The Hominini is a taxonomic tribe of the subfamily Homininae; it comprises three subtribes: Hominina, with its one genus Homo; Australopithecina, comprising at least three extinct genera; and Panina, with its one genus Pan, the chimpanzees. Members of the human clade, that is, the subtribe Hominina, include only the genus Homo; it is the "human" branch as depicted in an evolutionary tree chart (see below). Over the past several decades, a broad community of scientists and scholars have adopted the term "hominins" to emphasize the history of proto-humans speciating from the stem chimpanzees—which process began about eight million years ago; see chimpanzee-human last common ancestor (CHLCA). Thus, the genus Homo and those species of the australopithecines that arose after the split from the last common ancestor with chimpanzees are called "hominins". Not all hominins are directly related to the emergence of early Homo.
Temporal range: 7–0 Ma
|Skull of Sahelanthropus tchadensis, described as the earliest member of the hominin line, but this is debated.|
Researchers proposed the taxon Hominini on the basis that the least similar species of a trichotomy should be separated from the other two. The common chimpanzee and the bonobo of the genus Pan are the closest living evolutionary relatives to humans, sharing a common ancestor with humans about four to seven million years ago. 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.
The human clade, the subtribe Hominina, contains only Homo; it is the "human" branch as depicted in a tree of life or in an evolutionary tree chart (see below). It is a branch of the family Hominidae—the hominids—one of the families of the order primates known as the "family of great apes". One community of scientists and scholars emphasizes the long-drawn-out speciation of proto-humans from stem chimpanzees—which process started sometime after about eight million years ago (Mya) and may have taken until four Mya to completion (see Complex ... hybridization). This community applies the term hominin to all Homo species and those species of australopithecines and other genera that arose after the split from the line 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.
Until recently,[when?] much of the scientific community thought it likely that the australopithecines, dating to 4.4 Mya, gave rise to the earliest Homo genera. But, since the year 2000, the discovery of Orrorin tugenensis, dated as early as 6.2 Mya, has challenged critical elements of that hypothesis. The community now is divided or puzzled over some vital questions raised by Orrorin, including: Are humans descendants of the australopithecines or Orrorin, or both?; Are the australopithecines descendants of Orrorin?; Did pre-human bipedalism actually evolve in response to hominoids adapting to the dry and patchy-forest conditions of the East African savannah—or perhaps much earlier, with Orrorin-like hominoids adapting as tree-dwellers and tree-walkers in the arboreal setting of abundant forests? Reynolds (2012) is quoted:
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.
The earliest hominins also produced lines, whether of australopithecines or other genera, that arose after speciating from the line to chimpanzees but went extinct without developing into Homo; thus, not all hominins are ancestral to the emergence of early Homo. (See "Genera", classification infobox.)
All the listed fossil genera currently are being, or have been, 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, Kenyanthropus, 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.
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). So one or both of these specimens may yet prove to be "hybrid", and thereby ancestral to both Pan and Homo—or to Pan instead of Homo. Very few fossil specimens have been found on the Pan 'side' of the split from the common ancestral line. 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 radiometric (kyr).
Complex speciation and hybridizationEdit
Regarding the speciation of proto-humans from stem chimpanzees there is evidence that a complex speciation-hybridization process—rather than a clean split—occurred between the two lineages. Such a complex process greatly confuses estimating the actual age of the final Homo-Pan divergence, or split. Different chromosomes appear to have split at different times, over possibly as much as a four-million-year period; this indicates a long and drawn out speciation process 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).
Cladogram of the superfamily HominoideaEdit
This cladogram shows the clade of superfamily Hominoidea and its descendent clades; it features the hominids, which is the family Hominidae. The hominids comprise the orangutans, gorillas, and the clade of the tribe Hominini—which groups the subtribe Panina (chimpanzees), the extinct subtribe Australopithecina (australopithicenes), and the subtribe Hominina (humans).
