77-693: Otocephala is a clade of ray-finned fishes within the infraclass Teleostei that evolved some 230 million years ago. It is named for the presence of a hearing (otophysic) link from the swimbladder to the inner ear . Other names proposed for the group include Ostarioclupeomorpha and Otomorpha. The clade contains Clupeiformes (herrings) and Ostariophysi , a group of other orders including Cypriniformes (minnows and allies), Gymnotiformes (knifefish), and Siluriformes (catfish). Otocephala may also contain Alepocephaliformes (slickheads), but as yet (2016) without morphological evidence. The clade
154-534: A clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are
231-507: A predatory microorganism invaded a large prokaryote, probably an archaean , but instead of killing its prey, the attacker took up residence and evolved into mitochondria; one of these chimeras later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of plants ; and so on. After each endosymbiosis, the partners eventually eliminated unproductive duplication of genetic functions by re-arranging their genomes,
308-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
385-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
462-446: A compartment for abiogenic processes. A genomic analysis supports this hypothesis as they found 355 genes that likely traced to LUCA upon 6.1 million sequenced prokaryotic genes. They reconstruct LUCA as a thermophilic anaerobe with a Wood-Ljungdahl pathway, implying an origin of life at white smokers. LUCA would also have exhibited other biochemical pathways such as gluconeogenesis , reverse incomplete Krebs cycle , glycolysis , and
539-399: A different scenario a single last universal ancestor, e.g. a "first cell" or a first individual precursor cell has never existed. Instead, the early biochemical evolution of life led to diversification through the development of a multiphenotypical population of pre-cells from which the precursor cells ( protocells ) of the three domains of life emerged. Thus, the formation of cells was
616-649: A left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago). Ediacara biota appeared during the Ediacaran period, while vertebrates , along with most other modern phyla originated about 525 Ma during the Cambrian explosion . During the Permian period, synapsids , including the ancestors of mammals , dominated the land. The Permian–Triassic extinction event killed most complex species of its time, 252 Ma . During
693-443: A long way to go, since theoretical and empirical approaches are only beginning to make contact with each other. Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can catalyze both their own replication and
770-480: A particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters. Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles" and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of
847-583: A process which sometimes involved transfer of genes between them. Another hypothesis proposes that mitochondria were originally sulfur - or hydrogen -metabolising endosymbionts, and became oxygen-consumers later. On the other hand, mitochondria might have been part of eukaryotes' original equipment. There is a debate about when eukaryotes first appeared: the presence of steranes in Australian shales may indicate eukaryotes at 2.7 Ga; however, an analysis in 2008 concluded that these chemicals infiltrated
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#1732797845509924-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
1001-409: A successive process. See § Metabolism first: Pre-cells, successive cellularisation , below. Life on Earth is based on carbon and water . Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from carbon dioxide . There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; silicon ,
1078-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
1155-533: A very small percentage of species have been identified: one estimate claims that Earth may have 1 trillion species, because "identifying every microbial species on Earth presents a huge challenge." Only 1.75–1.8 million species have been named and 1.8 million documented in a central database. The currently living species represent less than one percent of all species that have ever lived on Earth. The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with
1232-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
1309-966: Is an excellent solvent and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have electrically negative and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as ammonia , are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages. Organisms based on alternative biochemistry may, however, be possible on other planets. Research on how life might have emerged from non-living chemicals focuses on three possible starting points: self-replication , an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes , which allow food to enter and waste products to leave, but exclude unwanted substances. Research on abiogenesis still has
1386-604: Is further evidence of possibly the oldest forms of life in the form of fossilized microorganisms in hydrothermal vent precipitates from the Nuvvuagittuq Belt , that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and after the Earth formed 4.54 billion years ago. These earliest fossils, however, may have originated from non-biological processes. Microbial mats of coexisting bacteria and archaea were
