Australopithecus

***Australopithecus*** (Latin for "southern ape") is a genus of extinct hominids closely related to humans.

There were many different species of _Australopithecus_. They all lived in the Late Miocene sub-epoch. The earliest _Australopithecus_ fossils are from around 4.4 million years ago; the most recent are from around 1.4 million years ago. All were found in East Africa.

The genus _Homo_, and hence human beings, probably developed from a group of hominids that included _Australopithecus_.

Famous australopithecine fossils include the Taung Child (which is around 2.8 million years old) and Lucy (around 3.2 million years old).

## Discovery

Raymond Dart was the first to describe _Australopithecus._ In 1925 he discovered and wrote about the Taung Child, an _Australopithecus_ fossil skull. It was the first fossil ever to show evidence that human ancestors walked on two feet.

## Forms

The genus _Australopithecus_ originally included two rather different forms. Scientists originally called them the _gracile australopithecines_ and the _robust australopithecines_.

### Gracile

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

### Robust

_See the main article: Paranthropus_

Originally, scientists identified another group called the robust australopithecines. Scientists now call these _Paranthropus._

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.

#### 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.

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. Fossil footprints 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.

### 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_. 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.

### 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.

#### 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, and _Orrorin tugenensis_ lived at least 6 million years ago. 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.

#### Theories

One theory suggests that the human and chimpanzee lineages diverged somewhat at first, then some populations interbred around one million years after diverging. 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. Researchers such as Ayala have challenged this assumption.

## Related pages

- Human timeline
- Human evolution
- Lucy (Australopithecus)
- _Paranthropus_ (the "robust australopithecines")