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Life is destructive. Our environment and our internal functions all wear and tear at our body over time. Evolutionarily speaking, natural selection rewards those who can survive such hardship. So why don’t we live forever—why age at all?
There have been numerous attempts to understand how and why we age—as recently as 1990, the biologist Zhores Medvedev tallied more than 300 possible hypotheses. But according to Steven Austad, a biogerontologist at the University of Alabama at Birmingham, one explanation has risen to the top: “Reproduction is the name of the game. Basically, we age because it’s not in nature’s best interest to perfectly repair our bodies. The main thing is to keep us reproductive as long as possible, and then let our bodies deteriorate.”
“Longevity is one of the most exciting areas of research because it really takes into account every aspect of a human being.”
—Winifred Rossi, National Institute on Aging
The rate of aging in humans and other mammals, Austad says, might be determined by how quickly we have to reproduce before we’re killed off by other factors. In general, the smaller the animal and the more hostile its environment, the shorter it lives. A field mouse, for example, must breed before a hawk snatches it up, and so its organs and immune system don’t need to last 50 years. On the flip side, elephants have few threats, so their bodies can keep going for decades. “In an evolutionary sense,” says Austad, “that is the timekeeper.”
Since 1900, average U.S. life expectancy has risen from 47 to 79. A lot of those gains come from a lower infant-mortality rate: A century ago, 1-in-10 babies born in the U.S. died before age 1, while today that figure is 1-in-170. But longevity gains in later years have also been substantial. This chart shows the expected age of death for those who make it to 65. All four nations shown here have gained about a decade. Women have also outpaced men, a trend Andrew Noymer, a demographer at the University of California at Irvine, ascribes to higher rates of smoking and drinking among males. In the past few decades, men have been closing the gap—meaning more golden years for everyone. —Katie Peek
Women outlive men, and the difference is growing
The average life spans of animals vary wildly from ours, but the mechanisms involved for each appear to be different. “There are multiple roads to longevity,” says Vadim Gladyshev, a geneticist at Harvard Medical School. Identifying strategies that nature uses to alter life span could help scientists figure out how to extend human lives.
