The grandmother hypothesis says that humans began to live longer because older individuals could increase their inclusive fitness by helping out. When did this evolve?
The authors used dental samples from four archeological populations representing four hominid groups (768 individuals total):
1. Later australopithecines (including both Australopithecus and Paranthropus)
2. Early and Middle Pleistocene Homo
3. Neandertals from Europe and Western Asia
4. Post-Neandertal Early Upper Paleolithic Europeans.
The primary metric was the Old to Young (OY) ratio -- the ratio old adults (at least twice the age of reproduction) to young adults.
The OY ratio kept increasing as the timeline got more recent, but more recent humans had a MUCH higher OY ratio than any previous groups (Table 1).
Discussion
"Recent models (Shennan, 2001) suggest demographic factors are responsible for the cultural innovations associated with modernity. Population expansion may have provided social pressures that led to extensive trade networks, increased mobility, and more complex systems of cooperation and competition between groups, resulting in increased personal ornamentation and other material expressions of individual and group identity."
and
"These results suggest a major increase in adult survivorship in the Upper Paleolithic. We suggest that this increase in longevity addresses the meaning of modernity itself. Modernity is a complex concept, incorporating both biological and cultural variables... However, if there is a single fundamental factor related to biology that underlies the behavioral innovations of modernity, this increase in adult survivorship may be it. We therefore think significant longevity came late in human evolution and was a fundamental demographic component tied to the population expansions and related behavioral innovations associated with modern humans."
