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Two fossils from a cave in South Africa have scientists excited and puzzled. The bones belong to creatures related to the famous Lucy fossil found in Ethiopia in the 1970s, but their owners lived more recently, just two million years ago. Scientists say the South African fossils, an adult female and a juvenile, could be the long-sought transition between ape-like ancestors and the first humans.

NPR's Christopher Joyce reports.

CHRISTOPHER JOYCE: Ask anthropologists what they dream about and many will tell you, the fossil of the last pre-human ancestor that led directly to us. Nobody's found it and any who claim to usually get publicly whacked by their peers.

Lucy and her kind, the diminutive ape-like Australopithecus that lived 3.2 million years ago may well have evolved into us, homo. But lots happened in between Lucy and the earliest humans, who emerged just over two million years ago. The true transitional species must have lived about the time we emerged.

Now, we have the South African fossils dated at 1.9 million years ago. They're called Australopithecus sediba. Anthropologist Lee Berger says these could be the one.

LEE BERGER: In our opinion, we put forth it's probably the best candidate ancestor for giving rise to our immediate ancestor.

JOYCE: The best candidate because Berger knows there were others at the time. Just before the earliest humans appeared, Africa was thick with all sorts of ape, but not ape, creatures. The evolution of big-brained, walking, tool-making primates was like a river splitting into numerous streams.

Berger, at the University of Witwatersrand in South Africa, says his two sediba fossils look like a real transition between Lucy's people, if you will, and something pretty human-like.

BERGER: It shows a small brain, but a brain that's beginning to reorganize in some ways that resemble our brain.

JOYCE: In fact, these two South African creatures were a mosaic, as if a child had assembled a creature by picking randomly from a box of Australopithecus bones and a box of human-like bones.

BERGER: You have things like a heel bone that's as primitive as a chimpanzee attached to the ankle bone that's as evolved as ours is.

JOYCE: Sediba's hand is unusually complete and also shows a mix-and-match anatomy.

Tracy Kivell of the Max Planck Institute in Germany is a member of Berger's team.

TRACY KIVELL: Sediba likely still used his hands for climbing in trees, but it was likely also capable of making the precision grips that we believe are necessary to make stone tools.

JOYCE: Berger assembled a large team of experts to examine these fossils and allowed many others to look at them. One reason is that proving you've found the last ancestor before humans is a high hurdle.

Brian Richmond at George Washington University is among anthropologists who says sediba is exciting, but not ready to be anointed yet.

BRIAN RICHMOND: They're starting to make the playing field look really crowded and right now, it's not really clear which contender will end up winning the prize of being the ancestor of homo.

JOYCE: There are fossils with human-like traits that are older than Berger's. Skeptics say sediba could have been one of many evolutionary experiments at the time, one that may have just died out, the stream just dried up.

Direct ancestor or not, the weird jumble of primitive and modern traits in sediba could rewrite textbooks. Connected features like the arm and the hand are thought to evolve together, but sediba had an ape's arm and a more human-like hand. And the pelvis - it's believed that our ancestors developed a roomy pelvis as their brains got bigger to deliver bigger-brained babies. The brain and pelvis supposedly evolved in lockstep. So why, asks Richmond, is sediba's pelvis big and roomy and its brain is small?

RICHMOND: So we have to go back now and rethink what actually leads to the evolution of the pelvis in a particular way.

JOYCE: In fact, not just the pelvis, but the whole human body. The research appears in the journal Science.

Christopher Joyce, NPR News.

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