Hominini

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Hominini
Temporal range: 6–0  Ma
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Ham-and-handler.jpg
Two hominins, according to the original definition by Gray: A human (Homo sapiens) holding a chimpanzee (Pan troglodytes)
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe:Hominini
Gray, 1824
Type species
Homo sapiens
Linnaeus, 1756
Genera

Panina

Hominina

The Hominini, or hominins, form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the genus Homo (humans), but excludes the genus Gorilla (gorillas). As of 2019, there is no consensus on whether it should include the genus Pan (chimpanzees and bonobos), the question being closely tied to the complex speciation process connecting humans and chimpanzees and the development of bipedalism in proto-humans.

In biology, a tribe is a taxonomic rank above genus, but below family and subfamily. It is sometimes subdivided into subtribes. By convention, all taxonomic ranks above genus are capitalized, including both tribe and subtribe.

Homininae Subfamily of mammals

Homininae, also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the Hominini tribe ―and 2) the Gorillini tribe (gorillas). Alternatively, the genus Pan is sometimes considered to belong to its own third tribe, Panini. Homininae comprises all hominids that arose after orangutans split from the line of great apes. The Homininae cladogram has three main branches, which lead to gorillas, and to humans and chimpanzees via the tribe Hominini and subtribes Hominina and Panina. There are two living species of Panina and two living species of gorillas, but only one extant human species. Traces of hypothetical Homo species, including Homo floresiensis and Homo denisova, have been found with dates as recent as 40,000 years ago. Organisms in this subfamily are described as hominine or hominines.

<i>Homo</i> Genus of mammals

Homo is the genus which emerged in the otherwise extinct genus Australopithecus that encompasses the extant species Homo sapiens, plus several extinct species classified as either ancestral to or closely related to modern humans, most notably Homo erectus and Homo neanderthalensis. The genus is taken to emerge with the appearance of Homo habilis, just over two million years ago. Genus Homo, together with the genus Paranthropus is probably sister to A. africanus in the genus Australopithecus, which itself had previously split from the lineage of Pan, the chimpanzees.

Contents

The tribe was originally introduced by John Edward Gray (1824), long before any details on the speciation of Pan and Homo were known. Gray's tribe Hominini by definition includes both Pan and Homo. This definition is still adhered to in the proposal by Mann and Weiss (1996), which divides Hominini into three subtribes, Panina (containing Pan), Hominina ("homininans", containing Homo "humans"), and Australopithecina (containing several extinct "australopithecine" genera). [2]

John Edward Gray British zoologist and philatelist

John Edward Gray, FRS was a British zoologist. He was the elder brother of zoologist George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray (1766–1828). The standard author abbreviation J.E.Gray is used to indicate this person as the author when citing a botanical name. The same is used for a zoological name.

Alternatively, Hominini is taken to exclude Pan. In this case, Panini ("panins", Delson 1977) [3] may be used to refer to the tribe containing Pan as its only genus. [4] [5]

Minority dissenting nomenclatures include Gorilla in Hominini and Pan in Homo (Goodman et al. 1998), or both Pan and Gorilla in Homo (Watson et al. 2001).

Terminology and definition

By convention, the adjectival term "hominin" (or nominalized "hominins") refers to the tribe Hominini, while the members of the Hominina subtribe (and thus all archaic human species) are referred to as "homininan" ("homininans"). [6] This follows the proposal by Mann and Weiss (1996), which presents tribe Hominini as including both Pan and Homo, placed in separate subtribes. The genus Pan is referred to subtribe Panina, and genus Homo is included in the subtribe Hominina (see above). [2] However, there is an alternative convention which uses "hominin" to exclude members of Panina, i.e. either just for Homo or for both human and australopithecine species. This alternative convention is referenced in e.g. Coyne (2009) [7] and in Dunbar (2014). [5] Potts (2010) in addition uses the name Hominini in a different sense, as excluding Pan, and uses "hominins" for this, while a separate tribe (rather than subtribe) for chimpanzees is introduced, under the name Panini. [4] In this recent convention, contra Gray, the term "hominin" is applied to Homo, Australopithecus, Ardipithecus , and others that arose after the split from the line that led to chimpanzees (see cladogram below); [8] [9] that is, they distinguish fossil members on the human side of the split, as "hominins", from those on the chimpanzee side, as "not hominins" (or "non-hominin hominids"). [7]

