorophylls is stored in the form of adenosine triphosphate (ATP). The rest of the energy is used to remove electrons from a substance such as water. These electrons are then used in the reactions that turn carbon dioxide into organic compounds. In plants, algae and cyanobacteria, this is done by a sequence of reactions called the Calvin cycle, but different sets of reactions are found in some bacteria, such as the reverse Krebs cycle in Chlorobium. Many photosynthetic organisms have adaptations that concentrate or store carbon dioxide. This helps reduce a wasteful process called photorespiration that can consume part of the sugar produced during photosynthesis.
The first photosynthetic organisms probably evolved about 3.5 billion years ago, early in the evolutionary history of life, when all forms of life on Earth were microorganisms and the atmosphere had much more carbon dioxide. They most likely used hydrogen or hydrogen sulfide as sources of electrons, rather than water. Cyanobacteria appeared later, around 3.0 billion years ago, and drastically changed the Earth when they began to oxygenate the atmosphere, beginning about 2.4 billion years ago. This new atmosphere enabled the evolution of complex life such as protists. Eventually, no later than 1.0 billion years ago, one of these protists formed a symbiotic relationship with a cyanobacterium, producing the ancestor of many plants and algae. The chloroplasts in modern plants are the descendants of these ancient symbiotic cyanobacteria.
Photosynthesis (from the Greek photo, "light," and synthesis, "putting together", "composition") is a process used by plants and other organisms to convert the light energy captured from the sun into chemical energy that can be used to fuel the organism's activities. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product.
Photosynthesis is vital for all aerobic life on Earth. In addition to maintaining normal levels of oxygen in the atmosphere, photosynthesis is the source of energy for nearly all life on earth, either directly, through primary production, or indirectly, as the ultimate source of the energy in their food, the exceptions being chemoautotrophs that live in rocks or around deep sea hydrothermal vents. The average rate of energy capture by photosynthesis globally is immense, approximately 130 terawatts, which is about six times larger than the power consumption of human civilization. As well as energy, photosynthesis is also the source of the carbon in all the organic compounds within organisms' bodies. In all, photosynthetic organisms convert around 100–115 thousand million metric tons (i.e., 100–115 petagrams) of carbon into biomass per year.
Although photosynthesis can happen in different ways in different species, some features are always the same. For example, the process always begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls. In plants, these proteins are held inside organelles called chloroplasts, while in bacteria they are embedded in the plasma membrane. Some of the light energy gathered by chlorophylls is stored in the form of adenosine triphosphate (ATP). The rest of the energy is used to remove electrons from a substance such as water. These electrons are then used in the reactions that turn carbon dioxide into organic compounds. In plants, algae and cyanobacteria, this is done by a sequence of reactions called the Calvin cycle, but different sets of reactions are found in some bacteria, such as the reverse Krebs cycle in Chlorobium. Many photosynthetic organisms have adaptations that concentrate or store carbon dioxide. This helps reduce a wasteful process called photorespiration that can consume part of the sugar produced during photosynthesis.
The first photosynthetic organisms probably evolved about 3.5 billion years ago, early in the evolutionary history of life, when all forms of life on Earth were microorganisms and the atmosphere had much more carbon dioxide. They most likely used hydrogen or hydrogen sulfide as sources of electrons, rather than water. Cyanobacteria appeared later, around 3.0 billion years ago, and drastically changed the Earth when they began to oxygenate the atmosphere, beginning about 2.4 billion years ago. This new atmosphere enabled the evolution of complex life such as protists. Eventually, no later than 1.0 billion years ago, one of these protists formed a symbiotic relationship with a cyanobacterium, producing the ancestor of many plants and algae. The chloroplasts in modern plants are the descendants of these ancient symbiotic cyanobacteria.
Friday, November 16, 2012
3.5 Billion B.C.T. - The First Primitive Life Forms Emerged
Around 3.5 Billion B.C.T., the first primitive life forms emerged on Earth. The fossil record contains prokaryotes (cells without nuclei) from up to 3.5 billion years ago, and eukaryotes (cells with nuclei) from up to 2.7 billion years ago.
The prokaryotes are a group of organisms whose cells lack a cell nucleus (karyon), or any other membrane-bound organelles. The organisms whose cells do have a nucleus are called eukaryotes. Most prokaryotes are unicellular organisms, although a few such as myxobacteria have multi-cellular stages in their life cycles or create large colonies like cyanobacteria. The word prokaryote comes from the Greek (pro-) "before" + (karyon) "nut or kernel".
Prokaryotes do not have a nucleus, mitochondria, or any other membrane-bound organelles. In other words, all their intra-cellular water-soluble components (proteins, DNA and metabolites) are located together in the same area enclosed by cell membrane, rather than separated in different cellular compartments.
