  Great apes, the other living members of our hominid family, become decrepit
before the age of forty and rarely outlive their fertile years. In contrast,
women - even in high mortality hunter-gatherer populations - usually remain
healthy and productive well beyond menopause. The grandmother hypothesis aims
to account for the evolution of this distinctive feature of human life history.
Our previous mathematical simulations of that hypothesis fixed the end of
female fertility at the age of 45, based on the similarities among living
hominids, and then modeled the evolution of human-like longevity from an
ancestral state, like that of the great apes, due only to grandmother effects.
A major modification here allows the age female fertility ends to vary as well,
directly addressing a version of the question, influentially posed by GC
Williams six decades ago: Why isn't menopause later in humans? Our model is an
agent-based model (ABM) that accounts for the coevolution of both expected
adult lifespan and end of female fertility as selection maximizes reproductive
value. We find that grandmother effects not only drive the population from an
equilibrium representing a great ape-like longevity to a new human-like
longevity, they also maintain the observed termination of women's fertility
before the age of 50.