<i>Pan</i> (genus) genus of mammals, includes common chimpanzee and bonobo

The genus Pan consists of two extant species: the common chimpanzee and the bonobo. Taxonomically, these two ape species are collectively termed panins; however, both species are more commonly referred to collectively using the generalized term chimpanzees, or chimps. Together with humans, gorillas, and orangutans they are part of the family Hominidae. Native to sub-Saharan Africa, common chimpanzees and bonobos are currently both found in the Congo jungle, while only the common chimpanzee is also found further north in West Africa. Both species are listed as endangered on the IUCN Red List of Threatened Species, and in 2017 the Convention on Migratory Species selected the common chimpanzee for special protection.

<i>Ardipithecus</i> genus of mammals

Ardipithecus is a genus of an extinct hominine that lived during the Late Miocene and Early Pliocene epochs in the Afar Depression, Ethiopia. Originally described as one of the earliest ancestors of humans after they diverged from the chimpanzees, the relation of this genus to human ancestors and whether it is a hominin is now a matter of debate. Two fossil species are described in the literature: A. ramidus, which lived about 4.4 million years ago during the early Pliocene, and A. kadabba, dated to approximately 5.6 million years ago. Behavioral analysis showed that Ardipithecus could be very similar to chimpanzees, indicating that the early human ancestors were very chimpanzee-like in behavior.

Cladogram

This cladogram shows the clade of superfamily Hominoidea and its descendent clades, focussed on the division of Hominini (omitting detail on clades not ancestral to Hominini). The family Hominidae ("hominids") comprises the tribes Ponginae (including orangutans), Gorillini (including gorillas) and Hominini, the latter two forming the subfamily of Homininae. Hominini is divided into Panina (chimpanzees) and Australopithecina (australopithecines). The Hominina (humans) are usually held to have emerged within the Australopithecina (which would roughly correspond to the alternative definition of Hominini according to the alternative definition which excludes Pan). Genetic analysis combined with fossil evidence indicates that hominoids diverged from the Old World monkeys about 25 million years ago (Mya), near the Oligocene-Miocene boundary. [10] The most recent common ancestors (MRCA) of the subfamilies Homininae and Ponginae lived about 15 million years ago. [11] In the following cladogram, the approximate time the clades radiated newer clades is indicated in millions of years ago (Mya).

Cladogram A diagram used to show relations among groups of organisms with common origins

A cladogram is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to descendants, nor does it show how much they have changed; nevertheless, many evolutionary trees can be inferred from a single cladogram. A cladogram uses lines that branch off in different directions ending at a clade, a group of organisms with a last common ancestor. There are many shapes of cladograms but they all have lines that branch off from other lines. The lines can be traced back to where they branch off. These branching off points represent a hypothetical ancestor which can be inferred to exhibit the traits shared among the terminal taxa above it. This hypothetical ancestor might then provide clues about the order of evolution of various features, adaptation, and other evolutionary narratives about ancestors. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational phylogenetics are now very commonly used in the generation of cladograms, either on their own or in combination with morphology.

Clade A group of organisms that consists of a common ancestor and all its lineal descendants

A clade, also known as monophyletic group, is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single "branch" on the "tree of life". Rather than the English term, the equivalent Latin term cladus is often used in taxonomical literature.

In biological classification, a subfamily is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end subfamily botanical names with "-oideae", and zoological names with "-inae".

