A hominid boy and woman who died in a South African cave 2m years ago show an extraordinary mixture of apelike and human features, according to an exhaustive analysis of their fossilised bones.
Scientists at the University of the Witwatersrand, who led the study published in the journal Science, said the “mosaic” character of hominid made it a “transitional” or “intermediary” species between the earlier and more apelike Australopithecus and the later Homo genus.
Lee Berger, the project leader who discovered the fossils in 2008, said he could not prove that the new species – called Australopithecus sediba – represented a direct ancestor of modern humans rather than an evolutionary dead-end. But he said: “The many advanced features found in the brain and body … make it possibly the best candidate ancestor for our genus Homo.
“The fossils demonstrate a surprisingly advanced but small brain, a very evolved hand with a long thumb like a human’s, a very modern pelvis, but a foot and ankle shape … that combines features of both apes and humans in one anatomical package,” Prof Berger added.
Most remarkable is the skull of the boy, who was around 10 to 13 years old when he died and has been nicknamed Karabo (which means “answer” in South Africa’s Setswana language).
The skull – still filled with rock – was examined at the European Synchrotron Radiation Facility in France, the world’s most powerful X-ray source for scanning fossils. This produced a very detailed impression of Karabo’s brain case, including its surface anatomy.
The brain turns out to be surprisingly small: at 420cc it is just 40cc larger than a chimpanzee’s brain and less than a third the size of a modern human brain at an equivalent age.
On the other hand, the brain’s overall shape and structure appear more human than apelike, with rewiring in progress toward a more modern-looking frontal lobe. The results cast doubt on the long-standing theory of gradual brain enlargement during the transition from Australopithecus to Homo between 3m and 1.5m years ago.
Instead it seems that neuronal changes in the brain enabled Australopithecus sediba to evolve more advanced behaviour – including manufacture and use of simple stone tools, to judge from the fossilised hand bones found at the site – while keeping its cranium small.
A separate study of the pelvic bones of the adult woman (who has not yet been nicknamed) comes to a similar conclusion. The pelvis already shows modern human features, before these were needed to accommodate the larger skulls of later Homo species.
This implies that pelvic changes were driven by bipedal locomotion – walking on two feet – rather than the birth of big-brained babies, said Steven Churchill from Duke University in North Carolina, a co-author of the publication.
The woman and child are likely to have lived in a warmer and wetter South Africa 2m years ago, walking through mountain forests and climbing trees, suggests Prof Berger. They may have died when falling into the partially buried Malapa cave.