Anthropologists Rachel Caspari and Sang-Hee Lee were astounded when they saw the numbers. After spending years studying fossilized teeth in museums around the world they were finally able to document one of the most dramatic turning points in the history of human evolution.
About 32,000 years ago, the researchers found, human longevity took a giant leap upward. In fact, the number of people surviving into old age more than quadrupled during what may have been a relatively brief period of time.
Why that happened is still a bit of a mystery, although the researchers have their hunches. But the consequences are clear. Population size grew dramatically, and with more and more people around, particularly older folks with a bit of wisdom and experience, new ideas began to emerge.
More people shared the chores of raising children and gathering food, and that left more time for leisure activities. People probably began to dream more, and shared more ideas with more people. Some became artists. Others, in time, musicians.
Population reached a "critical mass," as Caspari puts it, and from all those folks sitting around the campfire sharing ideas, modern humans began to emerge. It was not just evolution. It was revolution. This old planet would never be the same again.
Caspari, of the University of Michigan's Anthropology Museum, and Lee, of the University of California, Riverside, published their findings in a recent issue of the Proceedings of the National Academy of Sciences.
Lee, who is temporarily in South Korea as a visiting scholar at Chonnam National University, has described the events of that era as a period of "creative explosion." The research dovetails with other studies that show the importance of having elders around the campfire.
"The elders may have served the same purpose as computer hard drives serve today," Lee told the Riverside Press Enterprise. "They were the archives of vital information."
Caspari has been working on this project for 15 years, believing that key evidence about the mystery of human evolution could be found in old teeth. So she, and later Lee, traveled the Earth, visiting museum after museum, dating molars from 750 fossils. Those teeth, in the back of the mouth, were used to chomp through mammoths, so the amount of wear depended on how long the person lived.
That technique is not precise enough to come up with exact dates, but it's good enough to divide the fossils into two groups: those who died before or soon after reaching the age of reproductive maturity, about the age of 15, and those who lived at least twice as long, so they were old enough to become grandparents.
"We aren't talking about living into your 80s," Caspari says. Human life span in those days was pretty brief, under the best of circumstances, so even living into your 30s was quite an achievement.
The fossils came from Neanderthals, who later became extinct, and humans who ranged across Europe and western Asia when much of the world was covered with glaciers. Wear on the molars showed that life spans increased gradually across all groups until about 30,000 B.C., but at that time something quite remarkable happened.
The Neanderthals remained the same, with a gradual increase in longevity, but the ratio of older to younger persons took that giant leap forward among humans. And that, the researchers argue, gave humans an evolutionary edge in the battle for survival.