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updated 10/15/2012 7:11:26 PM ET 2012-10-15T23:11:26

Humans are living longer than ever, a life-span extension that occurred more rapidly than expected and almost solely from environmental improvements as opposed to genetics, researchers said Monday.

Four generations ago, the average Swede had the same probability of dying as a hunter-gatherer, but improvements in our living conditions through medicine, better sanitation and clean drinking water (considered "environmental" changes) decreased mortality rates to modern levels in just 100 years, researchers found.

In Japan, 72 has become the new 30, as the likelihood of a 72-year-old modern-day person dying is the same as a 30-year-old hunter-gatherer ancestor who lived 1.3 million years ago. Though the researchers didn't specifically look at the United States, they say the trends are not country-specific and not based in genetics.

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Quick jump in life span
The same progress of decreasing average probability of dying at a certain age in hunters-gatherers that took 1.3 million years to achieve was made in 30 years during the 21st century.

"I pictured a more gradual transition from a hunter-gatherer mortality profile to something like we have today, rather than this big jump, most of which occurred in the last four generations, to me that was surprise," lead author Oskar Burger, postdoctoral fellow at the Max Planck Institute for Demographic Research in Germany, told LiveScience.

Biologists have lengthened life spans of worms, fruit flies and mice in labs by selectively breeding for old-age survivorship or tweaking their endocrine system, a network of glands that affects every cell in the body. However, the longevity gained in humans over the past four generations is even greater than can be created in labs, researchers concluded. [Extending Life: 7 Ways to Live Past 100]

Genetics vs. environment
In the new work, Burger and colleagues analyzed previously published mortality data from Sweden, France and Japan, from present-day hunter-gatherers and from wild chimpanzees, the closet living relative to humans.

Humans have lived for an estimated 8,000 generations, but only in the past four have mortalities decreased to modern-day levels. Hunter-gatherers today have average life spans on par with wild chimpanzees.

The research suggests that while genetics plays a small role in shaping human mortality, the key in driving up our collective age lies with the advent of medical technologies, improved nutrition, higher education, better housing and several other improvements to the overall standards of living.

"This recent progress has been just astronomically fast compared to what we made since the split from chimpanzees," Burger said.

Most of the brunt of decreased mortality comes in youth: By age 15, hunters and gatherers have more than 100 times the chance of dying as modern-day people.

What's next?
"In terms of what's going on in the next four generations, I want to be very clear that I don't make any forecasts," Burger said. "We're in a period of transition and we don't know what the new stable point will be."

However, some researchers say that humans may have maxed out their old age.

"These mortality curves (that show the probability of dying by a certain age), they are now currently at their lowest possible value, which makes a very strong prediction that life span cannot increase much more," Caleb Finch, a neurogerontology professor at the University of Southern California who studies the biological mechanisms of aging, told LiveScience in an email.

Further, Finch, who was not involved in the current study, argues that environmental degradation, including climate change and ozone pollution, combined with increased obesity "are working to throw us back to an earlier phase of our improvements, they're regressive."

"It's impossible to make any reasonable predictions, but you can look, for example, in local environments in Los Angeles where the density of particles in the air predict the rate of heart disease and cancer," Finch said, illustrating the link between the environment and health.

The study was detailed Monday in the journal Proceedings of the National Academy of Sciences.

Editor's Note: This article was updated to clarify the difference between life span and mortality rate.

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