The division to prokaryotes and eukaryotes reflects two distinct levels of cellular organization rather than biological classification of species. Prokaryotes include two major classification domains: the bacteria and the archaea. Archaea were recognized as a domain of life in 1990. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.
The division to prokaryotes and eukaryotes is usually considered the most important distinction among organisms. The distinction is that eukaryotic cells have a "true" nucleus containing their DNA, whereas prokaryotic cells do not have a nucleus. One criticism of this classification points out that the word "prokaryote" is based on what these organisms are not (they are not eukaryotic), rather than what they are (either archaea or bacteria). Another difference is that ribosomes in prokaryotes are smaller than in eukaryotes. However, two organelles found in many eukaryotic cells, mitochondria and chloroplasts, contain ribosomes similar in size and makeup to those found in prokaryotes. This is one of many pieces of evidence that mitochondria and chloroplasts are themselves descended from free-living bacteria.
The genome in a prokaryote is held within a DNA/protein complex in the cytosol called the nucleoid, which lacks a nuclear envelope. The complex contains a single, cyclic, double-stranded molecule of stable chromosomal DNA, in contrast to the multiple linear, compact, highly organized chromosomes found in eukaryotic cells. In addition, many important genes of prokaryotes are stored in separate circular DNA structures called plasmids.
Prokaryotes lack distinct mitochondria and chloroplasts. Instead, processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic cell membrane. However, prokaryotes do possess some internal structures, such as prokaryotic cytoskeletons, and the bacterial order Planctomycetes have a membrane around their nucleoid and contain other membrane-bound cellular structures. Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein.
Prokaryotic cells are usually much smaller than eukaryotic cells. Therefore, prokaryotes have a larger surface-area-to-volume ratio, giving them a higher metabolic rate, a higher growth rate, and, as a consequently, a shorter generation time than eukaryotes.
In 1977, Carl Woese proposed dividing prokaryotes into the bacteria and archaea (originally eubacteria and archaebacteria) because of the major differences in the structure and genetics between the two groups of organisms. This arrangement of eukaryota (also called "eukarya"), bacteria, and archaea is called the three-domain system, replacing the traditional two-empire system.
The oldest known fossilized prokaryotes were laid down approximately 3.5 billion years ago, only about 1 billion years after the formation of the Earth's crust. Even today, prokaryotes are perhaps the most successful and abundant life-forms. Eukaryotes only appear in the fossil record later, and may have formed from endosymbiosis of multiple prokaryote ancestors. The oldest known fossil eukaryotes are about 1.7 billion years old. However, some genetic evidence suggests eukaryotes appeared as early as 3 billion years ago.
While Earth is the only place in the universe where life is known to exist, some have suggested that there is evidence on Mars of fossil or living prokaryotes; but this possibility remains the subject of considerable debate and skepticism.
Prokaryotes have diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct prokaryotic types. For example, in addition to using photosynthesis or organic compounds for energy, as eukaryotes do, prokaryotes may obtain energy from inorganic compounds such as hydrogen sulfide. This enables prokaryotes to thrive in harsh environments as cold as the snow surface of Antarctica, and as hot as undersea hydrothermal vents and land-based hot springs.
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A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the Greek eu ("good") and karyon ("nut" or "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and the Golgi apparatus. All large complex organisms are eukaryotes, including animals, plants and fungi. The group also includes many unicellular organisms.
Cell division in eukaryotes is different from that in organisms without a nucleus (Prokaryote). It involves separating the duplicated chromosomes, through movements directed by microtubules. There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (a cell having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.
Eukaryotes appear to be monophyletic, and thus make up one of the three domains of life. The two other domains, bacteria and archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things; even in a human body there are 10 times more microbes than human cells. However, due to their much larger size their collective worldwide biomass is estimated at about equal to that of prokaryotes.
The origin of the eukaryotic cell is considered a milestone in the evolution of life, since they include all complex cells and almost all multicellular organisms. The timing of this series of events is hard to determine. Some acritarchs are known from at least 1.65 billion years ago, and the possible algae Grypania has been found as far back as 2.1 billion years ago.
Organized living structures have been found in black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, dated at 2.1 billion years old. Eukaryotic life could have evolved at that time. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of a red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.
Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present in these rocks dated at 2.7 billion years old.
The prokaryotes are a group of organisms whose cells lack a cell nucleus (karyon), or any other membrane-bound organelles. The organisms whose cells do have a nucleus are called eukaryotes. Most prokaryotes are unicellular organisms, although a few such as myxobacteria have multi-cellular stages in their life cycles or create large colonies like cyanobacteria. The word prokaryote comes from the Greek (pro-) "before" + (karyon) "nut or kernel".
Prokaryotes do not have a nucleus, mitochondria, or any other membrane-bound organelles. In other words, all their intra-cellular water-soluble components (proteins, DNA and metabolites) are located together in the same area enclosed by cell membrane, rather than separated in different cellular compartments.
