Living longer: Information revolution, population expansion, and modern human origins.

T he dramatic increase in life expectancy and the rising numbers of old people so clearly visible in the 19th and 20th centuries are the result of medical and lifestyle changes that have important social and economic ramifications. Anthropologists have suspected for some time that these recent demographic changes build on a long-term trend that was continuous throughout human evolution. In this issue of PNAS, Drs. Rachel Caspari and Sang-Hee Lee (1) provide powerful evidence to demonstrate the antiquity of this trend toward increasing numbers of older people in society. Furthermore, they show evidence that a dramatic acceleration of this process occurred relatively recently (in evolutionary terms), along with the advent of modern human behavior in the Upper Paleolithic, beginning 30,000 years ago. Caspari and Lee base their conclusions on an analysis of a large sample of fossils representing the last 3 million years of human evolution. The trend for more people to live to older age throughout that time period accelerated sharply in the last part of the human evolutionary record, the Upper Paleolithic, when there was a 4-fold increase in the number of adults old enough to be grandparents. There are at least two significant consequences of having more older adults in a population: greater overlap between generations and higher fertility. Caspari and Lee argue that this life-history change had a major impact on human social and cultural life in enhancing the transfer of information between generations as grandparents helped to educate and enculturate the younger generation and contribute to extended families. It also means that adults lived longer in their reproductive years, increasing potential fertility and increasing the opportunity for passing on cultural knowledge. This demographic transformation was associated with major cultural changes (the ‘‘creative explosion’’) and fueled a dramatic population increase and geographic range expansion of humans that culminated in the colonization of most of the world by 15,000 years ago.