Australopithecus

Australopithecus^[1] is a genus of extinct hominids closely related to humans.

Quick facts Australopithecus Temporal range: Pliocene, Scientific classification ...

Australopithecus Temporal range: Pliocene

Australopithecus afarensis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Genus:	Australopithecus R.A. Dart, 1925
Species
†A. afarensis †A. africanus †A. anamensis †A. bahrelghazali †A. garhi

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Raymond Dart was the first to describe Australopithecus. In 1925 he discovered and wrote about the Taung Child, an Australopithecus fossil skull.

Their remains are mostly found in East Africa, and the first fossil is from 3.9 million years ago (mya).^[source?] The split from other apes happened earlier, perhaps about 5 mya.^[source?]

It is widely believed that the genus Homo, and hence human beings, developed from a group of hominids that included Australopithecus.^[2]

The genus Australopithecus originally included two rather different forms. One form was lightweight: the gracile australopithecines. The other form was bulkier: the robust australopithecines. Scholars do not agree on whether these should be put in separate genera. This article describes the gracile forms; the robust forms are described elsewhere as Paranthropus.

Gracile australopithecines shared several traits with modern apes and humans. They were widespread throughout Eastern and Northern Africa 3.9 to 3 million years ago.^[source?]

Morphology

Skull

Brain size

The brains of most species of Australopithecus were roughly 35% of the size of that of a modern human brain.^[source?] This is not much more than the brain of a chimpanzee. Brain size in hominins did not increase significantly until the genus Homo appeared.

Other features

The Taung specimen had short canine teeth, and the position of the foramen magnum was evidence that the Taung Child walked on two feet.^[3] Fossil footprints^[4] found in Laetoli, Tanzania also show that these apes had achieved bipedalism.

Skeleton

Most species of Australopithecus were small and gracile, usually standing between 1.2 and 1.4 m tall (approx. 4 to 4.5 feet). Australopithecines showed more sexual dimorphism than modern hominids do.

Modern humans display a low degree of sexual dimorphism, with males being only 15% larger (taller, heavier) than females, on average. In Australopithecus, however, the largest males could be up to to 50% larger than females.^[5]

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Evolution

Scholars used to think that Australopithecus africanus was the ancestor of the genus Homo (in particular Homo erectus). However, scientists have now found fossils from the genus Homo that are older than A. africanus. This leaves two possibilities. The genus Homo may have split off from the genus Australopithecus at an earlier date. Their latest common ancestor might have been A. afarensis or an even earlier form, possibly Kenyanthropus platyops. Another possibility is that both Homo and Australopithecus developed independently from the same common ancestor.

According to the Chimpanzee Genome Project, both human (Ardipithecus, Australopithecus and Homo) and chimpanzee (Pan troglodytes and Pan paniscus) lineages diverged from a common ancestor about 5 to 6 million years ago, if we assume a constant rate of evolution.^[source?]

However, hominins discovered more recently are somewhat older than the molecular clock would suggest. Sahelanthropus tchadensis, commonly called "Toumai", is about 7 million years old, and Orrorin tugenensis lived at least 6 million years ago.^[source?] Since little is known of them, they remain controversial because the molecular clock in humans has determined that humans and chimpanzees had an evolutionary split at least a million years later.

One theory suggests that the human and chimpanzee lineages diverged somewhat at first, then some populations interbred around one million years after diverging.^[6] More likely, the assumptions behind molecular clocks do not hold exactly. The key assumption behind the technique is that, in the long run, changes in molecular structure happen at a steady rate.^{[further explanation needed]} Researchers such as Ayala have challenged this assumption.^[7]^[8]^[9]