The division to prokaryotes and eukaryotes reflects two distinct levels of cellular organization rather than biological classification of species. Prokaryotes include two major classification domains: the bacteria and the archaea. Archaea were recognized as a domain of life in 1990. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.
The division to prokaryotes and eukaryotes is usually considered the most important distinction among organisms. The distinction is that eukaryotic cells have a "true" nucleus containing their DNA, whereas prokaryotic cells do not have a nucleus. One criticism of this classification points out that the word "prokaryote" is based on what these organisms are not (they are not eukaryotic), rather than what they are (either archaea or bacteria). Another difference is that ribosomes in prokaryotes are smaller than in eukaryotes. However, two organelles found in many eukaryotic cells, mitochondria and chloroplasts, contain ribosomes similar in size and makeup to those found in prokaryotes. This is one of many pieces of evidence that mitochondria and chloroplasts are themselves descended from free-living bacteria.
The genome in a prokaryote is held within a DNA/protein complex in the cytosol called the nucleoid, which lacks a nuclear envelope. The complex contains a single, cyclic, double-stranded molecule of stable chromosomal DNA, in contrast to the multiple linear, compact, highly organized chromosomes found in eukaryotic cells. In addition, many important genes of prokaryotes are stored in separate circular DNA structures called plasmids.
Prokaryotes lack distinct mitochondria and chloroplasts. Instead, processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic cell membrane. However, prokaryotes do possess some internal structures, such as prokaryotic cytoskeletons, and the bacterial order Planctomycetes have a membrane around their nucleoid and contain other membrane-bound cellular structures. Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein.
Prokaryotic cells are usually much smaller than eukaryotic cells. Therefore, prokaryotes have a larger surface-area-to-volume ratio, giving them a higher metabolic rate, a higher growth rate, and, as a consequently, a shorter generation time than eukaryotes.
In 1977, Carl Woese proposed dividing prokaryotes into the bacteria and archaea (originally eubacteria and archaebacteria) because of the major differences in the structure and genetics between the two groups of organisms. This arrangement of eukaryota (also called "eukarya"), bacteria, and archaea is called the three-domain system, replacing the traditional two-empire system.
The oldest known fossilized prokaryotes were laid down approximately 3.5 billion years ago, only about 1 billion years after the formation of the Earth's crust. Even today, prokaryotes are perhaps the most successful and abundant life-forms. Eukaryotes only appear in the fossil record later, and may have formed from endosymbiosis of multiple prokaryote ancestors. The oldest known fossil eukaryotes are about 1.7 billion years old. However, some genetic evidence suggests eukaryotes appeared as early as 3 billion years ago.
While Earth is the only place in the universe where life is known to exist, some have suggested that there is evidence on Mars of fossil or living prokaryotes; but this possibility remains the subject of considerable debate and skepticism.
Prokaryotes have diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct prokaryotic types. For example, in addition to using photosynthesis or organic compounds for energy, as eukaryotes do, prokaryotes may obtain energy from inorganic compounds such as hydrogen sulfide. This enables prokaryotes to thrive in harsh environments as cold as the snow surface of Antarctica, and as hot as undersea hydrothermal vents and land-based hot springs.
*****
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the Greek eu ("good") and karyon ("nut" or "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and the Golgi apparatus. All large complex organisms are eukaryotes, including animals, plants and fungi. The group also includes many unicellular organisms.
Cell division in eukaryotes is different from that in organisms without a nucleus (Prokaryote). It involves separating the duplicated chromosomes, through movements directed by microtubules. There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (a cell having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.
Eukaryotes appear to be monophyletic, and thus make up one of the three domains of life. The two other domains, bacteria and archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things; even in a human body there are 10 times more microbes than human cells. However, due to their much larger size their collective worldwide biomass is estimated at about equal to that of prokaryotes.
The origin of the eukaryotic cell is considered a milestone in the evolution of life, since they include all complex cells and almost all multicellular organisms. The timing of this series of events is hard to determine. Some acritarchs are known from at least 1.65 billion years ago, and the possible algae Grypania has been found as far back as 2.1 billion years ago.
Organized living structures have been found in black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, dated at 2.1 billion years old. Eukaryotic life could have evolved at that time. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of a red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.
Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present in these rocks dated at 2.7 billion years old.
Thursday, November 15, 2012
3.7 Billion B.C.T. - The Late Heavy Bombardment Ended
Around 3.7 Billion B.C.T., the Late Heavy Bombardment of the Earth came to an end.
