ANCESTRAL SPECIES?: New fossils from South Africa represent a previously unknown member of the human family, dubbed Australopithecus sediba. The remains include two partial skeletons, one of which preserves a largely complete skull, shown here. Image: Photo by Brett Eloff courtesy of Lee Berger and the University of the Witwatersrand
Scientists working in South Africa have unveiled fossils of a human species new to science that they say could be the direct ancestor of our genus, Homo. Discovered in Malapa cave—just 15 kilometers from the sites of Sterkfontein, Swartkrans and Kromdrai, which have yielded a number of important human fossils—the finds comprise two partial skeletons that are nearly 1.95 million years old. The researchers have given them the name Australopithecus sediba.
The pair—an adult female and juvenile male that may have been mother and son—appear to have fallen into the cave through a hole in the cave ceiling, perhaps while attempting to access a pool of water inside. So exceptional is the preservation of the skeletons, particularly the male, that the discovery is being likened to that of the famous Lucy fossil from Ethiopia and the Turkana boy from Kenya. But the startling mix of primitive and advanced traits evident in the remains is sparking debate over where on the family tree the new species belongs and raising important questions about the dawning of Homo.
The study of human origins has come a considerable way since German quarry workers discovered in 1856 the first fossil to be recognized as an early human (it was a Neandertal). Investigators have traced our roots back to perhaps seven million years ago, close to the point at which the human lineage diverged from that of our closest living relative, the chimpanzee; they have recovered extensive remains of australopithecines such as Lucy and her ilk, creatures that are transitional between apes and us; and they have unearthed fossils representing quite an array of species documenting the evolution of Homo. Considering the virtually nonexistent fossil trails of our cousins the chimps, bonobos and gorillas, the human fossil record is extraordinary.
There are, however, significant gaps in researchers' knowledge of how we came to be. One such blind spot is the origin of Homo. Most experts agree that Homo evolved from a species of Australopithecus—either A. afarensis (Lucy's species) or A. africanus. Connecting the dots between one of these australopithecine species and Homo has been difficult, though, because the oldest known Homo remains are so few and fragmentary.
Scientists’ best guess has been that Homo habilis, which lived between perhaps 2.3 million and 1.5 million years ago, signaled the debut of Homo and subsequently gave rise to H. erectus, the first hominin (member of the human lineage) to spread out from Africa across the globe and the one that is thought to have spawned later human species, including H. sapiens. But H. habilis is a relatively poorly understood species because the known specimens preserve few bones from below the head. In fact, some researchers wonder whether some of the fossils assigned to H. habilis might in fact belong to other species. "H. habilis has been on shaky footing for a long time," observes paleoanthropologist Leslie Aiello, president of the Wenner-Gren Foundation for Anthropological Research.
Into this morass falls A. sediba. In the paper describing the fossil remains, which will be published in the April 9 issue of Science along with a second paper detailing the geological context and age of the finds, Lee Berger of the University of the Witwatersrand in Johannesburg and his colleagues suggest that A. africanus gave rise to A. sediba, which in turn gave rise to the genus Homo. Intriguingly, the team hints that A. sediba might even be more closely related to H. erectus than H. habilis is, thus potentially relegating H. habilis to a side branch of the family tree, rather than a coveted spot on the line leading to us.