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FRIDAY, July 18 (HealthDayNews) -- Unlike many species, humans live well past their reproductive years.

Now, a University of California, Berkeley article offers a new evolutionary theory of aging that may explain the reason for that difference.

The classic theory of evolutionary aging proposes that fertility is the sole determinant of lifespan. Once a creature is past reproductive age and no longer able to pass on its genes, certain biological factors no longer act to extend the creature's lifespan.

But this new theory says that's not the case with humans and some other social animals who, even after they're no longer fertile, continue to make important contributions to younger generations in the form of care and nurturing. This is called the transfer effect.

According to the new evolutionary theory of aging, the lifespan of species that make no post-birth investment in offspring depends entirely on fertility. But in species that do provide a great deal of care for their offspring, life span depends heavily on the transfer effect.

That means that these species, such as humans and whales, live well beyond their reproductive years. In terms of the lifespan of human hunter-gatherers, this new theory makes more sense than the classic theory, the article says.

It appears in this week's issue of the Proceedings of the National Academy of Sciences.

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Here's where you can learn more about aging.

--Robert Preidt

SOURCE: Proceedings of the National Academy of Sciences, news release, July 16, 2003

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