The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time approximately 4.1 to 3.7 billion years ago during which a large number of impact craters were formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well. The LHB is "late" only in relation to the main period of accretion, when the Earth and the other three rocky planets first formed and gained most of their mass. In relation to the Earth or to Solar System history as a whole, it is still a fairly early phase. The evidence for this event comes primarily from the dating of lunar samples, which indicates that most impact melt rocks formed in this rather narrow interval of time. While many hypotheses have been put forth to explain a spike in the flux of either asteroidal or cometary materials in the inner Solar System, no consensus yet exists as to its cause. The Nice model, popular among planetary scientists, postulates that the gas giant planets underwent orbital migration at this time, scattering objects in the asteroid belt and/or Kuiper belt on eccentric orbits that crossed those of the terrestrial planets.
The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time approximately 4.1 to 3.7 billion years ago during which a large number of impact craters were formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well. The LHB is "late" only in relation to the main period of accretion, when the Earth and the other three rocky planets first formed and gained most of their mass. In relation to the Earth or to Solar System history as a whole, it is still a fairly early phase. The evidence for this event comes primarily from the dating of lunar samples, which indicates that most impact melt rocks formed in this rather narrow interval of time. While many hypotheses have been put forth to explain a spike in the flux of either asteroidal or cometary materials in the inner Solar System, no consensus yet exists as to its cause. The Nice model, popular among planetary scientists, postulates that the gas giant planets underwent orbital migration at this time, scattering objects in the asteroid belt and/or Kuiper belt on eccentric orbits that crossed those of the terrestrial planets.
3.7 Billion B.C.T. - Life Begins on Earth
Around 3.7 Billion B.C.T., life began on Earth.
Evidence suggests that life on Earth has existed for about 3.7 billion years, with the oldest traces of life found in fossils dating back 3.4 billion years. All known life forms share fundamental molecular mechanisms, reflecting their common descent; based on these observations, hypotheses on the origin of life attempt to find a mechanism explaining the formation of a universal common ancestor, from simple organic molecules via pre-cellular life to protocells and metabolism. Models have been divided into "genes-first" and "metabolism-first" categories, but a recent trend is the emergence of hybrid models that combine both categories.
There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the following observations:
However, since genes and proteins are both required to produce the other, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because of this, it is unlikely that genes and proteins arose independently.
A possibility, first suggested by Francis Crick, is that the first life was based on RNA, which has the DNA-like properties of information storage and the catalytic properties of some proteins. This is called the RNA world hypothesis, and it is supported by the observation that many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. Also, many critical co-factors are either nucleotides or substances clearly related to them. The catalytic properties of RNA had not yet been demonstrated when the hypothesis was first proposed, but they were confirmed by Thomas Cech in 1986.
One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic precursors is more difficult than for other organic molecules. One reason for this is that RNA precursors are very stable and react with each other very slowly under ambient conditions, and it has also been proposed that living organisms consisted of other molecules before RNA. However, the successful synthesis of certain RNA molecules under the conditions that existed prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction. This study makes the RNA world hypothesis more plausible.
In 2009, experiments demonstrated Darwinian evolution of a two-component system of RNA enzymes (ribozymes) in vitro. The work was performed in the laboratory of Gerald Joyce.
NASA findings in 2011, based on studies with meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules) may be formed extraterrestrially in outer space.
Evidence suggests that life on Earth has existed for about 3.7 billion years, with the oldest traces of life found in fossils dating back 3.4 billion years. All known life forms share fundamental molecular mechanisms, reflecting their common descent; based on these observations, hypotheses on the origin of life attempt to find a mechanism explaining the formation of a universal common ancestor, from simple organic molecules via pre-cellular life to protocells and metabolism. Models have been divided into "genes-first" and "metabolism-first" categories, but a recent trend is the emergence of hybrid models that combine both categories.
There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the following observations:
- The Miller-Urey experiment, and the work of Sidney Fox, show that conditions on the primitive Earth favored chemical reactions that synthesize amino acids and other organic compounds from inorganic precursors.
- Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
However, since genes and proteins are both required to produce the other, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because of this, it is unlikely that genes and proteins arose independently.
A possibility, first suggested by Francis Crick, is that the first life was based on RNA, which has the DNA-like properties of information storage and the catalytic properties of some proteins. This is called the RNA world hypothesis, and it is supported by the observation that many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. Also, many critical co-factors are either nucleotides or substances clearly related to them. The catalytic properties of RNA had not yet been demonstrated when the hypothesis was first proposed, but they were confirmed by Thomas Cech in 1986.
One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic precursors is more difficult than for other organic molecules. One reason for this is that RNA precursors are very stable and react with each other very slowly under ambient conditions, and it has also been proposed that living organisms consisted of other molecules before RNA. However, the successful synthesis of certain RNA molecules under the conditions that existed prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction. This study makes the RNA world hypothesis more plausible.
In 2009, experiments demonstrated Darwinian evolution of a two-component system of RNA enzymes (ribozymes) in vitro. The work was performed in the laboratory of Gerald Joyce.