The scientific community has not reached a consensus regarding a taxon for the probable ancestral stem between the subtribes Hominina and Australopithecina—hence the blank space (*) after the tribe-label "Hominini". However, the community has informally adopted a name that encompasses the humans and their extinct closest relatives and which excludes the chimpanzees; these humans, australopithecines, and other genera collectively are called hominins. They comprise the several Homo species—only one of which is not extinct—plus their extinct closest relatives that arose after the splitting of the proto-humans from the last common ancestor with the stem chimpanzees.
Morphological and radiometric analyses indicate that ancestral hominids, that is, the most recent common ancestors (MRCA, the equivalent of LCA) of the subfamilies Homininae and Ponginae, lived until at least about 14 million years ago (Mya), prior to the ancestors of orangutans speciating from the common ancestors. Previously, the common ancestors of hominids had speciated from the superfamily Hominoidae between 20 and 15 million years ago. Recent molecular analyses have refined some paleo-chronological dates for the emergence of specific clades.
Contending taxonomies for "hominins"Edit
By convention, the term "hominin", with its specified ending, is the taxonomical label assigned to any member of the tribe Hominini. But over recent decades, the role of this term has become caught up in a community-wide phenomenon of word appropriation, change-in-word-usage, and contending over taxonomies, all with resultant confusion to the reader—especially to the lay reader who doesn't 'stay up' with paleoanthropology. A brief review of that phenomenon follows.
A proposal by Mann and Weiss (1996) presents tribe Hominini as including both Pan and Homo, placed in separate subtribes. The genus Pan is referred to subtribe Panina; genus Homo—and (by inference) all bipedal apes—is referred to the subtribe Hominina. This taxonomy in effect excludes the chimpanzees from the clade of humans and their closest relatives and labels members of the Homo clade as "homininans"; see cladogram. However, Coyne (2009) noted the broad movement among researchers for assigning the label "hominins" to genus Homo and those relatives "after the split" from the line of the stem chimpanzees. Potts (2010) presents a taxonomy for this latter scenario, placing humans and their closest relatives as a group in the tribe Hominini and labeling them "hominins", while chimpanzees are placed in a separate tribe and dubbed "Panini" (not to be confused with panini).
Wood (2010) also reports different views for labeling the hominin taxonomy, noting that some researchers assign both Pan and the humans to tribe Hominini—they label the human clade as the subtribe Hominina and refer to the taxa and individuals within that clade as "homininans”. Other researchers use tribe Hominini to designate the human clade only and refer to all taxa and fossils within it as “hominins”; but Wood does not clarify how genus Pan is accounted for in this scheme. Dunbar (2014) reports that "all members of the lineage leading to modern humans that arose after the split with the LCA are referred to as hominins". The Australian National Museum provides a concise history of the muddle and the present (2016) state of flux. (See "Genera", classification infobox.)
As it happened over recent decades, a significant community of scientists and scholars adopted the change-in-usage of the term "hominin"; that is, they appropriated the term to designate all the members of genus Homo—both extant and extinct—plus the fossil members of australopithecines and those other bipedal genera that arose after the final split from the last common ancestor (LCA) with the stem chimpanzees; this de facto definition intentionally excluded the chimpanzees from the name "hominin". In effect, this construct treats the scientific term "hominin" as a common name applying not, as traditionally, to the tribe Hominini, but exclusively to the clade of humans and their closest relatives excluding chimpanzees. Similarly, the common name for the scientific label "Homo" is "human".
The change in usage of "hominin" does not conform to conventional rules for labeling taxa and affixing taxon-name endings to its members. For example, whenever Pan is listed as a member of the tribe Hominini, most taxonomists would routinely interpret chimpanzees as hominins. Further, there is no consensus among the community towards defining a taxon that would label the last common ancestor (LCA) of the greater hominin clade—all the humans and the australopithecines and other genera arising "after the split" from the line to chimpanzees; see cladogram at the blank (*) stem. What is factual now is that this greater clade is broadly called "hominins", thereby designating a de facto hominin clade outside the conventional rules of taxon labeling.
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|This section needs expansion. You can help by adding to it. (August 2017)|
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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.
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Sahelanthropus is the oldest and most primitive known member of the hominid clade, close to the divergence of hominids and chimpanzees.
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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.
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