Hominoidea  (20.4 Mya)

Hylobatidae (gibbons)

Hominidae  (15.7)

Ponginae (orangutans)

Homininae   (8.8)

Gorillini (gorillas)

Hominini  (6.3)

Panina (chimpanzees)

Australopithecines/ (4)

Ardipithecus (†)

Australopithecus

Praeanthropus (†)

Australopithecus/Paranthropus robustus (†2)

Australopithecus garhi (†2.5)

Homo (humans)

Hominina

Evolutionary history

Model of the phylogeny of Hominini over the past 10 million years. Hominini lineage.svg
Model of the phylogeny of Hominini over the past 10 million years.

Both Sahelanthropus and Orrorin existed during the estimated duration of the ancestral chimpanzee-human speciation events, within the range of eight to four million years ago (Mya). Very few fossil specimens have been found that can be considered directly ancestral to genus Pan . News of the first fossil chimpanzee, found in Kenya, was published in 2005. However, it is dated to very recent times—between 545 and 284 thousand years ago. [12] The divergence of a "proto-human" or "pre-human" lineage separate from Pan appears to have been a process of complex speciation-hybridization rather than a clean split, taking place over the period of anywhere between 13 million years ago (close to the age of the Hominini tribe itself) and some 4 million years ago. Different chromosomes appear to have split at different times, with broad-scale hybridization activity occurring between the two emerging lineages as late as the period 6.3 to 5.4 Mya, according to Patterson et al. (2006), [13] This research group noted that one hypothetical late hybridization period was based in particular on the similarity of X chromosomes in the proto-humans and stem chimpanzees, suggesting the final divergence even as recent as 4 Mya. Wakeley (2008) rejected these hypotheses; he suggested alternative explanations, including selection pressure on the X chromosome in the ancestral populations prior to the chimpanzee–human last common ancestor (CHLCA). [14] Most DNA studies find that humans and Pan are 99% identical, [15] [16] but one study found only 94% commonality, with some of the difference occurring in noncoding DNA. [17] It is most likely that the australopithecines, dating from 3 to 4.4 Mya, evolved into the earliest members of genus Homo. [18] [19] In the year 2000, the discovery of Orrorin tugenensis , dated as early as 6.2 Mya, briefly challenged critical elements of that hypothesis, [20] as it suggested that Homo did not in fact derive from australopithecine ancestors. [21] All the listed fossil genera are evaluated for: 1) probability of being ancestral to Homo, and 2) whether they are more closely related to Homo than to any other living primate—two traits that could identify them as hominins. Some, including Paranthropus , Ardipithecus , and Australopithecus , are broadly thought to be ancestral and closely related to Homo; [22] others, especially earlier genera, including Sahelanthropus (and perhaps Orrorin ), are supported by one community of scientists but doubted by another. [23] [24]

Speciation The evolutionary process by which populations evolve to become distinct species

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species. He also identified sexual selection as a likely mechanism, but found it problematic.

X chromosome The sex chromosome present in both sexes of species in which the male is the heterogametic sex. Two copies of the X chromosome are present in each somatic cell of females and one copy is present in males.

The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals, and is found in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system. The X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome, for the next letter in the alphabet, following its subsequent discovery.

Chimpanzee–human last common ancestor Chimpanzee–human last common ancestor

The chimpanzee–human last common ancestor, or CHLCA, is the last common ancestor shared by the extant Homo (human) and Pan genera of Hominini. Due to complex hybrid speciation, it is not possible to give a precise estimate on the age of this ancestral population. While "original divergence" between populations may have occurred as early as 13 million years ago (Miocene), hybridization may have been ongoing until as recently as 4 million years ago (Pliocene).

See also

Related Research Articles

Human evolution Hominin events for the last 10 million years

Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates—in particular genus Homo—and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes. This process involved the gradual development of traits such as human bipedalism and language, as well as interbreeding with other hominins, which indicate that human evolution was not linear but a web.

<i>Orrorin</i> 6-million-year-old hominid discovered in 2000 in Kenya

Orrorin tugenensis is a postulated early species of Homininae, estimated at 6.1 to 5.7 million years (Ma) and discovered in 2000. It is not confirmed how Orrorin is related to modern humans. Its discovery was an argument against the hypothesis that australopithecines are human ancestors, as much as it still remains the most prevalent hypothesis of human evolution as of 2012.

