Fossil Finds Complicate Search for Human Ancestor

Australopithecines lived in Africa some 4 million to 2 million years ago. Scientists speculate that the australopithecines gave rise to our own genus, Homo, sometime around 2 million years ago, but there’s not much fossil evidence to show exactly when or how this happened. But last year, scientists led by Lee Berger of the University of Witwatersrand announced they had found a possible candidate ancestor of Homo: Australopithecus sediba. The species lived 1.977 million years ago and resembled Homo in many ways.

This week, the researchers published five papers in the journal Science that provide a more in-depth look at the species. Experts are excited about the fossils, but do not agree on where A. sediba belongs in the human family tree—and in some sense, its discovery muddies the picture of human evolution at this critical transition 2 million years ago.

The new studies analyze two partial skeletons found in Malapa Cave in South Africa: a 12- to 13-year-old male and an adult female. Here’s a rundown of the key findings:

Brain: The researchers studied the size and shape of the young male’s brain by taking X-ray scans of his skull and creating a virtual 3-D endocast. A. sediba had a small brain—420 cubic centimeters—only slightly bigger than a chimpanzee brain or half the size of a Homo erectus brain. But the shape and organization of part of the frontal lobe appear similar to Homo. The team says this may mean brain reorganization came before a big jump in brain size in humans.

Pelvis: The pelvis had a mix of australopithecine- and Homo-like traits. This is interesting because some of A. sediba’s more advanced traits, like the shape and orientation of the ilium, were thought to have evolved in the genus Homo to accommodate bigger-brained babies as they came through the birth canal. But since A. sediba had these features and a small brain, another factor probably drove the evolution of these traits; they could be the result of spending even more time walking on the ground and less time in the trees, the researchers suggest.

Hands and Feet: The team found a nearly complete wrist and hand for the species as well as a partial foot and ankle. The foot had a unique mix of traits not seen in any other hominid, suggesting A. sediba had its own form of upright walking and probably still climbed trees. The hand also indicates A. sediba was a climber, but it shows that the hominid had the musculature and anatomy necessary for a “precision grip,” when the thumb meets the fingertips. This movement is what allows you to thread a needle or hold a pencil—and it probably enabled A. sediba to make and use stone tools, the researchers say, although they have not yet found any tools with the species.

Here’s why A. sediba complicates things. For the species to be the ancestor of Homo, it had to have lived before the first species of that genus. That’s just common sense. And it’s true for what the researchers call the “earliest uncontested evidence” of Homo: Homo erectus, at 1.9 million years ago.

But then there’s the contested evidence. At roughly 2.4 million years ago—before A. sediba— a species called H. habilis (“handy man”) lived in Africa, although the researchers say there is disagreement over what fossils should be included in this species. If this handy man is really the earliest member of Homo, it’s hard to call A. sediba an ancestor (unless, perhaps, additional fossil finds push back A. sediba’s age).

In some ways, H. habilis is more human-like than earlier hominids; it had a much larger brain, for example. But in other ways, such as the anatomy of the hand, A. sediba is more human-like than H. habilis, Berger and his colleagues say. What does this all mean? It’s unclear. But at the very least, several different types of Homo-like hominids probably all lived at about the same time—making it a “most challenging endeavor,” the researchers say, to figure out how these forms relate to each other and which if any best represents the ancestor of our genus.

As paleoanthropologists like to say, more fossils may help clarify things—or muddle them even more.

Erin Wayman | | READ MORE

Erin Wayman is a science and human evolution blogger for Hominid Hunting. She has M.As in biological anthropology and science writing.