The longevity of modern humans more than quadrupled around 30,000 BC., a time of "creative explosion," anthropologists say.

American researchers studied more than 750 fossil teeth from successive time periods, ranging from later australopithecines to post-Neanderthal Early Upper Paleolithic Europeans.

Sang-Hee Lee of the University of California, Riverside, and anthropologist Rachel Caspari of the University of Michigan surveyed the ratio of older to younger adults by analysing rates of molar wear.

To the researchers, "old" was considered to be at least double the age when humans can first reproduce and the third molars typically appear. Assuming this age was 15, for example, then a woman could become a grandmother at age 30.

They found a trend towards more older adults surviving throughout human evolution. Longevity during the Upper Paleolithic period was dramatically greater than in earlier periods, the pair reported in this week's online issue of the Proceedings of the National Academy of Sciences.

"Our paper makes a convincing case about the importance of older people in a society," said Lee in a release. "We were surprised ourselves to see that the time period when old adults outnumbered young adults is the time characterized by a creative explosion."

Sophisticated cave art was made during the Upper Paleolithic period.

The researchers speculate an increase in the number of older people allowed early modern humans to pass on specialized knowledge to later generations and strengthen their relationships.

The study offers evidence that modern humans had a competitive edge because they were "older and wiser," Caspari said.

The fossil results support the grandmother hypothesis, which says grandmothers can offer an evolutionary advantage by helping their adult children to breed earlier, more successfully and more frequently.