<i>Australopithecus</i> Genus of hominin ancestral to modern humans

Australopithecus ( OS-trə-lo-PITH-i-kəs; from Latin australis, meaning 'southern', and Greek πίθηκος, meaning 'ape', informal australopithecine or australopith   is a genus of hominins. From paleontological and archaeological evidence, the genus Australopithecus apparently evolved in eastern Africa around 4 million years ago before spreading throughout the continent and eventually becoming extinct two million years ago. Australopithecus is not literally extinct as the Kenyanthropus, Paranthropus and Homo genera probably emerged as sister of a late Australopithecus species such as Australopithecus africanus and/or A. sediba. During that time, a number of australopithecine species emerged, including Australopithecus afarensis, A. africanus, A. anamensis, A. bahrelghazali, A. deyiremeda, A. garhi, and A. sediba.

<i>Sahelanthropus</i> species of mammal (fossil)

Sahelanthropus tchadensis is an extinct species of the Hominini and is probably the ancestor to Orrorin that is dated to about 7 million years ago, during the Miocene epoch, possibly very close to the time of the chimpanzee–human divergence. Few specimens other than the partial skull, nicknamed Toumaï, are known.

Ape superfamily of mammals

Apes (Hominoidea) are a branch of Old World tailless simians native to Africa and Southeast Asia. They are the sister group of the Old World monkeys, together forming the catarrhine clade. They are distinguished from other primates by a wider degree of freedom of motion at the shoulder joint as evolved by the influence of brachiation. In traditional and non-scientific use, the term "ape" excludes humans, and is thus not equivalent to the scientific taxon Hominoidea. There are two extant branches of the superfamily Hominoidea: the gibbons, or lesser apes; and the hominids, or great apes.

Pongidae, or the Pongids, is an obsolete primate taxon containing gorillas, chimpanzees, and orangutans. They are sometimes called "great apes". Pongidae is now known to be paraphyletic. Pongids split from Hominina around seven mya. The corresponding crown group for this taxon is Hominidae. Pongidae has seven extant member species. This taxon is rarely used today but is of historical significance.

The Omo remains are a collection of hominin bones discovered between 1967 and 1974 at the Omo Kibish sites near the Omo River, in Omo National Park in south-western Ethiopia. The bones were recovered by a scientific team from the Kenya National Museums directed by Richard Leakey and others. The remains from Kamoya's Hominid Site (KHS) were called Omo I and those from Paul I. Abell's Hominid Site (PHS) Omo II.

Human taxonomy Classification of the human species

Human taxonomy is the classification of the human species within zoological taxonomy. The systematic genus, Homo, is designed to include both anatomically modern humans and extinct varieties of archaic humans. Current humans have been designated as subspecies Homo sapiens sapiens, differentiated from the direct ancestor, Homo sapiens idaltu.

Australopithecine subtribe of mammals

Australopithecines are generally Australopithecus, and it typically includes the earlier Ardipithecus. All these related species are now sometimes collectively classified as a subtribe of the Hominini tribe called Australopithecina. They are the extinct, close relatives of humans and, with the extant genus Homo, comprise the human clade. Members of the human clade, i.e. the Hominini after the split from the chimpanzees, are now called Hominina.

The Middle Awash is an archaeological site along the Awash River in Ethiopia's Afar Depression. A number of fossils of the earliest hominins, particularly of the Australopithecines, as well as some of the oldest known Olduwan stone artifacts, have been found at the site—all of late Miocene, the Pliocene, and the very early Pleistocene times, that is, about 5.6 million years ago (mya) to 2.5 mya. It is broadly thought that the divergence of the lines of the earliest humans (hominins) and of chimpanzees (hominids) was completed near the beginning of that time range, or sometime between seven and five mya. However, the larger community of scientists provide several estimates for periods of divergence that imply a greater range for this event, see CHLCA: human-chimpanzee split.

Human evolutionary genetics studies how one human genome differs from another human genome, the evolutionary past that gave rise to the human genome, and its current effects. Differences between genomes have anthropological, medical, historical and forensic implications and applications. Genetic data can provide important insights into human evolution.

