tortoise with a birthday hat on
Tortoises typically live well past 100 and might be able to survive even longer. In 2006, a giant tortoise thought to be 255 years old died at India's Calcutta Zoo. The Vorhees

This article was originally published in the March/April 2016 issue of Popular Science, as part of our “How To Live Forever” feature.

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.”

Extra Candles For The Cake

Life expectancy is rising

Expected age of death for 65-year-olds

Expected age of death for 65-year-olds

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

Years women outlive men, on average

Years women outlive men, on average

Animal Lessons

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.

Brandt’s Bat

Brandt’s Bats

Despite weighing only about as much as a nickel, Brandt’s bats can live as many as 41 years in the wild. In 2013, Gladyshev and colleagues found changes to genes that alter the animal’s response to growth hormones.
Bowhead Whale

Bowhead Whale

Possibly topping out at older than 200, the bowhead whale is the longest-lived mammal known. In 2015, a team of scientists found genetic variations relevant to aging, cancer protection, cell-cycle regulation, and DNA repair.
Naked Mole Rat

Naked Mole Rat

Hairless and wrinkled, naked mole rats can live to be 30 years old—many times the life span of other rodents. In 2014, research led by the University of Liverpool found changes in the rat’s genome related to cancer resistance.
Ocean Quahog

Ocean Quahog

In 2006, researchers found an ocean quahog thought to be 507 years old. “That is pretty remarkable when you figure that clams have beating hearts,” says Austad. His lab is looking for the reason its proteins can last so long.
Freshwater hydra


The freshwater hydra seems, under ideal conditions, to be immortal. It also has a seemingly endless supply of stem cells. German researchers have linked a longevity gene, also found in humans, to stem-cell production.