Two skeletons, nearly two million years old and unearthed in South Africa, bear traits from both lineages

Two skeletons nearly two million years old and unearthed in South Africa are part of a previously unknown species that scientists say fits the transition from ancient apes to modern humans.

The fossils bear traits from both lineages, and researchers have named them Australopithecus sediba, meaning “southern ape, wellspring” to indicate both their relation to earlier apelike forms and to their features later found in more modern people.

“These fossils give us an extraordinarily detailed look into a new chapter of human evolution and provide a window into a critical period when hominids made the committed change from dependency on life in the trees to life on the ground,” said Lee R. Berger of South Africa's University of Witwatersrand. “Australopithecus sediba appears to present a mosaic of features demonstrating an animal comfortable in both worlds.”

Mr. Berger and colleagues describe the find in Friday's issue of the journal Science.

Modern humans, known as Homo sapiens, descended over millions of years from earlier groups, such as Australopithecus, the best-known example of which may be the fossil Lucy, who lived about a million years before the newly discovered A. sediba.

Mr. Berger said the newly described fossils date between 1.95 million and 1.78 million years ago.

Some have characterised the find as a “missing link,” but that is a concept no longer accepted by science.

“The ‘missing link' made sense when we could take the earliest fossils and the latest ones and line them up in a row. It was easy back then,” explained Smithsonian palaeontologist Richard Potts. But now researchers know there was great diversity of branches in the human family tree rather than a single smooth line.

The two new fossils were found in a pit in what was once a cave, their bones preserved by hardened sediment that buried them in a flood shortly after they died, the researchers said.

One was a female estimated to have been in her late 20s or early 30s and the other was a male aged 8 or 9, according to the report. Two more have been found since this discovery, but Mr. Berger declined to detail them.

A. sediba could turn out to be a sort of Rosetta stone that helps unlock the secrets of the development of the genus Homo, Mr. Berger said, even if they turn out to be a side branch.

According to the researchers, A. sediba had an advanced hip bone and long legs able to stride like humans but long arms and powerful hands like an ape. Both the female and juvenile were 1.27 metres tall. The female would have weighed 33 kg and the child 27 kg.

“The brain size of the juvenile was between 420 and 450 cubic centimetres, which is small, but the shape of the brain seems to be more advanced than that of Australopithecines,” the researchers reported. Our human brains are about 1196 to 1606 cubic centimetres.

While the skeletons had traits of both genuses, the researchers said they chose to classify them conservatively as Australopithecus, rather than Homo, because of their upper body design and brain size.

Richard Potts, director of the Human Origins Project at the Smithsonian's National Museum of Natural History, noted that other examples with some Australopithecine and some Homo traits existed as much as a half-million years before this find. This particular combination has not been seen before, he said.

“It's part of the experimentation of evolution,” said Mr. Potts, who was not part of Mr. Berger's research team. Also, he cautioned, because there are only two examples there is no way to know if it were a gene pool that died out or was passed along to others.

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