Until fairly recently, little information existed about how long prehistoric people lived, on average. There were simply not enough fossilized human remains to draw broad conclusions about the age distribution or demographics of any given ancient population; only fragments of individual skeletons tended to be found in archeological sites. Tracking life expectancy as it increased, to see when human longevity first grew, was tough.
Struck by this challenge, anthropology professors Rachel Caspari of Central Michigan University and Sang-Hee Lee of the University of California-Riverside decided to take a different approach: rather than focus on the age of individuals at death, they concentrated on relative ages, that is, they calculated the different proportions of young and old. Using an established method of studying tooth wear in skeletons, they determined an “OY ratio”, or ratio of older adults to younger adults.
The team focused on the remains of 768 early people, in fossils accumulated over a three million year period, in four main sites. The earliest specimens were from East and South Africa, others were Neandertals dating back 130,000 - 30,000 years ago, and the most recent aggregate from Europe early in the Upper Paleolithic period, as recently as 20,000 years ago. Both artistic and functional tools are common artifacts from this period.
In a 2011 article in Scientific American, Caspari writes that the team was not prepared for how dramatic the results of their calculations would be. While longevity increased marginally over all of the time periods, so-called “modern” humans – living around 30,000 years ago – saw an increase in the ratio of older adults to younger ones that was five times greater than for earlier populations.
Caspari calls the shift the “evolution of grandparents”, as it appears to be a turning point in history, resulting in the coexistence of three generations for the first time. Though they cannot be sure which precise factors brought about this new trend, she and other colleagues have hypothesized that it’s not just healthier constitutions, or improving physiology. Rather, they suggest that older surviving members of a population contribute play a significant cultural and social role, passing on skills, knowledge and other resources that improve the longevity of their extended family – and setting the state for increased survivorship in modern society.
Caspari, R. “The Evolution of Grandparents.” Scientific American. 2011 vol:305 iss:2 pg:44 -9.
Rachel Caspari and Sang-Hee Lee. “Older Age Becomes Common Late in Human Evolution.” PNAS July 27, 2004 vol. 101 no. 30 10895-10900.