Why did Neanderthals disappear some 30,000 years ago, while Homo sapiens continued to thrive? At a paleontology conference last April, anthropologists Erik Trinkaus and Chris Ruff announced they had found some clues: the Neanderthals had stronger arms and they ran their children ragged.
The two anthropologists studied about two dozen fossil skeletons of Neanderthals and early modern humans, all from well-excavated sites in the Near East--the oldest dating from 150,000 years ago--where both populations lived at various times. Although anthropologists aren’t sure whether the two peoples ever overlapped, they clearly had much in common, says Trinkaus, who works at the University of New Mexico. In the Near East both humans and Neanderthals are found with archeological material that is, except in subtle details, indistinguishable, he says. They made the same type of tools, they hunted the same type of animals, they lived in very similar places.
Why then did one group vanish and the other prosper? The answer, says Trinkaus, may be found in their bones. He and Ruff, who teaches at Johns Hopkins, used X-rays and computer models of the fossils to study the stresses the bones were subjected to during life. They found that Neanderthal bones consistently seem to reflect more vigorous activity. The most likely explanation, says Trinkaus, given that both populations used essentially the same tools in the same environment, is that Neanderthals used their tools less efficiently. The extra effort put additional loads on the bones, which, as weight lifters know, strengthens them.
Using a computer program designed for orthopedic research, Ruff and Trinkaus measured the cross-sectional structure of the bones, a basic indicator of strength. They then factored in the Neanderthals’ and humans’ different proportions, to allow for the former’s greater weight and overall robustness. Finally they compared the relative strength of arm and leg bones from the two groups.
The strength of the leg bones of Neanderthals and early modern humans turned out to be equal. But the upper arm bones of Neanderthals were significantly stronger than those in humans. This means, Trinkaus thinks, that humans probably chose food that was easier to process or they used their tools more intelligently and less laboriously. One of the keys to evolutionary success, says Trinkaus, is the efficient use of energy. The more efficient a people, the better their chances of surviving. It takes only a very subtle difference in life-style to make a big difference in terms of evolutionary success, he says.
This work complemented earlier research by Trinkaus on differences in Neanderthal and human hipbones, particularly in the knobby neck of the femur, or thighbone, which fits into the hip socket. Modern studies have shown that this neck tends to get bent in active children. In Neanderthals, Trinkaus found, it was much more sharply bent than it was in humans. He interprets this to mean that Neanderthal children had to accompany adults on foraging trips, while human children remained at camp with other adults. Such a social structure may have given early human children a better chance of survival.
Critics say Ruff and Trinkaus may not be justified in inferring so much about behavior from skeletons. But Trinkaus says his work fits other evidence that early humans simply outcompeted Neanderthals for resources. We can say from the fossils that these populations were using their bodies in very different ways to manipulate their environment.