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

## Forms 
The genus *Australopithecus* originally included two rather different forms. Scientists originally called them the *gracile australopithecines* and the *robust australopithecines*.

### 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 in hominins did not increase significantly until the genus *Homo* appeared. 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.

## 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*. 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")

## References