1463-476: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
1540-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
1617-446: Is sequestered into calcium carbonate ( calcite ), phosphate concentrations are able to increase to levels necessary for facilitating biomolecule creation. Though carbonate-rich lakes have alkaline chemistry in modern times, models suggest that carbonate lakes had a pH low enough for prebiotic synthesis when placed in the acidifying context of Earth's early carbon dioxide rich atmosphere . Rainwater rich in carbonic acid weathered
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#17327978455091694-477: Is shown in the adjacent figure, where important evolutionary improvements are indicated by numbers. Wet-dry cycles at geothermal springs are shown to solve the problem of hydrolysis and promote the polymerization and vesicle encapsulation of biopolymers. The temperatures of geothermal springs are suitable for biomolecules. Silica minerals and metal sulfides in these environments have photocatalytic properties to catalyze biomolecules. Solar UV exposure also promotes
1771-512: Is sister to Euteleostei which contains the majority of bony fish alive today. In 2015, Benton and colleagues set a "plausible minimum" date for the origin of crown Otocephala as about 228.4 million years ago. They argued that since the oldest locality for any diversity of stem teleosts is the Carnian of Polberg bei Lunz, Austria, whose base is 235 million years old, a rough estimate for Otocephala can be made. The earliest known fossil member of
1848-402: Is solved in carbonate -rich environments. When in the presence of carbonate, calcium readily reacts to form calcium carbonate instead of apatite minerals. With the free calcium ions removed from solution , phosphate ions are no longer precipitated from solution. This is specifically seen in lakes with no inflow, since no new calcium is introduced into the water body. After all of the calcium
1925-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
2002-457: Is that the Earth and Moon started to coalesce at the same time but the Earth, having a much stronger gravity than the early Moon, attracted almost all the iron particles in the area. Until 2001, the oldest rocks found on Earth were about 3.8 billion years old, leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history
2079-398: Is water, which is much more plentiful than the geologically produced reducing agents required by the earlier non-oxygenic photosynthesis. From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes. Oxygen became a significant component of Earth's atmosphere about 2.4 Ga. Although eukaryotes may have been present much earlier,
2156-518: The Hadean . However, analysis of zircons formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an atmosphere , which may have been capable of supporting life. Evidence from the Moon indicates that from 4 to 3.8 Ga it suffered a Late Heavy Bombardment by debris that was left over from the formation of
2233-686: The Solar System , and the Earth should have experienced an even heavier bombardment due to its stronger gravity. While there is no direct evidence of conditions on Earth 4 to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although outgassing from volcanoes on Earth would have supplied at least half. However, if subsurface microbial life had evolved by this point, it would have survived
2310-602: The domain Archea and finally to the domain Eucarya . For the development of cells ( cellularisation ), the pre-cells had to be protected from their surroundings by envelopes (i.e. membranes, walls). For instance, the development of rigid cell walls by the invention of peptidoglycan in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonisation of virtually all habitats of
2387-477: The endosymbiont mitochondria provided a more abundant source of biological energy . Around 1.6 Ga, some eukaryotes gained the ability to photosynthesize via endosymbiosis with cyanobacteria, and gave rise to various algae that eventually overtook cyanobacteria as the dominant primary producers . At around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. While early organisms reproduced asexually ,
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2464-517: The pentose phosphate pathway , including biochemical reactions such as reductive amination and transamination . The Panspermia hypothesis does not explain how life arose originally, but simply examines the possibility of its coming from somewhere other than Earth. The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher Anaximander in
2541-415: The ability to tolerate and then to use oxygen, possibly via endosymbiosis , where one organism lives inside another and both of them benefit from their association. Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient, well-adapted to strike out on their own both as floating mats and as the first of the phytoplankton , provide
