Female chimps, gorillas and orangutans rarely survive long after their last pregnancy, but human women often live decades after menopause. Grandmothers may be the reason.

In a study published today, University of Utah and UCLA researchers argued that humans evolved certain traits - including long life spans - because grandmothers in ancient hunter-gatherer societies collected food for grandchildren.By helping feed weaned grandchildren, grandmas made it more likely the children would survive. Mothers could have babies more often because they could focus on breast-feeding newborns while grandmother fed older kids.

By increasing their daughters' fertility and helping grandchildren survive, vigorous grandmothers were more likely to pass on their genes to many grandchildren, female and male. So as humans evolved, natural selection favored longer life spans, including female longevity after menopause, according to the hypothesis.

"The tendency has been to believe that after menopause, you are sort of a third foot, and the real story is old, powerful men and young, nubile women," said Kristen Hawkes, who heads the U.'s anthropology department and led the new study. "This hypothesis says many of the things that make us human arise from what older women have been doing. Grandmothers may be the key to a whole array of things that make us distinctly human."

The study, published in the journal Proceedings of the National Academy of Sciences, was conducted by Hawkes, Utah biologist Eric Charnov, anthropologist Jim O'Connell, graduate student Helen Alvarez and UCLA anthropologist Nicholas Blurton Jones.

Critic Erik Trinkaus, of Washington University in St. Louis, said the theory may explain what happens in some hunter-gatherer cultures. But he said the researchers go too far in "trying to globalize it and apply it to everything."

In an earlier study, Hawkes, O'Connell and Blurton Jones outlined how the grandmother hypothesis worked among the Hadza hunter-gatherers in Tanzania. Hawkes and O'Connell lived with the Hadza during 1985-1986. They watched grandmothers in their 50s through 70s spend eight hours daily foraging for tubers, berries and other food for grandchildren.

That was key evidence for the hypothesis that grandmothering resulted in human women living many vigorous years after menopause, while other primates become frail soon after their last pregnancy and rarely live past 50.