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45-664: The theriodonts ( clade Theriodontia ) are a major group of therapsids which appeared during the Middle Permian and which includes the gorgonopsians and the eutheriodonts , itself including the therocephalians and the cynodonts . In 1876, Richard Owen named a suborder Theriodontia, which he divided into the Cynodontia and the Gomphodontia . The modern clade concept was devised by James Allen Hopson . In his system, Theriodontia fall into two main groups:

90-775: 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 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

135-479: 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

180-623: 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

225-494: A common genetic heritage, though the suggestion of substantial horizontal gene transfer during early evolution has led to questions about the monophyly (single ancestry) of life. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian . Universal common descent through an evolutionary process

270-401: A common original type, and thus a common parent. In 1794, Charles Darwin's grandfather, Erasmus Darwin asked: [W]ould it be too bold to imagine, that in the great length of time, since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament, which

315-486: A deceitful guide." And in the subsequent edition, he asserts rather, "We do not know all the possible transitional gradations between the simplest and the most perfect organs; it cannot be pretended that we know all the varied means of Distribution during the long lapse of years, or that we know how imperfect the Geological Record is. Grave as these several difficulties are, in my judgment they do not overthrow

360-528: A formal test, for the unity of life." Traditionally, these trees have been built using morphological methods, such as appearance, embryology , etc. Recently, it has been possible to construct these trees using molecular data, based on similarities and differences between genetic and protein sequences. All these methods produce essentially similar results, even though most genetic variation has no influence over external morphology. That phylogenetic trees based on different types of information agree with each other

405-422: 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

450-429: 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 ,

495-533: Is a recurring theme in many indigenous worldviews across the world. Later on, in the 1740s, the French mathematician Pierre Louis Maupertuis arrived at the idea that all organisms had a common ancestor, and had diverged through random variation and natural selection . In 1790, the philosopher Immanuel Kant wrote in Kritik der Urteilskraft ( Critique of Judgment ) that the similarity of animal forms implies

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540-499: 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,

585-491: Is convincing evidence of a single ancestry. 6,331 genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian . The genetic code (the "translation table" according to which DNA information is translated into amino acids , and hence proteins) is nearly identical for all known lifeforms, from bacteria and archaea to animals and plants . The universality of this code

630-416: Is generally regarded by biologists as definitive evidence in favor of universal common descent. The way that codons (DNA triplets) are mapped to amino acids seems to be strongly optimised. Richard Egel argues that in particular the hydrophobic (non-polar) side-chains are well organised, suggesting that these enabled the earliest organisms to create peptides with water-repelling regions able to support

675-476: 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

720-414: Is mentioned, above, that all living organisms may be descended from an original single-celled organism with a DNA genome , and that this implies a single origin for life. Although such a universal common ancestor may have existed, such a complex entity is unlikely to have arisen spontaneously from non-life and thus a cell with a DNA genome cannot reasonably be regarded as the origin of life. To understand

765-515: 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

810-489: 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

855-526: Is strong evidence of a real underlying common descent. Theobald noted that substantial horizontal gene transfer could have occurred during early evolution. Bacteria today remain capable of gene exchange between distantly-related lineages. This weakens the basic assumption of phylogenetic analysis, that similarity of genomes implies common ancestry, because sufficient gene exchange would allow lineages to share much of their genome whether or not they shared an ancestor (monophyly) . This has led to questions about

900-544: The Cretaceous–Paleogene extinction event , which wiped out the non- avian dinosaurs , allowing the mammals to diversify and dominate the Earth. [REDACTED] Clade In biological phylogenetics , 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

945-534: The Early Triassic . The remaining theriodonts, the cynodonts, also included carnivores, such as Cynognathus , as well as newly evolved herbivores ( Traversodontidae ). While traversodontids for the most part remained medium-sized to reasonably large (the length of the largest species was up to two meters), the carnivorous forms became progressively smaller as the Triassic progressed. They "miniaturised". By

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990-813: The Gorgonopsia and the Eutheriodontia . The latter consist of the Therocephalia and Cynodontia . Theriodonts appeared at the same time as their sister group within the Neotherapsida , the Anomodontia , about 270 million years ago, in the Middle Permian . Even these early theriodonts were more mammal-like than their anomodont and dinocephalian contemporaries. Early theriodonts may have been endothermic . Early forms were carnivorous, but several later groups became herbivorous during

1035-531: The Triassic . Theriodont jaws were more mammal-like than was the case of other therapsids, because their dentary was larger, which gave them more efficient chewing ability. Furthermore, several other bones that were on the lower jaw (found in reptiles ), moved into the ears , allowing the theriodonts to hear better and their mouths to open wider. This made the theriodonts the most successful group of synapsids . Eutheriodontia refers to all theriodonts except

1080-466: The laws of physics and chemistry - rather than through universal common descent - and therefore resulted in convergent evolution. In contrast, there is evidence for homology of the central subunits of transmembrane ATPases throughout all living organisms, especially how the rotating elements are bound to the membrane. This supports the assumption of a LUCA as a cellular organism, although primordial membranes may have been semipermeable and evolved later to

1125-656: The Late Triassic, the small cynodonts included the rodent-like Tritylodontidae (possibly related to or descended from traversodontids), and the tiny, shrew-like, Tritheledontidae , related to the Mammaliaformes . The tritheledontids died out during the Jurassic , and the tritylodontids survived into the Cretaceous , but their relatives, the mammals, continued to evolve. Many mammal groups managed to survive

