April 18, 2011 | 51
MINNEAPOLIS—It’s a great irony of paleoanthropology that for all the insights scientists have been able to glean from the fossil record about our early ancestors, the australopithecines (Lucy and her kin), they have precious little to document the origin of our own genus, Homo. They know that Homo descended from one of those australopithecine species and that over the course of that transition our ancestors evolved from chimp-size creatures with short legs and small brains into tall humans with long legs and large brains, among other hallmark traits. But the details of this evolutionary transformation—when the distinctive Homo characteristics arose and why—have remained elusive, because fossils of early Homo are rare and the ones that have turned up are generally too fragmentary to yield much information.
To that end, last spring Lee Berger of the University of the Witwatersrand in Johannesburg, South Africa, and his colleagues announced their discovery of two partial human skeletons (pictured above) from that mysterious period that might well revolutionize researcher’s understanding of how our genus got its start. The specimens, which date to around 1.95 million years ago, were said to exhibit a mosaic of traits linking them to both Australopithecus and Homo, leading the team to propose that they represent a previously unknown species of human—Australopithecus sediba—that could be the direct ancestor of Homo. The interpretation was controversial. Some critics argued that the fossils do belong in Australopithecus, but have no special relationship to Homo; others contended that they represent a dead-end branch of Homo, rather than ancestor of later species, including H. sapiens.
On April 12 at the annual meeting of the Paleoanthropology Society and on April 16 at the annual meeting of the American Association of Physical Anthropologists, Berger and his colleagues gave presentations on the results of their latest analyses of the A. sediba bones. The findings underscore the mosaic nature of the remains, and threaten to topple a leading model of human evolution.
Team member Kristian J. Carlson talked about the shape of A. sediba’s brain, as revealed by synchrotron scanning of the interior of the brain case. With an estimated cranial capacity of just 420 cubic centimeters, this species had about a third as much gray matter as we do. Indeed this tiny brain size—which lies in the lower end of the Australopithecus range—figured significantly in the team’s decision to place the fossils in the genus Australopithecus rather than Homo. Yet despite the diminutive size of the brain, its frontal lobe appears to have had a much more humanlike organization than that of the australopithecines. Carlson noted that this surprising finding hints that frontal lobe reorganization and the overall increase in brain size that characterizes Homo may not have occurred simultaneously, as was thought.
The mixture of primitive and advanced traits is apparent throughout the skeleton. Darryl de Ruiter of Texas A&M University reported that the skull exhibits a suite of traits in common with australopithecines, particularly A. africanus. Yet it also shares a number of skull traits in common with Homo—more, in fact, than any other australopithecine does. “The combination of primitive and derived cranial and postcranial [below the neck] characteristics in sediba highlight its intermediate nature,” de Ruiter observed, reiterating the team’s earlier claim that A. sediba could be the ancestor of Homo. Berger enumerated other mosaic traits—including the apelike ribcage and long arms combined with the humanlike hand, with its short fingers and long thumb.
But it was the pelvis of A. sediba that yielded perhaps the most startling revelation at the meetings. Many researchers have argued that increasing brain size in the Homo lineage was the driving factor in the evolution of the Homo pelvis from the australopithecine one, because in early Homo fossils a larger braincase accompanies the modified pelvis. According to a talk given by Berger on behalf of Steven Churchill of Duke University, however, A. sediba, with its tiny brain, has a pelvis that looks a lot like that of early Homo.
If ballooning brain size was not the driving factor in evolving a humanlike pelvis, then what was? “I would say it’s the shift from habitual bipedalism to more humanlike obligate bipedal locomotion,” Will Harcourt-Smith of the American Museum of Natural History in New York City, an expert not involved in the analysis, told Scientific American. He thinks bipedalism probably evolved in two stages: in the first stage, represented by Lucy’s species, early humans still spent a fair amount of time climbing in the trees in addition to walking upright on the ground. In the second, they lost their climbing ability and became fully bipedal.
“It’s very reasonable to see [A. sediba] as the ancestor of Homo,” Harcourt-Smith remarked, noting that he was much more on the fence until he saw the pelvis. “Am I 100 percent convinced? No, but it’s persuasive.”
Image: skeletons courtesy of Science/AAAS