n analysis of African fossils, including a few new specimens, has revealed a puzzling anatomical difference between two major species of the early human family, one from southern and the other from eastern Africa. The discoverers of the difference suggest that the evolution of the human body thus was more complicated than previously understood, possibly requiring some rearrangement of branches on the family tree.
| Could Lucy have a more apelike descendant? | |
"This is not what would be expected from progressive evolution," the scientists said of their findings. McHenry is a paleoanthropologist at the University of California at Davis, and Berger is at the University of Witwatersrand in Johannesburg.
The australopithecines included several species of hominids transitional between apes and humans. Since the Lucy discovery in the early 1970's, most specialists in human origins have come to accept the afarensis as the likely ancestral species in the lineage leading to the human genus, Homo, about 2.5 million years ago.
For Lucy and her kind to evolve into descendants with more apelike limbs, the scientists said, evolution would have to go backward, which rarely happens. One possible explanation for such an evolutionary reversal, they said, might have been to adapt to a more arboreal environment.
In a popular account of the findings in the August issue of National Geographic magazine, Berger said it was more likely that africanus did not descend from afarensis but that the two species evolved separately. They were apparently "sister species that share a missing-link ancestor."
Thus, only one of them could have been a direct ancestor of humans, Berger said, and africanus seemed more likely to have been that ancestor. It had developed a larger brain and somewhat more humanlike face and teeth than afarensis. Although the fossil evidence is scrappy, the first members of the Homo genus, Homo habilis, appeared to have had long arms and short legs, not unlike africanus.
McHenry agreed that africanus appeared to be "close to the ancestor of Homo." Throwing down the gauntlet before paleontologists working in East Africa, Berger said, "That reinforces my own conviction that Homo emerged from africanus in southern Africa and migrated north."
If africanus turned out to be on the main trunk of the family tree, then afarensis would be relegated to a dead-end branch, Berger said, culminating in A. boisei, which died out about a million years ago.
The analysis was based on a comparison of more than 100 fossil bones from a limestone quarry at Sterkfontein, South Africa, and arid badlands at Hadar, Ethiopia, where Lucy was excavated. The bones included the skeletons of Lucy herself and a male africanus. McHenry's main contribution was the development of a technique of inferring body weight and the length and diameter of limb bones from an analysis of a tiny fragment of a joint.
Like several other paleoanthropologists, Dr. Eric Delson of the American Museum of Natural History in New York said he found the research "a very intriguing piece of work and thought-provoking," but cautioned that it was too early to be redrawing the family tree. He said the fossil record for H. habilis was too scant to tie it to the A. africanus lineage.
Dr. Bernard Wood, a paleoanthropologist at George Washington University, said the analysis of joints in determining limb sizes was "quite ingenious, but the results are not earth-shattering." He was not surprised, for instance, that the findings did not appear to fit neatly into a pattern of progressive and linear relationships in evolution.
"My own view is that nature would have carried out many experiments," Wood said, referring to patterns of parallel evolution in which different lineages could arrive at different stages of development at different times. "We are still only scratching the surface of the complexity of human origins," he said, adding that it was unlikely the ancestor-descendant relationships would ever be reconstructed.