Grandmothers have driven the evolution of longer lifespans, extending them by around 25 years in a theoretical population of apes, according to a mathematical model that tests the 'grandmother hypothesis'.

According to this theory, women evolved lifespans that extend well past the menopause because older women can boost their daughters' fertility — by taking care of grandchildren. Kristen Hawkes at the University of Utah in Salt Lake City and her colleagues modelled a population of apes, and found that their lifespans evolved from an ape-like range of around 35 years to a human one of roughly 60 years when older females contributed to infant care. Grandmothers enabled mothers to have their next baby sooner without reducing their previous offspring's chances of survival.

Proc. R. Soc. B http://dx.doi.org/10.1098/rspb.2012.1751 (2012)