From Nature: "A model published this week questions a popular theory dubbed the 'grandmother hypothesis', which says that human females, unlike those of the other great apes, survive well past their reproductive prime because of the benefits that post-menopausal women offer to their grandchildren. ... Chimps almost never live into their forties in the wild, but most humans, if they're lucky enough to make it to adulthood, live beyond the childbearing years. ... Despite its anecdotal support and intuitive appeal, the grandmother hypothesis lacked much quantitative proof showing that it was possible for longevity to evolve from grandmothering ... [Researchers] ran a mathematical simulation to test the theory's plausibility. Their agent-based model, which simulates the actions and interactions of individuals, begins with a population of 1,000 people whose lifespans and reproductive windows are an inherited trait that mutates over time. ... After about 500 generations, the model demonstrated that the assistance of a grandmother during infancy shortened the interval between the times their daughters give birth, and led to shorter reproductive windows. However, compared with simulations in which grandmothers did not help out, the benefits never result in a change in longevity. ... In hindsight [the] result isn't as surprising at it might seem. Natural selection is strongest early in life, and its influence on a trait wanes as an organism ages. Therefore, the benefits of grandmothering would have to be enormous to extend human lifespan."