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Hominids[1]
Fossil range: 7–0 Ma
Late Miocene to Recent
Australopithecus africanus reconstruction
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorrhini
Infraorder: Simiiformes
Parvorder: Catarrhini
Superfamily: Hominoidea
Family: Hominidae
Gray, 1825
Genera
Synonyms
  • Pongidae

The Hominidae (anglicized hominids, also known as great apes[notes 1]) form a taxonomic family, including four extant genera: chimpanzees, gorillas, humans, and orangutans.[1]

A number of known extinct genera are grouped with humans in the Hominina subtribe, others with orangutans in the Ponginae subtribe. The most recent common ancestor of the Hominidae lived roughly 14 million years ago, when the ancestors of the orangutans speciated from the ancestors of the other three genera.[2] The ancestors of the Hominidae family had already speciated from those of the Hylobatidae family, perhaps 15-20 million years ago.[2][3]

Contents

History

On July 19, 2001, a 7-million-year-old fossil skull, nicknamed "Toumaï" by its discoverers, and formally classified as Sahelanthropus tchadensis, was discovered in Chad. 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. Some researchers have made the suggestion that this previously unknown species may in fact be a direct ancestor of modern humans (or at least closely related to a direct ancestor). Others contend that one fossil is not enough to make such a claim because it would overturn the conclusions of over 100 years of anthropological study. A report on this finding was published in the journal Nature on July 11, 2002. While some scientists claim that it is merely the skull of a female gorilla ancestor, others have called it the most important hominine fossil since Australopithecus.

Orangutan

Some experts use evidence from the genome in addition to the Toumaï fossil to argue that the species associated with the divergence between chimpanzees and proto-humans interbred over a long period of time, swapping genes, before making a final separation. A paper, whose authors include David Reich and Eric Lander (Harvard and MIT), was published in the journal Nature in May 2006.[4]

It is generally believed that the Pan/Homo separation occurred about 6.5 – 7.4 million years ago, but the molecular clock (a method of calculating evolution based on the rate at which genes mutate) suggests the genera division 5.4 – 6.3 million years ago.[citation needed] 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.[citation needed] The X sex chromosome is known to be vulnerable to selective pressure. 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.

An alternative minority viewpoint is that Homo diverged from a common ancestor with Pongo perhaps as early as 13 million years ago while 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 well corroborated such features compared with perhaps as little 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.[5][6] This theory has been proposed as an explanation as to why early hominids such as the australopiths not only look more like orangutans than either African ape, but also share characters unique to orangutans and their close fossil relatives such as a thickened posteror palate and anterior zygomatic roots.[7]

Taxonomic history

Humans, depicted here on a Pioneer plaque, are one of the four extant hominid genera

The classification of the great apes has been revised several times in the last few decades. Originally, the group was restricted to humans and their extinct relatives, with the other great apes being placed in a separate family, the Pongidae. This definition is still used by many anthropologists and by lay persons. However, that definition makes Pongidae paraphyletic because at least one great ape species appears to be more closely related to humans than other great apes. Most taxonomists nowadays encourage monophyletic groups so this would require the use of Pongidae to be restricted to one of the great ape groups only. Thus many biologists consider Hominidae to include Pongidae as the subfamily Ponginae, or restrict the latter to the orangutans and their extinct relatives like 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. Some researchers go so far as to include chimpanzees[8] and gorillas[9][10] in the genus Homo along with humans, but it is more commonly accepted to describe the relationships as shown here. Alternatively, those fossil relatives that are 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.[6][11]

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.[12]

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.[13] 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[14] and some say that the orangutan may also satisfy these criteria. These scientific debates take on political significance for advocates of Great Ape personhood.

Classification

Hominoid family tree
Skulls of an orangutan and a gorilla

The seven living species of great ape are classified in four genera. The following classification is commonly accepted:[1]

In addition to the extant species and subspecies above, archaeologists, paleontologists, and anthropologists have discovered numerous extinct great ape species. The list below are some of the genera of those discoveries.[1]

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.[15]

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. In contrast, humans consume a large proportion of highly processed, low fiber foods, unusual proportions of grains (only very recently) and vertebrate flesh, as well as a wide variety of other foodstuffs. The teeth are similar to those of the Old World monkeys and gibbons, although they are especially large in gorillas. The dental formula is:Upper: 2.1.2.3, lower: 2.1.2.3 Human teeth and jaws are markedly smaller for their size than those of other apes. Some scholars have speculated that humans' small teeth and jaws are adaptations to eating cooked food for more than a million years,[16] but this idea remains controversial.