2618-762: The assembly of vesicles. Exergonic reactions at hydrothermal vents are suggested to have been a source of free energy that promoted chemical reactions, synthesis of organic molecules, and are inducive to chemical gradients. In small rock pore systems, membranous structures between alkaline seawater and the acidic ocean would be conducive to natural proton gradients. Nucleobase synthesis could occur by following universally conserved biochemical pathways by using metal ions as catalysts. RNA molecules of 22 bases can be polymerized in alkaline hydrothermal vent pores. Thin pores are shown to only accumulate long polynucleotides whereas thick pores accumulate both short and long polynucleotides. Small mineral cavities or mineral gels could have been
2695-506: The atmosphere, leading to the Great Oxygenation Event around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles ) dates from 1.85 Ga, likely due to symbiogenesis between anaerobic archaea and aerobic proteobacteria in co-adaptation against the new oxidative stress . While eukaryotes may have been present earlier, their diversification accelerated when aerobic cellular respiration by
2772-428: The basis of most marine food chains. Eukaryotes may have been present long before the oxygenation of the atmosphere, but most modern eukaryotes require oxygen, which is used by their mitochondria to fuel the production of ATP , the internal energy supply of all known cells. In the 1970s, a vigorous debate concluded that eukaryotes emerged as a result of a sequence of endosymbiosis between prokaryotes . For example:
2849-464: The bombardment. The earliest identified organisms were minute and relatively featureless, and their fossils looked like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga. Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria, with geochemical evidence also seeming to show
2926-427: The bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there. Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms; oxygenic photosynthesis by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The source of hydrogen atoms used by oxygenic photosynthesis
3003-574: The capabilities of individual organisms. Ribozymes remain as the main components of ribosomes , the "protein factories" in modern cells. Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga. Although short self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA is possible. The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA , TNA or GNA , which would have been replaced later by RNA. In 2003, it
3080-534: The construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA. These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that offspring were likely to have different genomes from their parents, and evolution occurred at the level of genes rather than organisms. RNA would later have been replaced by DNA, which can build longer, more stable genomes, strengthening heritability and expanding
3157-624: The continuous exposure to sunlight as well as their cell walls with ion pumps to maintain their intracellular metabolism after they entered the oceans. Catalytic mineral particles and transition metal sulfides at these environments are capable of catalyzing organic compounds. Scientists simulated laboratory conditions that were identical to white smokers and successfully oligomerized RNA, measured to be 4 units long. Long chain fatty acids can be synthesized via Fischer-Tropsch synthesis . Another experiment that replicated conditions also similar white smokers, with long chain fatty acids present resulted in
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3234-479: The cytoplasm of modern cells. Fatty acids in acidic or slightly alkaline geothermal springs assemble into vesicles after wet-dry cycles as there is a lower concentration of ionic solutes at geothermal springs since they are freshwater environments, in contrast to seawater which has a higher concentration of ionic solutes. For organic compounds to be present at geothermal springs, they would have likely been transported by carbonaceous meteors. The molecules that fell from
3311-416: The dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth's crust but does not contain an iron -rich core like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars , throwing crust material into the orbit that formed the Moon. Another hypothesis
3388-466: The development of a multiphenotypical population of pre-cells , i.e. evolving entities of primordial life with different characteristics and widespread horizontal gene transfer . From this pre-cell population the founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life arose successively, leading first to the domain Bacteria , then to
3465-404: The dominant form of life in the early Archean eon, and many of the major steps in early evolution are thought to have taken place in this environment. The evolution of photosynthesis by cyanobacteria , around 3.5 Ga, eventually led to a buildup of its waste product, oxygen , in the oceans. After free oxygen saturated all available reductant substances on the Earth's surface , it built up in
3542-759: The earliest terrestrial ecosystems at least 2.7 Ga, the evolution of plants from freshwater green algae dates back to about 1 billion years ago. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event as early tree Archaeopteris drew down CO 2 levels, leading to global cooling and lowered sea levels, while their roots increased rock weathering and nutrient run-offs which may have triggered algal bloom anoxic events . Bilateria , animals having
