Australopithecus

Australopithecus (Latin for "southern ape"Latin australis = 'of the south', Greek pithekos = '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.Mai, L. L.; Owl, M. Y.; Kersting, M. P. (2005). The Cambridge Dictionary of Human Biology and Evolution. Cambridge & New York: Cambridge University Press. p. 45. ISBN 978-0-521-66486-8. 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.Leakey, Richard 1994. The origin of humankind. New York: BasicBooks. 

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.

 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. 

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.The foramen is a hole in the bottom of the skull, through which the spinal cord joins the brain. In apes it is positioned at the rear of the skull, in humans near the middle. The  foramen of a bipedal ape would be expected to be more like humans. Fossil footprintsThese footprints have been dated to 3.7 million years ago. 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.Jarmila Kukalová-Peck. 2008. Phylogeny of higher taxa in insecta: finding synapomorphies in the extant fauna and separating them from homoplasies. Evolutionary Biology 35, 4-51

 Related pages 
 Human timeline
 Human evolution
 Lucy (Australopithecus)
 Paranthropus (the "robust australopithecines")

 References 

Category:Australopithecines