<i>Ardipithecus kadabba</i> species of mammal (fossil)

Ardipithecus kadabba is the scientific classification given to fossil remains "known only from teeth and bits and pieces of skeletal bones," originally estimated to be 5.8 to 5.2 million years old, and later revised to 5.77 to 5.54 million years. According to the first description, these fossils are close to the common ancestor of chimps and humans. Their development lines are estimated to have parted 6.5–5.5 million years ago. It has been described as a "probable chronospecies" of A. ramidus. Although originally considered a subspecies of A. ramidus, in 2004 anthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim D. White published an article elevating A. kadabba to species level on the basis of newly discovered teeth from Ethiopia. These teeth show "primitive morphology and wear pattern" which demonstrate that A. kadabba is a distinct species from A. ramidus.

<i>Ardipithecus ramidus</i> species of mammal (fossil)

Ardipithecus ramidus is a species of hominin classified as an australopithecine of the genus Ardipithecus. A. kadabba was considered to be a subspecies of A. ramidus until 2004.

Hominidae Family of mammals

The Hominidae, whose members are known as great apes or hominids, are a taxonomic family of primates that includes eight extant species in four genera: Pongo, the Bornean, Sumatran and Tapanuli orangutan; Gorilla, the eastern and western gorilla; Pan, the chimpanzee and the bonobo; and Homo, of whom only modern humans remain, with several extinct relatives and ancestors, such as Homo erectus.

Evolution of primates The origin and diversification of primates through geologic time

The evolutionary history of the primates can be traced back 65 million years. One of the oldest known primate-like mammal species, the Plesiadapis, came from North America; another, Archicebus, came from China. Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene. Purgatorius is the genus of the four extinct species believed to be the earliest example of a primate or a proto-primate, a primatomorph precursor to the Plesiadapiformes, dating to as old as 66 million years ago.

Changes to the dental morphology and jaw are major elements of hominid evolution. These changes were driven by the types and processing of food eaten. The evolution of the jaw is thought to have facilitated encephalization, speech, and the formation of the uniquely human chin.