3619-451: The earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides , lipids and amino acids . A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including carbon monoxide and hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts . Most of
3696-481: The element directly below carbon on the periodic table , does not form very many complex stable molecules, and because most of its compounds are water-insoluble and because silicon dioxide is a hard and abrasive solid in contrast to carbon dioxide at temperatures associated with living things, it would be more difficult for organisms to extract. The elements boron and phosphorus have more complex chemistries but suffer from other limitations relative to carbon. Water
3773-498: The external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes , double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within
3850-451: The fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
3927-472: The geosphere and hydrosphere. This scenario may explain the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity of life), as well as the close relationship between the Archaea and the Eucarya. A scheme of the pre-cell scenario
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#17327978455094004-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
4081-1029: The group is Tischlingerichthys from the Late Jurassic of Germany . The 5th edition of Fishes of the World classifies the Otocephala as a cohort and subdivides it above the level of order as set out below, the classification of extant taxa from the level of order and below follows Eschmeyer's Catalog of Fishes : Taxonomy based on: Phylogeny of living groups based on: Clupeiformes ( herrings and relatives) [REDACTED] Alepocephaliformes ( slickheads and tubeshoulders ) [REDACTED] Gonorynchiformes ( milkfish and relatives) [REDACTED] Cypriniformes ( minnow , carp , loach) [REDACTED] Characiformes ( tetras and piranhas ) [REDACTED] Gymnotiformes (knifefish) [REDACTED] Siluriformes (catfish) [REDACTED] Clade In biological phylogenetics ,
4158-669: The lake - allowing the first prebiotic syntheses on Earth to occur. Microbial mats are multi-layered, multi-species colonies of bacteria and other organisms that are generally only a few millimeters thick, but still contain a wide range of chemical environments, each of which favors a different set of microorganisms. To some extent each mat forms its own food chain , as the by-products of each group of microorganisms generally serve as "food" for adjacent groups. Stromatolites are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water. There has been vigorous debate about
4235-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
4312-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
4389-420: The liposomes than outside. RNA is complex and there are doubts about whether it can be produced non-biologically in the wild. Some clays , notably montmorillonite , have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern; they are subject to an analogue of natural selection, as the clay "species" that grows fastest in
4466-406: The meteors were then accumulated in geothermal springs. Geothermal springs can accumulate aqueous phosphate in the form of phopshoric acid . Based on lab-run models, these concentrations of phoshate are insufficient to facilitate biosynthesis . As for the evolutionary implications, freshwater heterotrophic cells that depended upon synthesized organic compounds later evolved photosynthesis because of
4543-557: The most credible sources are Mars and Venus ; by alien visitors , possibly as a result of accidental contamination by microorganisms that they brought with them; and from outside the Solar System but by natural means. Experiments in low Earth orbit, such as EXOSTACK , have demonstrated that some microorganism spores can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years. Meteorite ALH84001 , which
4620-601: The origin of life since it is a critical component of nucleotides , phospholipids , and adenosine triphosphate . Phosphate is often depleted in natural environments due to its uptake by microbes and its affinity for calcium ions. In a process called ' apatite precipitation', free phosphate ions react with the calcium ions abundant in water to precipitate out of solution as apatite minerals. When attempting to simulate prebiotic phosphorylation , scientists have only found success when using phosphorus levels far above modern day natural concentrations. This problem of low phosphate
4697-522: The origin of life. Similar to the process predicted by geothermal hot spring hypotheses , changing lake levels and wave action deposited phosphorus-rich brine onto dry shore and marginal pools. This drying of the solution promotes polymerization reactions and removes enough water to promote phosphorylation, a process integral to biological energy storage and transfer. When washed away by further precipitation and wave action, researchers concluded these newly formed biomolecules may have washed back into
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#17327978455094774-445: The oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built. The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created selection pressure for organisms in this intermediate zone to acquire
