updated 10/15/2012 3:50:20 PM ET 2012-10-15T19:50:20

Babies born today are 200 times less likely to die in early life than if they were born a century ago.

At older ages, the chances of dying are also substantially lower now than they were just four generations ago.

A new study, which compared modern human lifespans with those of hunter-gatherers and chimpanzees, suggest that industrialization and development since 1900 -- not any genetic shift -- are responsible for our species' dramatic gains in life expectancy.

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By touching on why aging and death happen at the rates they do, studies like this one could ultimately help us figure out which public health strategies will work best to extend lifespans.

Still unresolved are debates about how long humans can ultimately expect to live and whether the previous century's drops in mortality might continue at the same rate.

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"In terms of the probability you'll live through the year, it's astronomical the improvement that we've made," said Oskar Burger, an evolutionary anthropologist at the Max Planck Institute for Demographic Research in Rostock, Germany.

"The probability that you'll live through the year in our evolutionary past that was experienced at age 30 or even 20 is now typical of people who are 70," he added. "Seventy-two is the new 30."

In the world's wealthiest nations, according to a 2002 study, life expectancy at birth has been increasing steadily at a rate of three months each year since 1840. That's a remarkable pattern, Burger said, especially for a trait that is so complicated and dependent on so many variables.

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In prior attempts to put those trends into perspective, scientists have compared life expectancies at one time and place to life expectancies at the same place a generation or two ago. Burger and colleagues wanted to get a deeper, more evolutionary perspective in order to better understand the underlying biology of what determines how long we live.

They began by gathering results from a study published in 2007 that compiled the life-expectancy patterns of several well-studied hunter-gatherer populations. Then, they compared those numbers with data from Sweden and Japan, where people today are born with some of the longest life expectancies in the world.

Compared to populations of hunter-gatherers who live like our ancestors did and rely exclusively on their own foraging skills, populations that have some access to Western clothing, food and medicine experience a big boost in survival, the researchers report today in the journal Proceedings of the National Academy of Sciences.

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For developed nations, improvements have been even more dramatic, with the biggest gains in infant mortality. For today's babies in the longest-lived places, the chances of surviving are 200 to 300-fold better than for people born into conditions like those our ancestors faced.

The scale of improvement drops in later decades, but the differences are still significant. A 65-year old hunter-gatherer has about a 5 percent chance of dying over the next year, for example, compared to a less than 1 percent chance of death for a 65-year-old person living in Japan.

Through age 15, hunter-gatherers experience rates of death more than 100 times higher than do today's Japanese. Over the course of the entire lifespan, mortality rates are 10 times higher. And a 15-year-old hunter-gatherer and a 69-year-old Swede face the same chances of dying in the next year.

Mortality patterns of today's hunter-gatherers are more similar to those of chimpanzees than they are to those of people in wealthy, developed nations, the study also found, though the difference matters most at young ages.

By zooming out to provide a broader perspective on changes in human mortality, the new study illustrates just how much things have changed, said Michael Gurven, an evolutionary anthropologist at the University of California, Santa Barbara, who conducted the research on hunter-gatherers that formed the crux of the new study.

The findings are likely to fuel an ongoing debate about the future of mortality and longevity.

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Since improvements have been so steady over the past century, some scientists argue that the trend will continue, leading to life-expectancies of 85, 90 or even 100 before too long.

Others, including Gurven, think that the future is unpredictable. That's especially true because society has made the greatest gains in eliminating deaths among infants and young children.

After age 70 or so, it gets harder to increase survivorship, and differences between populations drop dramatically.

"That really implies that there's a built-in design in the human body," Gurven said. "You can eliminate a lot of avoidable sources of mortality. You can clean up water, get rid of a lot of infections, try to make the environment safer so there aren't predators around, make traffic safer so that kids aren't getting run over by cars. But even if you eliminate all early mortality, it's becoming increasingly difficult to lower mortality at late ages."

"Things are getting better, but I think it's a little too optimistic to think things can continue to improve," he added. By age 90 or 100, "we're well beyond the warranted period of the design of the human body."

© 2012 Discovery Channel


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