Hominidae

Hominids Fossil range: Late Miocene to Present
Australopithecus africanus reconstruction.
Scientific classification
Class	Mammalia
Order	Primates
Suborder	Haplorrhini
Infraorder	Simiiformes
Parvorder	Catarrhini
Superfamily	Hominoidea
Family	Hominidae Gray, 1825
Genera	Subfamily Ponginae Pongo – orangutans Subfamily Homininae Gorilla – gorillas Homo – humans Pan – chimpanzees

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

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 some 13 million years ago, when the ancestors of the orangutans speciated from the ancestors of the other three genera. The ancestors of the Hominidae family in turn speciated from those of the Hylobatidae family some 15 million years ago.

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 and is possibly the earliest hominid fossil ever found. In addition to its age, Toumaï, unlike the 3 – 4 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 hominin fossil since Australopithecus.

In addition to the Toumaï fossil, some experts use evidence from the genome to argue that the species associated with the chimpanzees and proto-humans split 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.

It is generally believed that the Pan/Homo split occurred about 6.5–7.4 million years ago, but the molecular clock (a method of calculating evolution based on the speed at which genes mutate) suggests the genera split 5.4–6.3 million years ago. 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 split apart 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. The X sex chromosome is known to be vulnerable to selective pressure. Its age suggests there was an initial split 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 split 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. 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.

Taxonomic history

This 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 and gorillas 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.

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.

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.

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 and some say that the orangutan may also satisfy these criteria.

Classification

Hominoid family tree

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

• Family Hominidae: humans and other great apes; extinct genera and species excluded.
  • Subfamily Ponginae
    • Genus Pongo
      • Bornean Orangutan, Pongo pygmaeus
        • Pongo pygmaeus pygmaeus
        • Pongo pygmaeus morio
        • Pongo pygmaeus wurmbii
      • Sumatran Orangutan, Pongo abelii
  • Subfamily Homininae
    • Tribe Gorillini
      • Genus Gorilla
        • Western Gorilla, Gorilla gorilla
          • Western Lowland Gorilla, Gorilla gorilla gorilla
          • Cross River Gorilla, Gorilla gorilla diehli
        • Eastern Gorilla, Gorilla beringei
          • Mountain Gorilla, Gorilla beringei beringei
          • Eastern Lowland Gorilla, Gorilla beringei graueri
    • Tribe Hominini
      • Genus Pan
        • Common Chimpanzee, Pan troglodytes
          • Central Chimpanzee, Pan troglodytes troglodytes
          • West African Chimpanzee, Pan troglodytes verus
          • Nigerian Chimpanzee, Pan troglodytes vellerosus
          • Eastern Chimpanzee, Pan troglodytes schweinfurthii
        • Bonobo (Pygmy Chimpanzee), Pan paniscus
      • Genus Homo
        • Human, Homo sapiens sapiens

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.

• Subfamily Ponginae (or Pongidae)
  • Gigantopithecus†
  • Sivapithecus†
  • Lufengpithecus†
  • Ankarapithecus†
  • Ouranopithecus†
• Subfamily Homininae (or Hominidae)
  • Homo – immediate ancestors of modern humans
    • Homo habilis†
    • Homo rudolfensis†
    • Homo ergaster†
    • Homo georgicus†
    • Homo erectus†
    • Homo cepranensis†
    • Homo antecessor†
    • Homo heidelbergensis†
    • Homo rhodesiensis†
    • Homo neanderthalensis†
    • Homo sapiens
      • Homo sapiens idaltu†
      • Archaic Homo sapiens (Cro-magnon)†
    • Homo floresiensis†
  • Oreopithecus†
  • Paranthropus†
  • Australopithecus†
  • Sahelanthropus†(hominid status highly problematic)
  • Orrorin†
  • Ardipithecus† (hominid status highly problematic)
  • Kenyanthropus†

See also

• Ape extinction
• Evolution of Homo sapiens
  • Graphical timeline of human evolution

External links

• NPR News: Toumaï the Human Ancestor
• Hominid Species at TalkOrigins Archive
• For more details on Hominid species, including excellent photos of fossil hominids
• New Scientist 19 May 2003 – Chimps are human, gene study implies
• Scientific American Magazine (April 2006 Issue) Why Are Some Animals So Smart?