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.
Subscribe to:
Posts (Atom)