Two 1.9 million-year-old skeletons found in a South African cave have added a new and intriguing member to the primate family.

Dubbed Australopithecus sediba, it has many features — including long legs and a protruding nose — common to Homo, the genus that eventually spawned humans. Other features, such as extra-long forearms and flexible feet, date from deep in our primate past.

Paleontologists disagree over whether A. sediba is a direct human ancestor, or just looks like one. But whatever their lineage, the fossils provide rare insight into a period shrouded in paleontological mystery.

"We feel that A. sediba might be a Rosetta Stone for defining for the first time what the genus Homo is," said paleontologist Lee Berger of the University of Witwatersrand. "They're going to be a remarkable window, a time machine."

The skeletons, described April 8 in Science, were found — with a bit of help from Google Earth — two years ago in a South African cave, where they fell two million years ago.

On one side of that date in the fossil timeline are the various species of Australopithecus, the first great apes to walk on two feet. On the timeline's other side is the genus Homo, the first creatures one would recognize — with all due respect to Lucy's famous A. afarensis — as close to human.

In between is uncertainty. The fossil record is mostly bare. Some of the Australopithecus lineage split, with one branch becoming Homo. But the identity of that lineage, and the characteristics of early Homo, are unknown.

According to Berger's team, A. sediba's combination of old and new features make it a likely descendant of A. africanus — one of Lucy's direct descendants — and either a direct ancestor of early Homo and ultimately us, or what Berger calls "a very close side branch."

"It sits at a very critical moment in time," said Berger. It "fills a critical gap in the line."

Other paleontologists say Berger's fossils are a marvelous find. But as expected in a field where entire fossil records spanning millions of years could fit on a coffee table, and where the mostly missing A. sediba skeletons are considered remarkably complete, the new hominid's taxonomical position is being interpreted in many different ways.

While the Australopithecus designation is correct, "the proposed link between A. sediba and early Homo is forced and tenuous at best," said William Jungers, a Stony Brook University paleoanthropologist. He doesn't consider a juvenile specimen — the most complete of the two skeletons comes from the human equivalent of a teenager — a reliable indicator of adult features.

To this criticism, Berger said the teen's brain had "clearly reached about 95 to 98 percent of adult capacity." Few changes would be expected in its cranial size and shape, which are critical in characterizing a primate species.

Jungers also noted that the first Homo fossils predate A. sediba by 500,000 years, while Homo ergaster had reached western Asia just 200,000 years after A. sediba's known date. Both these figures suggest that Homo was established well before A. sediba came along, said Rick Potts, curator of anthropology at the Smithsonian National Museum of Natural History.

"The connection with the origin of Homo doesn't seem to hold much water," said Potts, and the confluence of some A. sediba traits with Homo is just coincidence. "Evolution produces a universe of features that are combined and recombined," he said.

According to Berger, however, A. sediba may have older roots than they think. "The site we found is simply a point in time. It doesn't represent the first appearance of this species," he said.

Meanwhile, Arizona State University paleoanthropologist William Kimbel argued that A. sediba should have been classified as Homo, though it may not have been a direct human ancestor.

"In my way of thinking, it belongs in Homo because of the brow ridge, the face, the pelvis," he said. "It's true that it has the small brain and long upper limbs indicative of Australopithecus, but those are signs of its ancestry, not its future."

These arguments may be settled as more A. sediba skeletons emerge. Berger is currently assembling at least two. However, taxonomic debates may ultimately prove less important than the questions A. sediba provokes.

Already the fossils suggest that Australopithecus didn't morph suddenly into Homo, but adapted in gradual, piecemeal fashion. What pressures led to these adaptations – and their relationship to tool use, cognitive developments, dietary shifts and climate changes – have yet to be determined.

"The significance is in the patterns and insights it provides," said Kimbel. "These specimens fall at the young end of a very puzzling million-year period in hominin evolution."

Whether or not A. sediba is our ancestor, "it could help us understand the dynamics that led to the split producing the lineage culminating ultimately in us," said Kimbel.

*Images: Lee Berger/*Science

See Also:

Citations: "Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa." By Lee R. Berger, Darryl J. deRuiter, Steven E. Churchill, Peter Schmid, Kristian J. Carlson, Paul H. G. M. Dirks, Job M. Kibii. Science, Vol. 328 No. 5975, April 9, 2010.

"Geological Setting and Age of Australopithecus sediba from Southern Africa." By Paul H. G. M. Dirks, Job M. Kibii, Brian F. Kuhn, Christine Steininger, Steven E. Churchill, Jan D. Kramers, Robyn Pickering, Daniel L. Farber, Anne-Sophie Mériaux, Andy I. R. Herries, Geoffrey C. P. King, Lee R. Berger. Science*, Vol. 328 No. 5975, April 9, 2010.*

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.