Hominid Species Discovery Shows Transition Between Apes, Humans

BY Lea Winerman  April 8, 2010 at 2:34 PM EDT

Two years ago, nine-year-old Matthew Berger accompanied his dad, paleoanthropologist Lee Berger, to an archeological excavation site in South Africa. Running after his dog, he noticed an interesting-looking bone protruding from the ground. He thought it might be from an antelope and held it up to show his dad. On Thursday, that bone was unveiled as part of a fossil of a new species of early hominid, Australopithecus sediba, that lived nearly 2 million years ago in Southern Africa.

The fossils show a transitional species that lived at a time when humanity’s ancestors were moving from a life in the trees to one on the ground, Berger said in a news conference. Australopithecus sediba was an animal “comfortable in both worlds,” he said.

The two fossils described Thursday in the journal Science come from an adult female and a young male of about 10 to 13 years old — possibly a mother and son. They were discovered just a few feet from one another in an area of South Africa called the Cradle of Humankind, which has long been rich with fossil finds. Berger, an American researcher now at the University of Witwatersrand in Johannesburg, has worked there for about 20 years.

Two million years ago, the spot where the fossils were found was a cave with an underground spring (“sediba” means “spring” in the local language Sotho). Berger and his team have worked with geologist Paul Dirks, of the University of Queensland in Australia, to map the cave system and to understand the geological record of the area.

“If we know where the caves are, and we can predict how the landscape developed, then maybe we can predict where the caves with fossils are,” Dirks explained in an article in the Cairns Post.

The researchers believe that the creatures might have fallen or climbed into the caves, then been swept away and eventually landed somewhere inaccessible enough to preserve their bones from scavengers.

Berger and his colleagues say the new species could be a direct ancestor of modern humans, or a closely related side branch. But — as with most fossil finds — researchers disagree about where precisely the fossils should be placed on the human family tree.

The well-preserved bones show a new mix of features from both the earliest human species and the apes that preceded them — the long arms and strong hands of earlier Australopithecines, good for climbing trees, but also the long legs and developed hips and pelvis that allow humans to walk upright.

“It shows that sediba is becoming a terrestrial biped, but also maintaining a sort of ‘reserve parachute’ in having very long ape-like arms,” Berger told reporters.

A. sebida’s face and head are also a mix. The facial features and small teeth are like those of later hominids. But the human-like features are set in a very small head, which held a small brain. “It would look almost like a pinhead,” Berger said.

Berger and his team have classified the new species as an Australopithecine, an earlier genus than Homo, of which modern humans (Homo sapiens) are part. But not all researchers agree with the classification. William Kimbel, director of the Institute of Human Origins at Arizona State University, says that the species should have been classified in the Homo genus. He also says that it’s unlikely to be the direct ancestor of modern humans — especially because there is already evidence of Homo species that lived hundreds of thousands of years earlier than A. sediba.

Berger says that whether or not the new species ends up being a direct ancestor of modern humans, the fossils will still provide insight into how humans developed.

“Regardless of whether they turn out to be a side branch or not, or a direct ancestor of the genus Homo, they are going to be a remarkable window into the evolutionary stresses going on between 1.8 and 2 million years ago,” he said.