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).
Thursday, November 29, 2012
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.
*****
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.
Subscribe to:
Posts (Atom)