Scientists say a set of 2-million-year-old fossils could be a key link in the process of evolution that led to modern human beings.
Some say the bones, which were found found in South Africa in 2008 and include a foot, hand, and parts of the pelvis and skull, could be a "game-changer" in understanding human evolution. They belonged to Australopithecus sediba, a type of pre-human who scientists have said could be an ancestor of modern Homo sapiens. It has characteristics of older members of the Australopithecus family, but also much in common with the newer Homo class, which includes today's human beings.
"This is what evolutionary theory would predict, this mixture of Australopithecene and Homo," said Darryl J. DeRuiter of Texas A&M University. "It's strong confirmation of evolutionary theory."
The bones belonged to an adult female and a child, probably 10-13 years old. It is not possible to determine if they were related.
They have been examined extensively since they were excavated from a South African cave, and are described in detail in five separate studies in this week's edition of the journal Science.
"The team says the new species may be the best candidate yet for the immediate ancestor of our genus, Homo," wrote Michael Balter of the Science staff in an overview piece. "That last claim is a big one, and few scientists are ready to believe it themselves just yet."
Scientists have long talked about a "missing link" between very old fossils, more than 3 million years old, and much newer ones that they believe are clearly ancestors of today's human beings. There is a gap in the fossil record, so far unexplained. Does Australopithecus sediba help fill the gap? Not on its own, say most researchers, but it helps.
The brain was small. The shape of the feet suggested that the two were able to walk upright, but probably spent a lot of time climbing trees, both for food and protection from predators.
"But at the same time, there are features in the hand which reflect a greater emphasis on tool use," said Steven Churchill of Duke University, one of the researchers, "and in particular, there are things which seem to suggest the production and use of stone tools."
That's something not seen before except in direct human ancestors.
There will probably be vigorous debate now over the importance of Australopithecus sediba. The scientists publishing their work today say it could be critical. But researchers in the past have made many finds that turned out to be dead ends.
After the fossils were discovered three years ago, children in South Africa were invited to come up with a name for the fossil that clearly had been a young person. They came up with "Karabo." In the local Tswana language, it means "answer."
The Associated Press contributed to this story. More information from the journal Science.