Academics still debate whether certain African hominid species of this time, such as A. robustus and A. boisei, constitute members of the same genus. If so, they would be considered robust australopiths while the others would be gracile australopiths. However, if these species do constitute their own genus, they may be given their own name, Paranthropus.
Archaeologists and palaeontologists widely hold that the australopiths played a significant part in human evolution, being the first of the hominins to show presence of a gene that causes increased length and ability of neurons in the brain, the duplicated SRGAP2 gene. One of the australopith species eventually evolved into the Homo genus in Africa around two million years ago, which contained within it species like Homo habilis, H. ergaster, and eventually the modern human species, H. sapiens sapiens
Gracile australopiths shared several traits with modern apes and humans, and were widespread throughout Eastern and Northern Africa around 3.5 million years ago. The earliest evidence of fundamentally bipedal hominids can be observed at the site of Laetoli in Tanzania. This site contains hominid footprints that are remarkably similar to those of modern humans and have been dated to as old as 3.6 million years. The footprints have generally been classified as australopith because that is the only form of prehuman known to have existed in that region at that time.
Australopithecus anamensis, Australopithecus afarensis and Australopithecus africanus are among the most famous of the extinct hominins. A. africanus were once regarded as ancestral to the genus Homo (in particular Homo erectus). 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 yet possibly unknown 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 five to six million years ago, if we assume a constant rate of evolution. It is theoretically more likely for evolution to happen more slowly, as opposed to more quickly, from the date suggested by a gene clock (the result of which is given as a youngest common ancestor, i.e., the latest possible date of divergence.) However, hominins discovered more recently are somewhat older than the molecular clock would theorize.
Sahelanthropus tchadensis, commonly called "Toumai" is about seven million years old and Orrorin tugenensis lived at least six million years ago. Since little is known of them, they remain controversial among scientists since the molecular clock in humans has determined that humans and chimpanzees had a genetic 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.
The brains of most species of Australopithecus were roughly 35% of the size of that of a modern human brain. Most species of Australopithecus were diminutive and gracile, usually standing between 1.2 to 1.4 m (3 ft 11 in to 4 ft 7 in) tall. In several variations of Australopithecus there is a considerable degree of sexual dimorphism, in this case males being larger than females.
According to A. Zihlman, Australopithecus body proportions closely resemble those of bonobos (Pan paniscus), leading evolutionary biologists like Jeremy Griffith to suggest that bonobos may be phenotypically similar to Australopithecus.
Modern hominids do not appear to display sexual dimorphism to the same degree — particularly, modern humans display a low degree of sexual dimorphism, with males being only 15% larger than females, on average. In Australopithecus, however, males can be up to 50% larger than females. New research suggests that sexual dimorphism may be less pronounced than this, but there is still debate on the subject.