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Prehuman Skeleton Offers Clues to Evolution

Scientists digging in Ethiopia's desert have found the most complete skeleton yet of a prehistoric ape-man species thought to be a forerunner of humans. It is of a very young female who died of unknown causes more than three million years ago. Her bones fill an important gap in understanding this particular species and potentially its place in human evolution.

The rare skeleton is of a three-year-old female from the primitive pre-human species called Australopithecus afarensis. The first specimen of this group is the famous fossil named Lucy, a 3.2-million-year-old adult female discovered nearby in 1974. The baby skeleton is about 150,000 years older than Lucy and far more complete, with fingers, a foot, and a torso in addition to a skull.

"This discovery is making an enormous contribution to our understanding of the biology of Australopithecus afarensis as a species as well as other early hominids," said Zeresenay Alemseged.

This is one of the discoverers, Zeresenay Alemseged of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany speaking to the journal Nature, which published the research. He says no previous fossil of a baby pre-human has consisted of more than a partial skull, jawbone, or some teeth. Because this one is in such good shape, Alemseged says it could reveal what this human ancestor really looked like.

"Based on this fossil, we are able to address some key questions regarding the developmental patterns of the face and the skull of Australopithecus afarensis," he said. "Before today, we didn't know how the baby Australopithecus looked, and thus it has implications for our understanding of changes that take place from baby morphology to the adult morphology."

The skeleton offers new clues about how the afarensis species blurs the line between ape and human. The angle of the thigh bone from knee to hip is very human, implying she walked on two legs. But her torso is ape-like, as noted by another co-author of the research, Fred Spoor of University College London.

"My best bet would be that it's largely a chimp," said Fred Spoor. "There are very few hints that it's actually more human in the way it would grow up."

Spoor says this group of pre-humans were basically walking chimps. Zeresenay Alemseged points out that brain size of his specimen is that of a chimp, her neck is short and thick, her fingers are curved, and her shoulder blades, or scalpulae, are like a gorilla's.

"So would this mean that Australopithecus afarensis was climbing trees? This question remains unanswered at this stage, but the fact that the morphology of the scalpulae looks like that of a gorilla was a big surprise," he said.

Alemseged's personal view is that afarensis was basically a ground forager that climbed trees when necessary.

But anthropologist Owen Lovejoy of Kent State University, who did not take part in the discovery, says these beings were purely terrestrial. He argues that some traits suggestive of tree climbing are actually non-functioning genetic vestiges, such as the curved fingers.

"This is a really young skeleton," said Owen Lovejoy. "The finger bones are already curved, which tells us this is a genetic adaptation and it's not a signal that this animal was actually in the trees. It takes a long time to get rid of some of these adaptations and it takes positive selection to make them shift. So what we have in this skeleton are a lot of clear adaptations to terrestrial walking. They are so substantial that it implies that this is an animal that spent virtually all of its life on the ground."

In a Nature magazine commentary, George Washington University anthropologist Bernard Wood says that whatever the answer to this question is, the infant skeleton has the potential to provide a wealth of information about the growth and function of this apeman species and its place in the human lineage.