Sept. 18, 2007 — -- Scientists think they've found one of the reasons why humans defy evolution theory and live well beyond their reproductive life. It's all those old guys latching on to younger women and passing their good genes down to their kids.
For decades now evolutionary biologists have argued over a conundrum. There isn't much reason for members of any species to hang around after they can no longer reproduce, because according to theory they've already fulfilled their role in life. But humans tend to stick around for a long time after their traditional childbearing years are over.
Years ago the issue caught the attention of one of the pillars of evolutionary biology, William Hamilton, who died in 2000 after contracting malaria during a biological expedition to Africa. Hamilton wrestled with what came to be known as the "wall of death." Once reproduction ends, according to the wall theory, any biological system winds down abruptly.
Hamilton argued that when an organism stops reproducing, it is more susceptible to mutations that reduce its chances of survival. Thus death should come quickly. But all he had to do was glance around his home in England to see that there was something wrong with his own theory. People weren't dropping dead after females entered menopause. So Hamilton started looking for other factors that might influence human longevity.
One of the theories that came out of that is the "grandmother hypothesis." In early human history, some older women made themselves useful by helping their daughters, and even their granddaughters, raise their kids. That helping hand gave the kids a better chance to survive, thus preserving and ultimately passing along some of grandma's genes.
And since grandma lived long enough to help raise her great grandkids, then she must have had some pretty good longevity genes.
Makes sense, and several other studies over the past few decades continued to focus on the role of women in human longevity, since there is somewhat of a consistency in the age when women stop reproducing.
But now three scientists -- all males, by the way -- have taken a fresh look at the issue and produced a lot of data suggesting that men are part of the equation, too. Their study, "Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan," was published in a recent edition of PLoS, an online public access science journal. The study was conceived by Shripad Tuljapurkar, an internationally known expert on population at Stanford University.
Cedric Puleston, a doctoral candidate in biology at Stanford and a co-author of the study, said the researchers wanted to see if men also played a role in increasing the human lifespan.
"If men and women have different life histories, in terms of survival and reproduction, doesn't it make sense to look at what's going on in both sexes?" Puleston said. For help, Tuljapurkar and Puleston turned to a colleague, anthropologist Michael D. Gurven of the University of California, Santa Barbara. Gurven has compiled extensive records of longevity and fertility of several hunter-gatherer groups, including the Dobe !Kung of the Kalahari and the Ache of Paraguay, one of the most isolated populations in the world.
These people live a lifestyle that is believed to be very similar to the hunter-gatherers in the earliest phase of human history. Probably not exactly, but it's the best scientists have to work with.
"Much of what we know about early human life is based on what we know from modern hunter-gatherers," Puleston said.
The research showed that in very primitive societies, men remained reproductive much later in life than women. They usually mated with women who were much younger, and they tended to do that over and over again. Male fertility tended to taper off with age, but it didn't end suddenly as it did with women.
"That's true in every population we looked at," Puleston said. That continues to this day, with some men in their 80s siring children. Puleston said he knows of one man who became a father at 95.
Older men helped increase the birth rate in the population as a whole, because more old guys were remaining useful, and it also meant gramps was passing along a pretty good set of genes. And it's not just a matter of passing on good genes.
"It's probably more accurate to talk about a lack of bad genes passing on," Puleston said.
That contributed to a longer human lifespan over centuries of evolution. Of course, grandma probably did her part, too. And cultural factors, like the availability of antibiotics, have helped extend longevity.
But were there really that many old guys hanging around way back then? Could they possibly have done their part to add a few years to our lives today?
"There were probably some old people from the beginning of human history," Puleston said. "We didn't invent old age."
In their study, the researchers put it this way:
"Our analysis of mating patterns shows that productive mating between men older than the age of female menopause and younger women was likely a feature of early human life."
By the way, the researchers note that it's not going to add any years to an old man's life if he takes on a young mate.
Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.