We can thank the likes of David Letterman and Paul McCartney for their tiny contribution to increasing the human lifespan. These elder daddies and their prehistoric equivalents ward off early death for the rest of us simply by remaining fertile, according to a study from Stanford University.

Most animals die at the age when females are no longer fertile, but we humans live well into our eighties. The researchers point the longevity finger at men who father children in their fifties and beyond. Because men can reproduce well into their silver years, there is an evolutionary incentive for them to live longer and remain amorous. "Selection favors keeping people alive while they still actively reproduce," says Cedric Puleston, a co-author of the study.

Women also prosper when men of advanced age father children. Beneficial longevity genes are probably carried on the X and Y chromosomes so dads pass them down to both their female and male children, spawning long-living men and women.

Puleston notes that his male-centered theory of longevity doesn't rule out female contributions to extended lifespan. The "grandmother hypothesis" has long held that women live past menopause because grannies who help their daughters and granddaughters raise children propagate their own genes. "They could both be right."