1170-538: The Origin of Species , were that it was probable that there was only one progenitor for all life forms: Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed. But he precedes that remark by, "Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. But analogy may be

1215-629: The ancestral population two species have in common, the more closely are they related. The most recent common ancestor of all currently living organisms is the last universal ancestor, which lived about 3.9 billion years ago . The two earliest pieces of evidence for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia . All currently living organisms on Earth share

1260-456: The effect of protein - and RNA- enzymes , then translated into proteins by (highly similar) ribosomes , with ATP , NADPH and others as energy sources. Analysis of small sequence differences in widely shared substances such as cytochrome c further supports universal common descent. Some 23 proteins are found in all organisms, serving as enzymes carrying out core functions like DNA replication. The fact that only one such set of enzymes exists

1305-456: The essential electron exchange ( redox ) reactions for energy transfer. Similarities which have no adaptive relevance cannot be explained by convergent evolution , and therefore they provide compelling support for universal common descent. Such evidence has come from two areas: amino acid sequences and DNA sequences. Proteins with the same three-dimensional structure need not have identical amino acid sequences; any irrelevant similarity between

1350-501: The general notion of common descent. It should come as no surprise, then, that the scientific community at large has accepted evolutionary descent as a historical reality since Darwin's time and considers it among the most reliably established and fundamentally important facts in all of science. All known forms of life are based on the same fundamental biochemical organization: genetic information encoded in DNA , transcribed into RNA , through

1395-473: The gorgonopsians (the most "primitive" group). They included the therocephalians and the cynodonts. The cynodonts include the mammals. The eutheriodonts have larger skulls, accommodating larger brains and improved jaw muscles. The eutheriodontian theriodonts are one of the two synapsid survivors of the great Permian–Triassic extinction event , the other being the dicynodonts . Therocephalians included both carnivorous and herbivorous forms; both died out after

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1440-477: The great First Cause endued with animality, with the power of acquiring new parts attended with new propensities, directed by irritations, sensations, volitions, and associations; and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end? Charles Darwin 's views about common descent, as expressed in On

1485-546: 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"

1530-590: 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"

1575-518: 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

1620-448: The membranes of modern bacteria, and on a second path to those of modern archaea also. Another important piece of evidence is from detailed phylogenetic trees (i.e., "genealogic trees" of species) mapping out the proposed divisions and common ancestors of all living species. In 2010, Douglas L. Theobald published a statistical analysis of available genetic data, mapping them to phylogenetic trees, that gave "strong quantitative support, by

1665-495: The origin of life, it has been proposed that DNA based cellular life descended from relatively simple pre-cellular self-replicating RNA molecules able to undergo natural selection . During the course of evolution, this RNA world was replaced by the evolutionary emergence of the DNA world. A world of independently self-replicating RNA genomes apparently no longer exists (RNA viruses are dependent on host cells with DNA genomes). Because

1710-458: The positioning of introns and pseudogenes , provide strong evidence of common ancestry. Biologists often point to the universality of many aspects of cellular life as supportive evidence to the more compelling evidence listed above. These similarities include the energy carrier adenosine triphosphate (ATP), and the fact that all amino acids found in proteins are left-handed . It is, however, possible that these similarities resulted because of

1755-453: The redundant codons, and since the correct amino acids would already have been in place, natural selection would not have driven any change in the codons, however much time was available. Genetic drift could change the codons, but it would be extremely unlikely to make all the redundant codons in a whole sequence match exactly across multiple lineages. Similarly, shared nucleotide sequences, especially where these are apparently neutral such as

1800-659: The same environmental conditions to evolve similar biochemistry convergently , they might independently have acquired similar genetic sequences. Theobald's "formal test" was accordingly criticised by Takahiro Yonezawa and colleagues for not including consideration of convergence. They argued that Theobald's test was insufficient to distinguish between the competing hypotheses. Theobald has defended his method against this claim, arguing that his tests distinguish between phylogenetic structure and mere sequence similarity. Therefore, Theobald argued, his results show that "real universally conserved proteins are homologous ." The possibility

1845-443: The sequences is evidence for common descent. In certain cases, there are several codons (DNA triplets) that code redundantly for the same amino acid. Since many species use the same codon at the same place to specify an amino acid that can be represented by more than one codon, that is evidence for their sharing a recent common ancestor. Had the amino acid sequences come from different ancestors, they would have been coded for by any of

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1890-453: The single ancestry of life. However, biologists consider it very unlikely that completely unrelated proto-organisms could have exchanged genes, as their different coding mechanisms would have resulted only in garble rather than functioning systems. Later, however, many organisms all derived from a single ancestor could readily have shared genes that all worked in the same way, and it appears that they have. If early organisms had been driven by

1935-521: 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. Common ancestor Common descent is an effect of speciation , in which multiple species derive from a single ancestral population. The more recent

1980-427: The theory of descent from a few created forms with subsequent modification". Common descent was widely accepted amongst the scientific community after Darwin's publication. In 1907, Vernon Kellogg commented that "practically no naturalists of position and recognized attainment doubt the theory of descent." In 2008, biologist T. Ryan Gregory noted that: No reliable observation has ever been found to contradict

2025-617: Was first proposed by the British naturalist Charles Darwin in the concluding sentence of his 1859 book On the Origin of Species : There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. The idea that all living things (including things considered non-living by science) are related

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