NASA findings in 2011, based on studies with meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules) may be formed extraterrestrially in outer space.
Wednesday, November 14, 2012
3.8 Billion B.C.T. - The Archean Eon Begins
Around 3.8 Billion B.C.T., the Archean Eon began.
The Archean (also spelled Archaean) is a geologic eon before the Proterozoic Eon, before 2.5 billion years ago. The Archean era is generally agreed to have started at 3.8 billion years ago, but this boundary is informal.
If you were able to travel back to visit the Earth during the Archean Eon, you would likely not recognize it as the same planet we inhabit today. The atmosphere was very different from what we breathe today. At that time, it was likely a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. Also, during this time, the Earth's crust cooled enough that rocks and continental plates began to form.
It was early in the Archean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. In fact, all life during the more than one billion years of the Archean was bacterial. The Archean coast was home to mounded colonies of photosynthetic bacteria called stromatolites. Stromatolites have been found as fossils in early Archean rocks of South Africa and western Australia. Stromatolites increased in abundance throughout the Archean, but began to decline during the Proterozoic. They are not common today, but they can still be found in Shark Bay, Australia.
The Archean (also spelled Archaean) is a geologic eon before the Proterozoic Eon, before 2.5 billion years ago. The Archean era is generally agreed to have started at 3.8 billion years ago, but this boundary is informal.
If you were able to travel back to visit the Earth during the Archean Eon, you would likely not recognize it as the same planet we inhabit today. The atmosphere was very different from what we breathe today. At that time, it was likely a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. Also, during this time, the Earth's crust cooled enough that rocks and continental plates began to form.
It was early in the Archean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. In fact, all life during the more than one billion years of the Archean was bacterial. The Archean coast was home to mounded colonies of photosynthetic bacteria called stromatolites. Stromatolites have been found as fossils in early Archean rocks of South Africa and western Australia. Stromatolites increased in abundance throughout the Archean, but began to decline during the Proterozoic. They are not common today, but they can still be found in Shark Bay, Australia.
Tuesday, November 13, 2012
4.0 Billion B.C.T. - A Meteor Struck Mars
Around 4 Billion B.C.T., conditions for the creation of life on Earth began to be maximized. Also, around this time, a meteor struck Mars.
When a meteor perhaps as large as a hundred miles in diameter smashed into Mars four billion years ago, the force of the impact re-contoured much of the southern Martian hemisphere. The collision punched a hole in the surface 1,300 miles across and 5 miles deep. The fallout of debris, enough to theoretically deposit a mile-thick layer across the continental United States, stretched 2,500 miles from the basin's center.
The Martian meteor impact of four billion years ago has a bearing on life on Earth because the debris from such impacts wound up landing on Earth and may possibly have contributed to the organic chemistry -- the chemistry of life -- that came to exist.
As of 2012, of the over 53,000 meteorites that have been found on Earth, 99 have been identified as Martian. These meteorites are thought to be from Mars because they have elemental and isotopic compositions that are similar to rocks and atmosphere gases analyzed by spacecraft on Mars.
Some planetary scientists think that the impact of asteroids and comets on Mars sent chunks of Martian rock hurtling into space, where they orbited the sun for millions of years. Finally, some of these bits of Mars came crashing down in Antarctica and other places on Earth. However, the impact on Mars some 4 billion years ago may have led to a more direct contribution of Martian materials to the milieu of what was then the nascent terraforming of Earth.
When a meteor perhaps as large as a hundred miles in diameter smashed into Mars four billion years ago, the force of the impact re-contoured much of the southern Martian hemisphere. The collision punched a hole in the surface 1,300 miles across and 5 miles deep. The fallout of debris, enough to theoretically deposit a mile-thick layer across the continental United States, stretched 2,500 miles from the basin's center.
The Martian meteor impact of four billion years ago has a bearing on life on Earth because the debris from such impacts wound up landing on Earth and may possibly have contributed to the organic chemistry -- the chemistry of life -- that came to exist.
As of 2012, of the over 53,000 meteorites that have been found on Earth, 99 have been identified as Martian. These meteorites are thought to be from Mars because they have elemental and isotopic compositions that are similar to rocks and atmosphere gases analyzed by spacecraft on Mars.
Some planetary scientists think that the impact of asteroids and comets on Mars sent chunks of Martian rock hurtling into space, where they orbited the sun for millions of years. Finally, some of these bits of Mars came crashing down in Antarctica and other places on Earth. However, the impact on Mars some 4 billion years ago may have led to a more direct contribution of Martian materials to the milieu of what was then the nascent terraforming of Earth.
Tuesday, November 6, 2012
4.56 Billion B.C.T. - The Hadean Eon Begins
Around 4.56 Billion Years B.C.T., the Hadean Eon began. During this period the Earth-Moon system would be formed.
