By Paul Rincon
BBC News Online science staff
The human brain may have started evolving its unique characteristics much earlier than has previously been supposed, according to new research.
Australopithecine brains were similar in size to those of chimps
Hominid brains were being reorganised before the growth in brain size thought to have established a gulf between human and ape abilities, it is claimed.
The conclusions come from analysis of a small-brained fossil hominid - or human-like primate - from South Africa.
The authors report their findings in the journal Comptes Rendus Palevol.
Because the brain creates a mirror image of its surface inside the skull, scientists can create a cast - or endocast - by applying several layers of rubber paint to the cavity. When dry, this leaves a hollow rubber model of the brain that can be removed.
The researchers studied an endocast of the brain of Stw 505, a hominid specimen belonging to the species Australopithecus africanus that was unearthed in the Sterkfontein caves in South Africa in the 1980s.
Stw 505 is between two and three million years old.
When differences in size are discounted, the overall form of human and ape brains is remarkably similar.
One of a handful of differences between them is the position of the primary visual striate cortex (PVC), an area of the brain devoted exclusively to vision.
The boundary of this region is defined by an arching depression in the brain's surface called the lunate sulcus.
In the ape brain, this feature is situated further forward than it is in human brains, making the PVC larger.
Other researchers had claimed the PVC only decreased in size once the brain had grown substantially in size.
This occurred once big-brained Homo - the hominid group that includes humans - appeared on the scene around 2.4 million years ago, not before, they claimed. Australopithecines evolved before Homo, and their brains were similar in size to those of chimpanzees.
But the lunate sulcus of Stw 505 was positioned further back, as it is in humans.
This suggests australopithecine brains were already evolving towards a more human condition.
Anthropologist Raymond Dart first raised this possibility in 1925 in his description of the Taung skull, a juvenile Australopithecus africanus. But his claims were vehemently challenged by prominent scientists of the day.
"If you reduce the size of the lunate sulcus, then whatever is (in front) must expand to accommodate it," co-author Professor Ralph Holloway, of Columbia University in New York, told BBC News Online.
What lies in front is the posterior parietal cerebral cortex, which is associated in humans with a variety of complex behaviours such as the appreciation of objects and their qualities, facial recognition and social communication.
"I would speculate that (australopithecines) were under selection to extend their ecological niche into something beyond leaves and fruit," says Professor Holloway.
One eventual outcome of this process would be a dietary reliance on meat and the crafting of stone tools.
These developments drove, and were in turn influenced by, a rapid increase in brain size, with all the advanced capabilities this conferred.