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 the mother must care for them 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 8–13 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.[15]

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. By contrast, orangutans are generally solitary. The social structure of humans is complex and highly variable. DNA studies suggest that, as in other apes, female humans are the sex that leaves the group at maturity, or at least has done so over evolutionary time.[citation needed]

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 non-human 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.[17]

See also

Notes

  1. ^ "Great ape" is a common name rather than a taxonomic label and there are differences in usage. Subtly, it may seem to exclude human beings ("humans and the great apes") or to include them ("humans and non-human great apes").

References

  1. ^ a b c d e Groves, C. (2005). Wilson, D. E., & Reeder, D. M. ed. Mammal Species of the World (3rd ed.). Johns Hopkins University Press. pp. 181-184. ISBN 0-801-88221-4. http://www.bucknell.edu/msw3/browse.asp?id=12100786. 
  2. ^ a b Dawkins R (2004) The Ancestor's Tale.
  3. ^ http://www.timetree.org/time_e_query.php?taxon_a=Hominidae&taxon_b=Hylobatidae
  4. ^ Patterson, Nick; Richter, Daniel J.; Gnerre, Sante; Lander, Eric S.; Reich, David (2006), "Genetic evidence for complex speciation of humans and chimpanzees", Nature 441: 1103–1108, doi:10.1038/nature04789, http://www.nature.com/nature/journal/v441/n7097/abs/nature04789.html 
  5. ^ Schwartz, J.H. (2005) The Red Ape. Westview Press.
  6. ^ a b Grehan, J.R. (2006) Mona Lisa Smile: The morphological enigma of human and great ape evolution. Anatomical Record 289B: 139-157.
  7. ^ Schwartz, J.H. (2004) Barking up the wrong ape - australopiths and the quest for chimpanzee characters in hominid fossils. Colloquim Antropologicum Supplement 28: 87-100.
  8. ^ Pickrell, John (2003-05-20). "Chimps Belong on Human Branch of Family Tree, Study Says". National Geographic. http://news.nationalgeographic.com/news/2003/05/0520_030520_chimpanzees.html. Retrieved 2007-08-04. 
  9. ^ Relationship Humans-Gorillas.
  10. ^ Watson, E. E. et al. (2001). "Homo genus: a review of the classification of humans and the great apes". in eds. Tobias, P. V. et al.. Humanity from African Naissance to Coming Millennia. Florence: Firenze Univ. Press. pp. Pp. 311–323. 
  11. ^ Schwartz, J.H. (1986) Primate systematics and a classification of the order. Comparative primate biology volume 1: Systematics, evolution, and anatomy (ed. by D.R. Swindler, and J. Erwin), pp. 1-41, Alan R. Liss, New York.
  12. ^ Schwartz, J.H. (2004b) Issues in hominid systematics. Zona Arqueología 4, 360-371.
  13. ^ Heyes, C. M. (1998). "Theory of Mind in Nonhuman Primates". Behavioral and Brain Sciences 21. doi:10.1017/S0140525X98000703. bbs00000546. http://www.bbsonline.org/documents/a/00/00/05/46/index.html. 
  14. ^ Van Schaik, C.P. et al. (2003) Orangutan cultures and the evolution of material culture. Science 299: 102-105.
  15. ^ a b Harcourt, A.H., MacKinnon, J. & Wrangham, R.W. (1984). Macdonald, D.. ed. The Encyclopedia of Mammals. New York: Facts on File. pp. 422–439. ISBN 0-87196-871-1. 
  16. ^ Richard Wrangham. "The Cooking Enigma". in Charles Pasternak (ed.). What Makes Us Human?. Oxford: Oneworld Press. 
  17. ^ "Spanish parliament to extend rights to apes". Reuters. 2008-06-25. http://www.reuters.com/article/scienceNews/idUSL256586320080625. Retrieved 2008-07-11. 

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