Humans began to live long and prosper only about 30,000 years ago, researchers report. Results published online this week by the Proceedings of the National Academy of Sciences reveal a striking increase in human longevity during the Upper Paleolithic Period when the number of people surviving to old age increased four-fold.

Rachel Caspari of the University of Michigan and Sang-Hee Lee of the University of California at Riverside, examined 768 hominid dental samples from a variety of locations and time periods. Included in the selection were fossils from Australopithecus, Homo erectus, and Neandertals from Europe and Western Asia. The researchers analyzed the amount of wear on the molars to determine the age of the individuals and defined as old those who reached double the age of reproductive maturation. A comparison of the number of old and young individuals in each time period revealed a dramatic increase in life span that occurred about 30,000 years ago. (The above photograph shows the skull of an early modern human from the site of Cro-Magnon in France.) Significant longevity came late in human evolution and its advantages must have compensated somehow for the disabilities and diseases of older age, when gene expressions uncommon in younger adults become more frequent, the scientists write.

The findings support the so-called grandmother hypothesis, which posits that older women no longer responsible for their own children help support the group by strengthening social bonds and providing greater opportunities to pass on specialized knowledge. "There has been a lot of speculation about what gave modern humans their evolutionary advantage, Caspari remarks. This research provides a simple explanation for which there is now concrete evidence: modern humans were older and wiser.