By Ewen Callaway of Nature magazine
New descriptions of Australopithecus sediba fossils have added to debates about the species' place in the human lineage. Five papers published today in Science describe the skull, pelvis, hands and feet of the ancient hominin unearthed three years ago in South Africa.
The papers reveal a curious mix of traits, some found in apes and earlier Australopithecus fossils, and others thought to be unique to Homo erectus--the tall, thin-boned hominin that emerged around 2 million years ago in eastern Africa and colonized Europe and Asia--and its descendants, including modern humans.
This mix of features has left palaeoanthropologists unsure of how A. sediba relates to other ancient human relatives. Lee Berger, a palaeoanthropologist at the University of Witwatersrand in Johannesburg, South Africa, whose team discovered A. sediba, proposes that A. sediba may have evolved into H. erectus, but many other researchers are sceptical of that claim.
Berger's 9-year-old son Matthew happened on the first A. sediba fossil in August 2008, while the two were exploring Malapa, a collapsed cave system not far from the Sterkfontein and Swartkrans sites, which have also yielded a wealth of ancient human remains. The fossil turned out to be a collar bone. The team went on to find more than 220 bones from at least five individuals, including infants, older children and adults.
The researchers concluded that the fossils represented a new species of hominin, which they named after the Sotho word for wellspring -- sediba. The species' small brain and limb proportions matched those seen in the genus Australopithecus, whose most famous representative is the 3-million-year-old Australopithecus afarensis skeleton known as Lucy. Other palaeoanthropologists argue that A. sediba should have been placed in the genus Homo because of its modern features, including its hand and pelvis.
"That is exactly what you'd expect when you find a very transitional form: 50% of the field saying they're right, it's an Australopithecine, the other half saying, put this in the genus Homo," said Berger during a conference call announcing the new A. sediba papers.
At around 420 cubic centimeters, A. sediba's puny brain compares to those of other Australopithecus specimens and chimpanzees. But a high-resolution synchrotron scan of the brain's impression on the skull shows enlarged frontal areas that are normally associated with humans and linked to higher cognitive abilities, such as planning.
A. sediba's pelvis also looks wider than those of other australopiths, raising doubts about the idea that the human pelvic shape evolved to accommodate large-brained babies. "Whatever is driving a relatively human-like shape of the pelvis, it is not a big brain," says Berger.
The orientations of its leg and ankle bones suggest that A. sediba walked upright, and its nearly complete ankle resembles that of a human. But its long arms, and some features of its feet and shin bones, are similar to those of a chimpanzee. Taken together, these features suggest that A. sediba was adapted for both bipedalism and tree-dwelling.
Attached to those ape-like arms are human-like hands with strong thumbs perfect for gripping. "That hand is the most human-like hand outside of Homo sapiens or Neanderthals that's been discovered," Berger says. The researchers have not reported stone tools associated with A. sediba, but Berger says the hands suggest it would have been capable of making and using them.
Berger says this hotch-potch suggests that A. sediba is a direct ancestor of H. erectus. Alternatively, he says the new fossils may mark a late-surviving species of Australopithecus that went extinct.
"They're obviously fabulous fossils and I'm cautiously receptive to their big picture overview of it" as a predecessor of early Homo, says Dean Falk, a neuroanatomist at Florida State University in Tallahassee and the School for Advanced Research in Santa Fe, New Mexico. However, she would like to see the brain's impression in the skull compared to those of many other Australopithecus species before accepting that its brain shape resembles that of a human.
Donald Johanson, a palaeoanthropologist at Arizona State University in Tempe, would also like to see A. sediba thoroughly compared to other ancient human fossils, in particular those of earlier Homo species, such as Homo habilis. "It is possible it's another sprig on the evolutionary tree," he says of A. sediba.
Bernard Wood, a palaeoanthropologist at the George Washington University in Washington, D.C., is also sceptical. "It's perfectly possible that A. sediba is an ancestor of Homo," he says. "Do I think it's likely? No, I don't."
But Wood says the species' unique mix of primitive and modern anatomy, particularly its foot, underscores the difficulty in determining whether any fossil represents a direct human ancestor or an evolutionary dead-end with some human traits. "I think we had this crazy notion that our morphology and our behaviour were so special that they couldn't have conceivably evolved more than once," he says, adding that the papers "will make identifying human ancestors a hell of a lot more difficult today than it was yesterday."
Berger's team is still excavating Malapa, and have plans to describe remains of other individuals from the cave. They recovered material surrounding some of the fossils that may represent preserved skin and soft tissue, something never before seen in human fossils this old.
Instead of keeping tight-lipped about that material, Berger's team has put out an open call for help from other researchers in determining whether the material really is skin and, if so, what it might have to say about ancient humans. The project is in its infancy, but Berger says they may detail the work and its conclusion online before official publication.