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Hominidae

The Hominidae ( /h??m?n?di?/; anglicized hominids, also known as great apes[notes 1]), as the term is used here, form a taxonomic family of primates, including four extant genera: chimpanzees (Pan), gorillas (Gorilla), humans (Homo), and orangutans (Pongo).[1] The term "hominid" is also used in the more restricted sense of humans and relatives of humans closer than chimpanzees.[2] In this usage, all hominid species other than Homo sapiens are extinct.

A number of known extinct genera are grouped with humans in the Homininae subfamily, others with orangutans in the Ponginae subfamily. The most recent common ancestor of the Hominidae lived roughly 14 million years ago,[3] when the ancestors of the orangutans speciated from the ancestors of the other three genera.[4] The ancestors of the Hominidae family had already speciated from those of the Hylobatidae family, perhaps 15 million to 20 million years ago.[4][5]

History

On July 19, 2001, a 7-million-year-old fossil skull, classified as Sahelanthropus tchadensis and nicknamed "Toumaï", was discovered in Chad, Africa. It is possibly the earliest hominine fossil ever found. In addition to its age, Toumaï, unlike the three- to four-million-year younger gracile australopithecine dubbed "Lucy", has a relatively flat face without the prominent snout seen on other pre-Homo hominids.[6] There is some dispute over the importance of Sahelanthropus tchadensis. It could be that this species is a direct ancestor of modern humans (or at least closely related to a direct ancestor). In the absence of additional fossil species from this era, certainty is impossible.[7]

Researchers, such as David Reich and Eric Lander, have found evidence that the divergence between proto-chimpanzees and proto-humans was marked by episodes of gene flow over millions of years, before the final separation.[8] It is generally believed that the Pan/Homo separation occurred about 6 – 8 million years ago,[9][10][11] but the molecular clock (a method of calculating evolution based on the rate at which genes mutate) suggests the genera division 4.6 – 6.2 million years ago.[12] Previous studies looked at average genetic differences between human and chimp. The new study compares the ages of key sequences of genes of modern humans and modern chimps. Some sequences are younger than others, indicating that chimps and humans gradually separated over a period of 4 million years. The youngest human chromosome is the X chromosome which is about 1.2 million years more recent than the 22 autosomes.[8] The X sex chromosome responds more quickly to selective pressure, which may explain the discrepancy.[13] Its age suggests there was an initial division between the two species, followed by gradual divergence and interbreeding that resulted in younger genes, and then a final separation.

A minority viewpoint is that Homo diverged from a common ancestor with Pongo perhaps as early as 13 million years ago, so that Pan is more closely related to Gorilla. This alternative is supported by characteristics uniquely shared between humans and orangutans, such as dental structure, thick enamel, shoulder blade structure, thick posterior palate, single incisive foramen, high estriol production, and beard and mustache. There are at least 28 such well-corroborated features compared with perhaps as few as one unique feature shared between humans and chimpanzees. It is widely believed that these physical features are misleading, but an alternative possibility is that orangutans have undergone more genetic change than humans and African apes have since their divergence from the common ancestor. If this had happened, then the apparent genetic similarity between humans and chimpanzees would not necessarily be due to a close evolutionary relationship.[14][15] This hypothesis has been proposed as an explanation as to why early hominids, such as the australopiths, not only look more like orangutans than modern African apes, but also share characters unique to orangutans and their close fossil relatives, such as a thickened posterior palate and anterior zygomatic roots.[16]

Taxonomic history
The classification of the great apes has been revised several times in the last few decades. These various revisions have led to a varied use of the word "hominid" – the original meaning of Hominidae referred only to the modern meaning of Hominina, i.e. only humans and their closest relatives. The meaning of the taxon changed gradually, leading to the modern meaning of "hominid", which includes all great apes and humans.

The primatological term hominid is easily confused with a number of very similar words:

  • A hominoid, commonly called an ape, is a member of the superfamily Hominoidea: extant members are the lesser apes (gibbons) and great apes (hominids).
  • A hominine is a member of the subfamily Homininae: gorillas, chimpanzees, humans (excludes orangutans).
  • A hominin is a member of the tribe Hominini: bonobos, chimpanzees and humans.
  • A hominan is a member of the subtribe Hominina: modern humans and their extinct relatives.
  • A human is a member of the genus Homo, of which Homo sapiens is the only extant species, and within that Homo sapiens sapiens is the only surviving subspecies.

    Many scientists, including anthropologists, continue to use the term hominid to mean humans and their direct and near-direct bipedal ancestors.

    As mentioned, Hominidae was originally the name given to humans and their extinct relatives, with the other great apes being placed in a separate family, the Pongidae. However, that definition made Pongidae paraphyletic because at least one great ape species appears to be more closely related to humans than to other great apes. Most taxonomists nowadays encourage monophyletic groups – this would require the use of Pongidae to be restricted to one of the great ape groups (containing Pongo, the orangutans) only. Thus, many biologists consider Hominidae to include Pongidae as the subfamily Ponginae, or restrict the latter to the orangutans and their extinct relatives, such as Gigantopithecus. The taxonomy shown here follows the monophyletic groupings according to the two theories of human and great ape relationships.

