"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.