4851-537: The presence of life 3.8 Ga. However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported. While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life. Evidence for fossilized microorganisms considered to be 3.77 billion to 4.28 billion years old
4928-688: The present day. Earth formed about 4.5 billion years ago (abbreviated as Ga , for gigaannum ) and evidence suggests that life emerged prior to 3.7 Ga. The similarities among all known present-day species indicate that they have diverged through the process of evolution from a common ancestor . The earliest clear evidence of life comes from biogenic carbon signatures and stromatolite fossils discovered in 3.7 billion-year-old metasedimentary rocks from western Greenland . In 2015, possible "remains of biotic life " were found in 4.1 billion-year-old rocks in Western Australia . There
5005-463: The primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants ), is sexual reproduction , the fusion of male and female reproductive cells ( gametes ) to create a zygote . The origin and evolution of sexual reproduction remain a puzzle for biologists, though it is thought to have evolved from a single-celled eukaryotic ancestor. While microorganisms formed
5082-580: The recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs , dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 Ma killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity . Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. Only
5159-417: The rock on the surface of the Earth at rates far greater than today. With high phosphate influx, no phosphate precipitation, and no microbial usage of phosphate at this time, models show phosphate reached concentrations approximately 100 times greater than they are today. Modeled pH and phosphate levels of early Earth carbonate-rich lakes nearly match the conditions used in current laboratory experiments on
5236-438: The rocks less than 2.2 Ga and prove nothing about the origins of eukaryotes. Fossils of the algae Grypania have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised ), indicating that eukaryotes with organelles had already evolved. A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga. The earliest known fossils of fungi date from 1.43 Ga. Plastids ,
5313-437: The sixth century BCE . In the twentieth century it was proposed by the physical chemist Svante Arrhenius , by the astronomers Fred Hoyle and Chandra Wickramasinghe , and by molecular biologist Francis Crick and chemist Leslie Orgel . There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar System via fragments knocked into space by a large meteor impact, in which case
5390-543: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Evolutionary history The history of life on Earth traces the processes by which living and extinct organisms evolved, from the earliest emergence of life to
5467-423: The steps required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents. In this scenario, the biochemical evolution of life led to diversification through
5544-415: The superclass of organelles of which chloroplasts are the best-known exemplar, are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1.5 Ga and enabled eukaryotes to carry out oxygenic photosynthesis . Three evolutionary lineages of photosynthetic plastids have since emerged: chloroplasts in green algae and plants, rhodoplasts in red algae and cyanelles in
5621-407: The synthesis of biomolecules like RNA nucleotides. An analysis of hydrothermal veins at a 3.5 Gya (giga years ago or 1 billion years) geothermal spring setting were found to have elements required for the origin of life, which are potassium, boron, hydrogen, sulfur, phosphorus, zinc, nitrogen, and oxygen. Mulkidjanian and colleagues find that such environments have identical ionic concentrations to
5698-400: The validity of alleged stromatolite fossils from before 3 Ga, with critics arguing that they could have been formed by non-biological processes. In 2006, another find of stromatolites was reported from the same part of Australia, in rocks dated to 3.5 Ga. In modern underwater mats the top layer often consists of photosynthesizing cyanobacteria which create an oxygen-rich environment, while
5775-563: Was found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada, although the evidence is disputed as inconclusive. Some biologists reason that all living organisms on Earth must share a single last universal ancestor , because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms. According to
5852-496: Was once part of the Martian crust, shows evidence of carbonate-globules with texture and size indicative of terrestrial bacterial activity. Scientists are divided over the likelihood of life arising independently on Mars, or on other planets in our galaxy . One theory traces the origins of life to the abundant carbonate-rich lakes which would have dotted the early Earth . Phosphate would have been an essential cornerstone to
5929-411: Was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents . Under this hypothesis, lipid membranes would be the last major cell components to appear and, until then, the protocells would be confined to the pores. It has been suggested that double-walled "bubbles" of lipids like those that form
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