The Hadean Eon is the first geologic eon of the Earth and lies before the Archean Eon. It began with the formation of the Earth about 4.56 billion years ago and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, the Greek name for the underworld. The name is in reference to the "hellish" conditions on Earth at the time: the planet had just formed and was still very hot due to high volcanism, a partially molten surface and frequent collisions with other Solar System bodies. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth.
The Moon is the only natural satellite of the Earth, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having 27% the diameter and 60% the density of Earth, resulting in 1⁄81 its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter.
The Moon is in synchronous rotation with the Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a reflectance similar to that of coal. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. This matching of apparent visual size is a coincidence. Earlier in the Earth's history, the Moon was closer to the Earth, and had an apparent visual size greater than that of the Sun.
The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body. The Moon is the only celestial body other than the Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history.
Although the Moon itself is devoid of air, water, and life, its creation and existence has proven to be essential to the origin of life on Earth. It is, after all, our unusually large moon which exerts a stabilizing influence on the Earth's spin axis that has prevented the planet from wobbling like an out-of-control top -- and has thereby saved the Earth from wild climate fluctuations that would have been hostile to life. Indeed, it is generally conceded that we, the inhabitants of the Earth, are in an exceptional state. We owe our very existence to the terrestrial stability caused by the nightly present Moon.
The tilt of the Earth's spin axis, called the obliquity, is what gives us seasons. If the Earth were spinning like a perfectly upright top, perpendicular to the plane of its orbit around the Sun (an obliquity of zero), there would hardly be any seasons at all because every point on the planet would receive a constant amount of sunlight all year long. However, if the Earth were rolling on its side (an obliquity of 90 degrees), each pole would swing between extreme heat and total darkness every year. Because the Earth's spin axis is only slightly tilted, by about 23.5 degrees, the planet enjoys moderate seasonal variations.
This slight tilt of the Earth's spin axis has been the case for eons and it has been so because of the Moon. Because the Earth spins, the planet bulges at the equator. The Sun and the other planets exert a gravitational pull on this bulge, causing the Earth's axis to rock slowly. As the planets move in their orbits -- and as they deform one another's orbits through their gravitational interactions -- the overall strength of the various forces would cause the Earth's spin axis to oscillate in an inherently unpredictable, chaotic way. Computer modeling indicates that the oscillation could vary anywhere from 0 to 85 degrees -- an amount that would make the possibility of life on Earth very unlikely.
However, with the Moon, chaos is averted and stability is imposed. The Moon packs enough gravitational pull to effectively cancel most of the other forces on the Earth's spin axis. With just a little help from the Sun, the Moon makes the Earth's axis settle into a regular motion, causing it to precess in a small circle every 26,000 years. The tilt of the axis does change over time, but only by 1.3 degrees instead of 85.
It is important to note that a mere one degree change in obliquity is not entirely harmless. Indeed, many scientists think that such a change could trigger an ice age. Given that such a traumatic climatic change could accompany only a minor oscillation, an obliquity that fluctuated by tens of degrees would have played havoc with the world's environment. The climate would have swung between epochs with extreme seasonal variations and epochs with none at all. Eco-systems would not have been able to stabilize long enough for advanced life forms such as humans to evolve.
It is now a generally accepted theory that the stability of the Earth's spin axis is a key to life. And as further proof of the validity of this theory, scientists point to Mars. Even though Mars currently tilts at a comfortable 25 degrees, that situation is probably temporary. Mars has only two Manhattan size satellites. Even together, these moons exert a puny gravitational force -- hardly enough to counteract other planetary influences.
The Hadean Eon is the first geologic eon of the Earth and lies before the Archean Eon. It began with the formation of the Earth about 4.56 billion years ago and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, the Greek name for the underworld. The name is in reference to the "hellish" conditions on Earth at the time: the planet had just formed and was still very hot due to high volcanism, a partially molten surface and frequent collisions with other Solar System bodies. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth.
The Moon is the only natural satellite of the Earth, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having 27% the diameter and 60% the density of Earth, resulting in 1⁄81 its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter.
The Moon is in synchronous rotation with the Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a reflectance similar to that of coal. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. This matching of apparent visual size is a coincidence. Earlier in the Earth's history, the Moon was closer to the Earth, and had an apparent visual size greater than that of the Sun.
The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body. The Moon is the only celestial body other than the Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history.
Although the Moon itself is devoid of air, water, and life, its creation and existence has proven to be essential to the origin of life on Earth. It is, after all, our unusually large moon which exerts a stabilizing influence on the Earth's spin axis that has prevented the planet from wobbling like an out-of-control top -- and has thereby saved the Earth from wild climate fluctuations that would have been hostile to life. Indeed, it is generally conceded that we, the inhabitants of the Earth, are in an exceptional state. We owe our very existence to the terrestrial stability caused by the nightly present Moon.
