WASHINGTON - Grandmothers are more than hugs, cookies and free baby-sitting. According to recent research, they might have played a crucial role in human evolution.
That's because in primitive societies, older women provide most of the food their family needs, freeing their grown daughters to have more and healthier children.
"Grandmothers raise the rate of baby production," said Kristen Hawkes, an anthropologist at the University of Utah who studies the lifestyles of modern-day hunter-gatherers in Africa and South America.
The extra help from postmenopausal women gave an advantage to early members of the genus Homo, the group that includes our own species, Homo sapiens, and our extinct relatives, such as the Neanderthals.
Hawkes says this previously unrecognized "novel role for grandmothers underlies the shift from an apelike ancestral pattern to one more like our own."
This "grandmothering hypothesis" was developed by Hawkes and her colleagues, James O'Connell, also at Utah, and Nicholas Blurton Jones, at the University of California in Los Angeles. In reports in scientific journals, they describe how grandmothers help feed their daughters' offspring in tribes not practicing agriculture.
Men in those tribes hunt game, but that's an inefficient way of feeding their families compared with the women's work of gathering and preparing edible roots, berries and fruits. Based on archaeological evidence, a similar pattern held among archaic humans about 2 million years ago.
"Old people are good to have around," British population researcher Spencer Wells wrote in a forthcoming book, The Journey of Man.
Our ancestors matured later than apes, developed bigger bodies and brains, learned to speak, mastered the use of simple tools and spread out of their African homeland to Europe, Asia and the Middle East.
And unlike apes, which usually die when their childbearing years end, human females, both ancient and modern, live long after menopause.
"The striking difference between us and the other great apes lies in the low adult mortalities that give us long average life spans after menopause," O'Connell reported in the Proceedings of the National Academy of Science.
The grandmother hypothesis challenges the conventional theory that males were primarily responsible for the evolutionary leap to Homo sapiens. In the traditional view, men of the clan provided food by hunting and women minded children.
But studies of modern hunter-gatherers - notably the Bushmen of South Africa, a study led by anthropologists Irvin Devore of Harvard University and Richard Lee of the University of Toronto - showed that 80 percent of their diet consisted of vegetables: berries, fruits and starchy roots known as tubers. Meat was an occasional treat.
Big game hunting, in prehistoric times and now, was "a poor strategy for feeding a family," Hawkes said.
"Meat from large animals probably cannot have been a key component in early human subsistence, let alone the main catalyst for the evolution of the genus," O'Connell reported in last December's Journal of Human Evolution. "The quantities involved simply would have been too small."
Nancy Devore, an anthropologist who worked with her husband, Irvin, on the Bushmen project and is not a member of the Hawkes group, supported the grandmother hypothesis.
"It makes perfect sense," she said. "Why else should women live that long after the last child is born? The men were out hunting game, but they often didn't get any, so they foraged a bit on their way back. But the women were out there gathering all day."