References

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  2. 1 2 Mann, Alan; Mark Weiss (1996). "Hominoid Phylogeny and Taxonomy: a consideration of the molecular and Fossil Evidence in an Historical Perspective". Molecular Phylogenetics and Evolution. 5 (1): 169–181. doi:10.1006/mpev.1996.0011. PMID   8673284.
  3. Delson, Journal of Human Evolution 6 (1977), p. 450.
  4. 1 2 Potts (2010). “What Does It Mean to Be Human?”, pp. 34. ISBN   978-1-4262-0606-1. National Geographic Society, Washington.
  5. 1 2 "Conventionally, taxonomists now refer to the great ape family (including humans) as 'hominids', while all members of the lineage leading to modern humans that arose after the split with the [Homo-Pan] LCA are referred to as 'hominins'. The older literature used the terms hominoids and hominids respectively."Dunbar, Robin (2014). Human evolution. ISBN   9780141975313.
  6. Bernard Wood (11 May 2010), "Reconstructing human evolution: Achievements, challenges, and opportunities", Proceedings of the National Academy of Sciences of the United States of America, 107 (Supplement 2): 8902–8909, Bibcode:2010PNAS..107.8902W, doi:10.1073/PNAS.1001649107, PMC   3024019 , PMID   20445105 ,  Wikidata Q28743412
  7. 1 2 Coyne, Jerry A. (2009) Why Evolution Is True, pp.197-208, 244, 248. ISBN   978-0-670-02053-9(hc), ISBN   978-0-14-311664-6(pbk). Penguin Books Ltd, London. "Anthropologists apply the term hominin to all the species on the "human" side of our family tree after it split from the branch that became modern chimps." (p.197)
  8. Brenda J. Bradley (1 April 2008), "Reconstructing phylogenies and phenotypes: a molecular view of human evolution", Journal of Anatomy, 212 (4): 337–353, doi:10.1111/j.1469-7580.2007.00840.x, PMC   2409108 , PMID   18380860 ,  Wikidata Q24646554
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  10. "Fossils May Pinpoint Critical Split Between Apes and Monkeys". redOrbit.com. 15 May 2013.
  11. The most well-known fossil genus of Ponginae is Sivapithecus, consisting of several species from 12.5 million to 8.5 million years ago. It differs from orangutans in dentition and postcranial morphology. >Taylor, C. (2011). "Old men of the woods". Palaeos . Retrieved 2013-04-04.
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  14. Wakeley J (March 2008). "Complex speciation of humans and chimpanzees". Nature. 452 (7184): E3–4, discussion E4. Bibcode:2008Natur.452....3W. doi:10.1038/nature06805. PMID   18337768. "Patterson et al. suggest that the apparently short divergence time between humans and chimpanzees on the X chromosome is explained by a massive interspecific hybridization event in the ancestry of these two species. However, Patterson et al. do not statistically test their own null model of simple speciation before concluding that speciation was complex, and—even if the null model could be rejected—they do not consider other explanations of a short divergence time on the X chromosome. These include natural selection on the X chromosome in the common ancestor of humans and chimpanzees, changes in the ratio of male-to-female mutation rates over time, and less extreme versions of divergence with gene flow. I therefore believe that their claim of hybridization is unwarranted."
  15. Mary-Claire King (1973) Protein polymorphisms in chimpanzee and human evolution, Doctoral dissertation, University of California, Berkeley.
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  20. Potts, Richard and Sloan, Christopher. “What Does It Mean to Be Human?”, pp. 38-39. ISBN   978-1-4262-0606-1. National Geographic Society, Washington.
  21. Reynolds, Sally C; Gallagher, Andrew (2012-03-29). African Genesis: Perspectives on Hominin Evolution. ISBN   9781107019959.:"The discovery of Orrorin has ... radically modified interpretations of human origins and the environmental context in which the African apes/hominoid transition occurred, although ... the less likely hypothesis of derivation of Homo from the australopithecines still holds primacy in the minds of most palaeoanthropologists."
  22. Potts, Richard and Sloan, Christopher. “What Does It Mean to Be Human?”, pp. 31-42. ISBN   978-1-4262-0606-1. National Geographic Society, Washington.
  23. Brunet, Michel; Guy, F; Pilbeam, D; MacKaye, H. T.; Likius, A; Ahounta, D; Beauvilain, A; Blondel, C; Bocherens, H; Boisserie, JR; De Bonis, L; Coppens, Y; Dejax, J; Denys, C; Duringer, P; Eisenmann, V; Fanone, G; Fronty, P; Geraads, D; Lehmann, T; Lihoreau, F; Louchart, A; Mahamat, A; Merceron, G; Mouchelin, G; Otero, O; Pelaez Campomanes, P; Ponce De Leon, M; Rage, J. C.; et al. (July 2002), "A new hominid from the Upper Miocene of Chad, Central Africa", Nature, 418 (6894): 145–151, doi:10.1038/nature00879, PMID   12110880, Sahelanthropus is the oldest and most primitive known member of the hominid clade, close to the divergence of hominids and chimpanzees.
  24. Wolpoff, Milford; Senut, Brigitte; Pickford, Martin; Hawks, John (October 2002), "Sahelanthropus or 'Sahelpithecus'?" (PDF), Nature, 419 (6907): 581–582, Bibcode:2002Natur.419..581W, doi:10.1038/419581a, PMID   12374970, Sahelanthropus tchadensis is an enigmatic new Miocene species, whose characteristics are a mix of those of apes and Homo erectus and which has been proclaimed by Brunet et al. to be the earliest hominid. However, we believe that features of the dentition, face and cranial base that are said to define unique links between this Toumaï specimen and the hominid clade are either not diagnostic or are consequences of biomechanical adaptations. To represent a valid clade, hominids must share unique defining features, and Sahelanthropus does not appear to have been an obligate biped.