Well past age 50, men can still impregnate women of childbearing age. That lengthy period of fertility spurred the evolution of relatively long lives in both sexes, a new study suggests.
In modern hunter-gatherer societies, a substantial number of men in their 60s and 70s continue to father children, say Stanford University biologist Shripad D. Tuljapurkar and his colleagues. A mathematical model that they describe in the August PLoS ONE indicates that, if this mating pattern had occurred throughout human evolution, it would have preserved genes that favor both male and female survival for as long as men can reproduce, until roughly age 70.
That conclusion builds on a review of mortality patterns in hunter-gatherer and other nonindustrial societies published in the June Population and Development Review. Anthropologists Michael D. Gurven of the University of California, Santa Barbara and Hillard Kaplan of the University of New Mexico in Albuquerque conclude that nearly one in three members of such populations live past age 55, many into their 70s. Older, high-status men often monopolize access to reproductive-age women in these groups, Gurven and Kaplan say.
The same mating pattern applied to many Stone Age groups, Tuljapurkar proposes.
A female life span extending beyond menopause also evolved as a result of grandmothers increasingly assisting their daughters in caring for their own offspring, asserts anthropologist Kristen Hawkes of the University of Utah in Salt Lake City.
However, this grandmother effect can’t explain why menopause evolved in the first place, notes University of New Mexico anthropologist Kim Hill. In a majority of hunter-gatherer groups, moreover, men who are older than 55 generally father few children, undermining Tuljapurkar’s conclusions, Hill contends.