Temporal range: 6-0 Ma
|Two hominins, according to the original definition by Gray: A human (Homo sapiens) holding a chimpanzee (Pan troglodytes)|
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 2020, 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.
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).
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).
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"). 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). 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) and in Dunbar (2014). 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. 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); 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").
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. The most recent common ancestors (MRCA) of the subfamilies Homininae and Ponginae lived about 15 million years ago. In the following cladogram, the approximate time the clades radiated newer clades is indicated in millions of years ago (Mya).
|Hominoidea (20.4 Mya)|
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. 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), 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). Most DNA studies find that humans and Pan are 99% identical, but one study found only 94% commonality, with some of the difference occurring in noncoding DNA. It is most likely that the australopithecines, dating from 3 to 4.4 Mya, evolved into the earliest members of genus Homo. In the year 2000, the discovery of Orrorin tugenensis, dated as early as 6.2 Mya, briefly challenged critical elements of that hypothesis, as it suggested that Homo did not in fact derive from australopithecine ancestors. 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; others, especially earlier genera, including Sahelanthropus (and perhaps Orrorin), are supported by one community of scientists but doubted by another.
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 is Homo 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.
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 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.