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Temporal range: Pliocene
R.A. Dart, 1925
The genus Australopithecus originally included two rather different forms. One form was lightweight: the gracile australopithecines. The other form was bulkier, the robust australopithecines.
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.
The brains of most species of Australopithecus were roughly 35% of the size of that of a modern human brain. This is not much more than the brain of a chimpanzee. Brain size in hominins does not increase significantly until the arrival of the genus Homo.
Most species of Australopithecus were diminutive and gracile, usually standing between 1.2 and 1.4 m tall (approx. 4 to 4.5 feet). There is a considerable degree of sexual dimorphism. Modern hominids do not show sexual dimorphism to the same degree — particularly, modern humans display a low degree of sexual dimorphism, with males being only 15% larger (taller, heavier) than females, on average.
In Australopithecus, however, males can be up to 50% larger than females, though usually less pronounced than this.
However, fossils assigned to the genus Homo have been found that are older than A. africanus. Thus, the genus Homo either split off from the genus Australopithecus at an earlier date (the latest common ancestor being A. afarensis or an even earlier form, possibly Kenyanthropus platyops), or both developed from a common ancestor independently.
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.
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.
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.
- Latin australis = 'of the south', Greek pithekos = 'ape'
- Leakey, Richard 1994. The origin of humankind. New York: BasicBooks.
- 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.
- Beck, Roger B. (1999). World history: patterns of interaction. Linda Black, Larry S. Krieger, Phillip C. Naylor, Dahia Ibo Shabaka. Evans ton, IL: McDougal Littell. .
- These footprints have been dated to 3.7 million years ago.
- Bower, Bruce (May 20, 2006). "Hybrid-driven evolution: genomes show complexity of human-chimp split". Science News 169 (20): 308. https://www.jstor.org/pss/4019102.
- Ayala F.J. (1999). "Molecular clock mirages". BioEssays 21 (1): 71–75. . http://www3.interscience.wiley.com/cgi-bin/abstract/60000186/ABSTRACT?CRETRY=1&SRETRY=0.
- Schwartz J.H. and Maresc B. (2006). "Do molecular clocks run at all? A critique of molecular systematics". Biological Theory 1: 357–371. Lay summary – Science Daily..
- 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