The tilt of the Earth's spin axis, called the obliquity, is what gives us seasons. If the Earth were spinning like a perfectly upright top, perpendicular to the plane of its orbit around the Sun (an obliquity of zero), there would hardly be any seasons at all because every point on the planet would receive a constant amount of sunlight all year long. However, if the Earth were rolling on its side (an obliquity of 90 degrees), each pole would swing between extreme heat and total darkness every year. Because the Earth's spin axis is only slightly tilted, by about 23.5 degrees, the planet enjoys moderate seasonal variations.
This slight tilt of the Earth's spin axis has been the case for eons and it has been so because of the Moon. Because the Earth spins, the planet bulges at the equator. The Sun and the other planets exert a gravitational pull on this bulge, causing the Earth's axis to rock slowly. As the planets move in their orbits -- and as they deform one another's orbits through their gravitational interactions -- the overall strength of the various forces would cause the Earth's spin axis to oscillate in an inherently unpredictable, chaotic way. Computer modeling indicates that the oscillation could vary anywhere from 0 to 85 degrees -- an amount that would make the possibility of life on Earth very unlikely.
However, with the Moon, chaos is averted and stability is imposed. The Moon packs enough gravitational pull to effectively cancel most of the other forces on the Earth's spin axis. With just a little help from the Sun, the Moon makes the Earth's axis settle into a regular motion, causing it to precess in a small circle every 26,000 years. The tilt of the axis does change over time, but only by 1.3 degrees instead of 85.
It is important to note that a mere one degree change in obliquity is not entirely harmless. Indeed, many scientists think that such a change could trigger an ice age. Given that such a traumatic climatic change could accompany only a minor oscillation, an obliquity that fluctuated by tens of degrees would have played havoc with the world's environment. The climate would have swung between epochs with extreme seasonal variations and epochs with none at all. Eco-systems would not have been able to stabilize long enough for advanced life forms such as humans to evolve.
It is now a generally accepted theory that the stability of the Earth's spin axis is a key to life. And as further proof of the validity of this theory, scientists point to Mars. Even though Mars currently tilts at a comfortable 25 degrees, that situation is probably temporary. Mars has only two Manhattan size satellites. Even together, these moons exert a puny gravitational force -- hardly enough to counteract other planetary influences.
It is estimated that the obliquity of Mars has probably varied between 0 and 60 degrees. That suggests a Martian history (and future) of wild climate fluctuations. Indeed, photographs of Mars' polar regions from the twin Viking orbiters reveal layers of ice and dust stacked unevenly like hotcakes -- a possible geologic record of the advance and retreat of the polar ice caps over hundreds of millions of years.
There are two implications of the association of the Moon with life on Earth that are quite profound. While planets like the Earth may well be abundant in the Universe, astronomers now concede that Earth-size planets with moons as large as ours are likely to be extremely rare. Thus, if a large moon is a prerequisite for the evolution of life, beings like us might turn out to be exceedingly rare as well.
The other implication is that the Earth can count on its stable climate only as long as it has a large moon nearby. However, the Moon, as a result of its gravitational interaction with the Earth, is gradually being accelerated into a higher orbit. The Moon is receding from the Earth at a rate of about one inch per year. In about one billion years, the pull of the Moon will be so weak that the Earth's obliquity will begin to fluctuate chaotically. When that happens the Earth's climate will surely undergo wild variations making life on the planet extremely difficult and probably impossible -- at least based upon our current perspective and upon the currently known abilities of the beings known as Homo sapiens.
There are two implications of the association of the Moon with life on Earth that are quite profound. While planets like the Earth may well be abundant in the Universe, astronomers now concede that Earth-size planets with moons as large as ours are likely to be extremely rare. Thus, if a large moon is a prerequisite for the evolution of life, beings like us might turn out to be exceedingly rare as well.
The other implication is that the Earth can count on its stable climate only as long as it has a large moon nearby. However, the Moon, as a result of its gravitational interaction with the Earth, is gradually being accelerated into a higher orbit. The Moon is receding from the Earth at a rate of about one inch per year. In about one billion years, the pull of the Moon will be so weak that the Earth's obliquity will begin to fluctuate chaotically. When that happens the Earth's climate will surely undergo wild variations making life on the planet extremely difficult and probably impossible -- at least based upon our current perspective and upon the currently known abilities of the beings known as Homo sapiens.
4.560 - 4.550 Billion Years B.C.T. – A Proto-Earth formed at the outer (cooler) edge of the habitable zone of the Solar System. At this stage the solar constant of the Sun was only about 73% of its current value, but liquid water may have existed on the surface of the Proto-Earth, probably due to the greenhouse warming of high levels of methane and carbon dioxide present in the atmosphere. The early bombardment began. Because the solar neighborhood is rife with large planetoids and debris, Earth experiences a number of giant impacts that help to increase its overall size.
