By Bob Holmes
Hunting skills may not after all have triggered the tremendous burst of human evolution at the beginning of the ice ages nearly two million years ago. Instead of man the hunter, the driving force behind this evolutionary surge may have been woman the gatherer, with both mother and grandmother playing a vital role.
For 40 years, anthropologists have leaned toward the notion that rich, nourishing meat - brought home by hunters and shared out - played a crucial role in human origins. This would explain why evolution selected for larger, smarter hunters with lighter jaws and teeth: precisely the changes seen as Homo erectus arose in eastern Africa.
The hunter-driven scenario also included the formation of nuclear family groups, in which men hunted while women gathered plants and cared for their children, thus kicking off humans' social evolution as well.
But this picture may be wrong on several counts. To begin with, early men probably were not bringing meat home to the family. Most evidence of hunting by early African Homo erectus comes from archaeological sites containing both animal bones and primitive stone tools. But most of these lie next to rivers, the kind of predator-filled habitat that today's Hadza hunter-gatherers in Tanzania call a "city of lions".
"They're certainly not places where early humans were spending the night," says James O'Connell, an anthropologist at the University of Utah in Salt Lake City. O'Connell is lead author of a critique of the hunting hypothesis published in the latest issue of the Journal of Human Evolution (vol 43, p 831).
Instead, the remains probably represent temporary meal sites - perhaps a convenient patch of shade - where the group gathered around a fallen animal, O'Connell and his team suggest.
Most likely, the "hunters" were not actually hunting either. Many of the bones bear both cut-marks from primitive stone tools and the tooth marks of animals. When the researchers compared these with marks on bones made in modern experiments, they found that the pattern of marks and the mix of bones were similar to those left by human scavengers (see graphic).
This suggests that early humans drove other predators away from freshly killed carcasses - a view now gaining support among palaeoanthropologists. But O'Connell's team went a step further. They wanted to know what kind of a living early African Homo erectus made if in fact they were scavengers, not hunters.
The Hadza people today scavenge avidly in the same way, and studies in the late 1980s noted that they found an average of one carcass every two to three weeks. Based on that observation, the team estimated that early humans might have picked up a carcass every few days in the wettest areas, but in drier areas might have got as little as one a month: nowhere near enough to live on.
If fathers were not feeding their children meat most of the time, that means mothers and, perhaps, grandmothers must have been. Older women might have proved crucial in feeding children, the researchers say, allowing the mothers to get pregnant again more quickly.
Evolution would thus favour a long lifespan, which is closely linked to large body size and delayed maturity. Suddenly, all the major changes in human life history are explained by foraging, not hunting.
Critics point out that even if the meat supply was not reliable enough to live on, it must have been important in evolutionary terms. Humans have been top carnivores - a highly unusual role for a primate - since at least the Stone Age.
"Something special did happen with regard to carnivory," says Robert Blumenschine, a palaeoanthropologist at Rutgers University in New Brunswick, New Jersey. "The extent to which it shaped human evolution remains in question, but I would think it must have had some strong influence."
New Scientist Monday January 6, 02:00 PM