Stone age role revolution: modern humans may have divided labor to conquer.

Chalk up modern humanity's rise and the extinction of Neandertals to a geographic accident. That's the implication of a new analysis of material from previously excavated Stone Age sites.

Homo sapiens evolved in Africa's resource-rich tropics. As a result, a division of labor arose beginning around 40,000 years ago that roughly corresponds to the arrangement found in most foraging societies today, say Steven L. Kuhn and Mary C. Stiner, both archaeologists at the University of Arizona in Tucson. Men in these societies hunt small and large game, while women and children gather tubers, berries, and other foods.

In contrast, Neandertals evolved in Europe and Asia, where large animals were the most abundant food source. Kuhn and Stiner suspect that individuals of both sexes and all ages collaborated in hunting. The high risks of killing the large beasts kept Neandertals' numbers low, the researchers propose.

H. sapiens' switch to a division of labor for procuring different foods prompted population growth, the researchers say. As humans migrated north and the two species jockeyed for survival in the same areas, humans enjoyed a competitive advantage over Neandertals.

Kuhn and Stiner say that humans' survival at Neandertals' expense hinged not on being uniquely clever, as many scientists have assumed, but on a fortunate social structure. Their investigation appears in the December Current Anthropology.

Kuhn and Stiner reviewed evidence from well-excavated Neandertal and modern-human sites in Italy, Israel, and Turkey dating mainly between 100,000 and 10,000 years ago. Neandertals lived from around 250,000 to 30,000 years ago.

Before 45,000 years ago, large- and medium-size game--including gazelles, deer, and wild horses--account for virtually all the animal remains and potential food sources at most locations. Large game declined slightly in importance--to about 80 percent of all prey--beginning around 45,000 years ago. The decline appeared primarily at modern-human sites.

Kuhn and Stiner then determined that the range and amount of small game increased dramatically after 45,000 years ago at modern-human sites. Such prey included birds, rabbits, and fish. Furthermore, many of these sites contain evidence of elaborate clothing and specialized artifacts, including sewing implements likely used by women.

After 15,000 years ago, following Neandertals' extinction, the importance of large game dropped sharply--to about 30 percent of all prey--and evidence of plant foods became more prominent.

Kuhn and Stiner suggest that men and women first adopted specific social roles in tropical African regions where modern H. sapiens originated. However, scientists have examined only a handful of such sites.

Archaeologist John J. Shea of the State University of New York at Stony Brook regards Kuhn and Stiner's argument as a "reasonable hypothesis." Still, he cautions, much is unknown about the extent to which modern-human and Neandertal behavior varied from one region to another.

Archaeologist Olga Soffer of the University of Illinois at Urbana-Champaign remains unconvinced. Kuhn and Stiner underestimate Neandertals' reliance on small game such as turtles and birds, and men and women alike may have used sewing implements to mend clothes, she says.