4.533 Billion Years B.C.T. – The Precambrian, now termed a "supereon" but formerly an era, is split into three geological time intervals called eons: Hadean, Archaean and Proterozoic. The latter two are sub-divided into several eras as currently defined. In total, the Precambrian comprises some 85% of geological time from the formation of Earth to the time when creatures first developed exoskeletons (i.e., hard outer parts) and thereby left abundant fossil remains.
4.533 Billion Years B.C.T. – The Hadean Eon, Precambrian Supereon, and unofficial Cryptic era start as the Earth-Moon system forms, possibly as a result of a glancing collision between proto-Earth and the hypothetical protoplanet Theia (the Earth was considerably smaller than now, before this impact). This impact vaporized a large amount of the crust, and sent material into orbit around Earth, which lingered as rings, similar to those of Saturn, for a few million years, until they coalesced to become the Moon. The Moon geology pre-Nectarian period starts. Earth was covered by a magmatic ocean 200 kilometers (120 miles) deep resulting from the impact energy from this and other planetesimals during the early bombardment phase, and energy released by the planetary core forming. Outgassing from crustal rocks gives Earth a reducing atmosphere of methane, nitrogen, hydrogen, ammonia, and water vapor, with lesser amounts of hydrogen sulfide, carbon monoxide, then carbon dioxide. With further full outgassing over 1000–1500 K, nitrogen and ammonia become lesser constituents, and comparable amounts of methane, carbon monoxide, carbon dioxide, water vapor, and hydrogen are released.
4.5 Billion Years B.C.T. – The Sun entered a main sequence: a solar wind swept the Earth-Moon system clear of debris (mainly dust and gas). The end of the Early Bombardment Phase. The Basin Groups Era begins on Earth.
Basin Groups refers to 9 subdivisions of the lunar Pre-Nectarian geologic period. It is the second era of the Hadean.
The motivation for creating the Basin Groups subdivisions was to place 30 pre-Nectarian impact basins into 9 relative age groups. The relative age of the first basin in each group is based on crater densities and superposition relationships, whereas the other basins are included based on weaker grounds.[1] Basin Group 1 has no official age for its base, and the boundary between Basin Group 9 and the Nectarian period is defined by the formation of the Nectaris impact basin.
The age of the Nectaris basin is somewhat contentious, with the most frequently cited numbers placing it at 3.92 Ga, or more infrequently at 3.85 Ga.[2] Recently, however, it has been suggested that the Nectaris basin could be, in fact, much older and might have formed at ~4.1 Ga.[3] Basin Groups are not used as a geologic period on any of the United States Geological Survey lunar geologic maps. Basin Groups 1-9 and the earlier (informal) Cryptic era together make up the totality of the Pre-Nectarian period.
Since little or no geological evidence on Earth exists from the time spanned by the Pre-Nectarian period of the Moon, the Pre-Nectarian has been used as a guide by at least one notable scientific work[4] to subdivide the unofficial terrestrial Hadean eon. In particular, it is sometimes found that the Hadean eon is subdivided into the Cryptic era and Basin Groups 1-9 (which collectively make up the Pre-Nectarian), and the Nectarian and Lower Imbrian. The first lifeforms (self replicating RNA molecules, see RNA world hypothesis) may have evolved on earth around 4 bya during this era.
- c. 4,450 Ma – 100 million years after the Moon formed, the first lunar crust, formed of lunar anorthosite, differentiates from lower magmas. The earliest Earth crust probably forms similarly out of similar material. On Earth the pluvial period starts, in which the Earth's crust cools enough to let oceans form.
- c. 4,404 Ma – First known mineral, found at Jack Hills in Western Australia. Detrital zircons show presence of a solid crust and liquid water. Latest possible date for a secondary atmosphere to form, produced by the Earth's crust outgassing, reinforced by water and possibly organic molecules delivered by comet impacts and carbonaceous chondrites (including type CI shown to be high in a number of amino acids and polycyclic aromatic hydrocarbons (PAH)).
- c. 4,300 Ma – Nectarian Era begins on Earth.
- c. 4,250 Ma – Earliest evidence for life, based on unusually high amounts of light isotopes of carbon, a common sign of life, found in Earth's oldest mineral deposits located in the Jack Hills of Western Australia.[4]
- c. 4,100 Ma – Early Imbrian Era begins on Earth. Late heavy bombardment of the Moon (and probably of the Earth as well) by bolides and asteroids, produced possibly by the planetary migration of Neptune into the Kuiper belt as a result of orbital resonances between Jupiter and Saturn.[5] "Remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia.[6][7] According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."[6]
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