After millennia of sluggish progress, humans abruptly showed signs of complex culture about 30,000 years ago. Anthropologist Rachel Caspari of the University of Michigan at Ann Arbor may have uncovered the cause of the rapid innovations in art, tools, and civilization: a dramatic increase in the number of old people.
By examining teeth from 768 ancient skulls, Caspari and colleague Sang-Hee Lee of the University of California at Riverside were able to estimate the age distributions of various prehistoric populations. “What you see is a steady increase in the number of people living into their thirties—and then there is an explosion,” Caspari says.
Around 2 million years ago, only about one in 10 Australopithecines—the modest-brained hominids exemplified by the famous fossil Lucy— who made it to adulthood lived to twice the age of sexual maturity. That number increased to about 1 in 5 when early Homo species appeared a million years ago and increased again to nearly 4 in 10 by the age of the Neanderthals, some 130,000 to 30,000 years ago.
“Then around the 30,000-year mark, all of a sudden you see the reverse, where you have about two older adults for every young adult,” Caspari says. She suggests that the older faction, particularly postmenopausal women, sparked a cultural revolution. Having enough elders around to help rear the young would have provided a survival advantage and allowed larger populations. Elders would have also been invaluable in cultural transmission, passing along ideas and traditions. Caspari’s findings add to the growing support for the grandmother hypothesis, which suggests that human females (unlike those of most species) live far past their reproductive years because their presence aids their grandchildren. “It offers a simple explanation, with concrete evidence, for a very puzzling cultural phenomenon,” Caspari says.