Australopithecus

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

Forms

The genus Australopithecus originally included two rather different forms. Scientists originally called them the gracile australopithecines and the robust australopithecines.^[2]

Gracile

The "gracile" australopithecines were lightweight. This article describes this type of australopithecine. Gracile australopithecines shared several traits with modern apes and humans.^[6]

Robust

See the main article: Paranthropus

Originally, scientists identified another group called the robust australopithecines. Scientists now call these Paranthropus.^[7]

Scholars do not agree on whether Australopithecus and Paranthropus should be put in separate genera. Some say Paranthropus is just another species of Australopithecus, not its own genus.^[7]

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Morphology

Skull

Brain size

All australopithecines had smaller brains than modern humans do. For example, Australopithecus africanus had an average brain volume of 450 cc, about the same size as a modern chimpanzee's brain.^[8]

Brain size in hominins did not increase significantly until the genus Homo appeared.^[8]

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.^[9] Fossil footprints^[10] 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.^[11] In Australopithecus, however, the largest males could be up to to 50% larger than females.^[12]

Evolution

Ancestors of Homo?

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.^[13] 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^[14]^[15] or an even earlier form, possibly Kenyanthropus platyops^[16]. Another possibility is that both Homo and Australopithecus developed independently from the same common ancestor.

When did human and chimps split?

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?]

Toumai and Orrorin

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^[17], and Orrorin tugenensis lived at least 6 million years ago.^[18] 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.^[source?]

Theories

One theory suggests that the human and chimpanzee lineages diverged somewhat at first, then some populations interbred around one million years after diverging.^[19] 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.^[20]^[21]^[22]