    Especially close human relatives form a subfamily, the Homininae. A few researchers go so far as to include chimpanzees[17] and gorillas[18][19] in the genus Homo along with humans. Alternatively, those fossil relatives more closely related to humans than the nearest living great ape species represent members of Hominidae without necessarily assigning subfamily or tribal categories. If the orangutan is the closest living relative of humans, there would be a sister group relationship between Hominidae and Pongidae, with the African apes comprising a separate family (Panidae) according to the morphological evidence.[15][20]

    Many extinct hominids have been studied to help understand the relationship between modern humans and the other extant hominids. Some of the extinct members of this family (as defined to encompass humans and chimpanzees) include Gigantopithecus, Orrorin, Ardipithecus, Kenyanthropus, and the australopithecines Australopithecus and Paranthropus. In the orangutan model of human origin, the Hominidae would include the australopiths, and possibly Orrorin and Kenyanthropus, but not Ardipithecus and Sahelanthropus that lack features necessary to provide strong corroboration as hominids.[21]

    The exact criteria for membership in the Homininae under the chimpanzee theory of human origins are not clear, but the subfamily generally includes those species that share more than 97% of their DNA with the modern human genome, and exhibit a capacity for language or for simple cultures beyond the family or band. The theory of mind including such faculties as mental state attribution, empathy and even empathetic deception is a controversial criterion distinguishing the adult human alone among the hominids. Humans acquire this capacity at about four and a half years of age, whereas it has neither been proven nor disproven that gorillas and chimpanzees develop a theory of mind.[22] This is also the case for some New World monkeys outside the family of great apes, as, for example, the capuchin monkeys.

    However, without the ability to test whether early members of the Homininae (such as Homo erectus, Homo neanderthalensis, or even the australopithecines) had a theory of mind, it is difficult to ignore similarities seen in their living cousins. Orangutans have also been shown to have culture comparable to that of chimpanzees,[23] and some say the orangutan may also satisfy these criteria. These scientific debates take on political significance for advocates of Great Ape personhood.

    Physical description

    The great apes are large, tailless primates, with the smallest living species being the bonobo at 30–40 kilograms in weight, and the largest being the gorillas, with males weighing 140–180 kilograms. In all great apes, the males are, on average, larger and stronger than the females, although the degree of sexual dimorphism varies greatly among species. Although most living species are predominantly quadrupedal, they are all able to use their hands for gathering food or nesting materials, and, in some cases, for tool use.[24]

    Most species are omnivorous, but fruit is the preferred food among all but humans. Chimpanzees and orangutans primarily eat fruit. When gorillas run short of fruit at certain times of the year or in certain regions, they resort to eating shoots and leaves, often of bamboo, a type of grass. Gorillas have extreme adaptations for chewing and digesting such low-quality forage, but they still prefer fruit when it is available, often going miles out of their way to find especially preferred fruits. Humans, since the neolithic revolution, consume mostly cereals and other starchy foods, including increasingly highly processed foods, as well as many other domesticated plants (including fruits) and meat. Hominid teeth are similar to those of the Old World monkeys and gibbons, although they are especially large in gorillas.

    Gestation in great apes lasts 8–9 months, and results in the birth of a single offspring, or, rarely, twins. The young are born helpless, and they must be cared for, for long periods of time. Compared with most other mammals, great apes have a remarkably long adolescence, not being weaned for several years, and not becoming fully mature for eight to thirteen years in most species (longer in humans). As a result, females typically give birth only once every few years. There is no distinct breeding season.[24]

    Gorillas and chimpanzees live in family groups of around five to ten individuals, although much larger groups are sometimes noted. Chimpanzees live in larger groups that break up into smaller groups when fruit becomes less available. When small groups of female chimpanzees go off in separate directions to forage for fruit, the dominant male(s) can no longer control them and the females often mate with other subordinate males, whether by choice or not. In contrast, groups of gorillas stay together regardless of the availability of fruit. When fruit is hard to find, they resort to eating leaves and shoots. Because gorilla groups stay together, the male is able to monopolize the females in his group. This fact is related to gorillas' greater sexual dimorphism than chimpanzees'. In both chimpanzees and gorillas, the groups include at least one dominant male, and females leave the group at maturity.

    Legal status

    Due to the close genetic relationship between humans and other great apes, certain animal rights organizations, such as the Great Ape Project, argue that nonhuman great apes are persons and should be given basic human rights. Some countries have instituted a research ban to protect great apes from any kind of scientific testing.

    On 25 June 2008, the Spanish parliament supported a new law that would make "keeping apes for circuses, television commercials or filming" illegal.[27]

    References

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    This article uses material from the Wikipedia article "Hominidae", which is released under the Creative Commons Attribution-Share-Alike License 3.0.