| Hominini | |
|---|---|
 | |
| Two hominins: Ahuman holding achimpanzee (Joseph V. Brady andHam the chimp) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Suborder: | Haplorhini |
| Infraorder: | Simiiformes |
| Family: | Hominidae |
| Subfamily: | Homininae |
| Tribe: | Hominini Arambourg, 1948[1] |
| Type genus | |
| Homo Linnaeus, 1758 | |
| Genera | |
TheHominini (hominins) form ataxonomic tribe of the subfamilyHomininae (hominines). They comprise two extant genera:Homo (humans) andPan (chimpanzees andbonobos), and in standard usage exclude the genusGorilla (gorillas), which is grouped separately within the subfamily Homininae.
The term Hominini was originally introduced byCamille Arambourg (1948), who combined the categories ofHominina andSimiina pursuant toGray's classifications (1825).
Traditionally,chimpanzees,gorillas andorangutans were grouped together, excluding humans, aspongids. Since Gray's classifications, evidence accumulating from geneticphylogeny confirmed that humans, chimpanzees, and gorillas are more closely related to each other than to the orangutan.[3] The orangutans were reassigned to the familyHominidae (great apes), which already included humans; and the gorillas were grouped as a separate tribe (Gorillini) of the subfamily Homininae.[3] Still, details of this reassignment remain contested, and of publishing since (on tribe Hominini), not every source excludes gorillas and not every source includes chimpanzees.
Humans are the only extant species in theAustralopithecine branch (subtribe), which also contains many extinct close relatives of humans.
Concerning membership, when Hominini is taken to excludePan,Panini ("panins")[4] may refer to the tribe containingPan as its only genus.[5][6] Or perhaps placePan with otherdryopithecine genera, making the whole tribe or subtribe of Panini or Panina together. Minority dissenting nomenclatures includeGorilla in Hominini andPan inHomo (Goodman et al. 1998), or bothPan andGorilla inHomo (Watson et al. 2001).
By convention, the adjectival term "hominin" (or nominalized "hominins") refers to the tribe Hominini, whereas the members of the subtribe Hominina (and thus all archaic human species) are referred to as "homininan" ("homininans").[7][8][9] This follows the proposal by Mann and Weiss (1996), which presents tribe Hominini as including bothPan andHomo, placed in separate subtribes. The genusPan is referred to subtribePanina, and genusHomo is included in the subtribeHominina (seebelow).[10]
The alternative convention uses "hominin" to exclude members of Panina: forHomo; or for human and australopithecine species. This alternative convention is referenced in e.g. Coyne (2009)[11] and in Dunbar (2014).[6] Potts (2010) in addition uses the name Hominini in a different sense, as excludingPan, and uses "hominins" for this, while a separate tribe (rather than subtribe) for chimpanzees is introduced, under the name Panini.[5] In this recent convention,contra Arambourg, the term "hominin" is applied toHomo,Australopithecus,Ardipithecus, and others that arose after the split from the line that led to chimpanzees (see cladogram below);[12][13] 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-homininhominids").[11]
Thiscladogram shows theclade ofsuperfamily Hominoidea and its descendant clades, focused on the division of Hominini (omitting detail on clades not ancestral to Hominini). Thefamily Hominidae ("hominids") comprises thetribes Ponginae (includingorangutans), Gorillini (includinggorillas) and Hominini, the latter two forming thesubfamily 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 excludesPan).
Genetic analysis combined with fossil evidence indicates that hominoids diverged from theOld World monkeys about 25 million years ago (Mya), near the Oligocene-Miocene boundary.[14] Themost recent common ancestors (MRCA) of the subfamilies Homininae and Ponginae lived about 15 million years ago. The best-known fossil genus of Ponginae isSivapithecus, consisting of several species from 12.5 million to 8.5 million years ago. It differs from orangutans in dentition and postcranial morphology.[15] In the following cladogram, the approximate time the clades radiated newer clades is indicated in millions of years ago (Mya).
| Hominoidea (20.4 Mya) |
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−10 — – −9 — – −8 — – −7 — – −6 — – −5 — – −4 — – −3 — – −2 — – −1 — – 0 — |
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BothSahelanthropus andOrrorin 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 genusPan. 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.[16] The divergence of a "proto-human" or "pre-human" lineage separate fromPan appears to have been a process of complexspeciation-hybridization rather than a clean split, taking place over the period of anywhere between 13 Mya (close to the age of the tribe Hominini itself) and some 4 Mya. 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),[17] This research group noted that one hypothetical late hybridization period was based in particular on the similarity ofX chromosomes in the proto-humans and stem chimpanzees, suggesting that the final divergence was 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 thechimpanzee–human last common ancestor (CHLCA).[18]
MostDNA studies find that humans andPan are 99% identical,[19][20] but one study found only 94% commonality, with some of the difference occurring innon-coding DNA.[21] It is most likely that the australopithecines, dating from 4.4 to 3 Mya, evolved into the earliest members of genusHomo.[22][23] In the year 2000, the discovery ofOrrorin tugenensis, dated as early as 6.2 Mya, briefly challenged critical elements of that hypothesis,[24] as it suggested thatHomo did not in fact derive from australopithecine ancestors.[25]
All the listed fossil genera are evaluated for two traits that could identify them as hominins:
Some, includingParanthropus,Ardipithecus, andAustralopithecus, are broadly thought to be ancestral and closely related toHomo;[26] others, especially earlier genera, includingSahelanthropus (and perhapsOrrorin), are supported by one community of scientists but doubted by another.[27][28]
Extant species are in bold.
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However, overwhelming genetic evidence has since demonstrated thathumans,chimpanzees, andgorillas are much more closely related to each other than to theorangutan ... Thus, there is no genetic support for grouping thegreat apes together in a distinct group from humans. For this reason, many researchers now place all species of greatape and human within a single family, Hominidae – making them all proper 'hominids'.
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.
Anthropologists apply the termhominin to all the species on the "human" side of our family tree after it split from the branch that became modern chimps." (p.197)
Thus human evolution is the study of the lineage, or clade, comprising species more closely related to modern humans than to chimpanzees. Its stem species is the so-called 'common hominin ancestor', and its only extant member isHomo sapiens. This clade contains all the species more closely related to modern humans than to any other living primate. Until recently, these species were all subsumed into a family, Hominidae, but this group is now more usually recognised as a tribe, the Hominini.
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.
AfterA. afarensis, the fossil record shows a confusing melange of gracile australopithecine species lasting up to about two million years ago. … [T]he late australopithecines, already bipedal, were beginning to show changes in teeth, skull, and brain that presage modern humans. It is very likely that the lineage that gave rise to modern humans included at least one of these species.
The discovery ofOrrorin 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 ofHomo from the australopithecines still holds primacy in the minds of most palaeoanthropologists.
Sahelanthropus is the oldest and most primitive known member of the hominid clade, close to the divergence of hominids and chimpanzees.
Sahelanthropus tchadensis is an enigmatic new Miocene species, whose characteristics are a mix of those of apes andHomo 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, andSahelanthropus does not appear to have been an obligate biped.
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