The Evolution of Menopause and the Grandmother Hypothesis

Throughout the natural world trade-offs between reproductive success and longevity have been noted by scientists for many years. Most wild animals remain fertile until death, as do human males, and although human males do become less fertile with age, they in general experience no complete shutdown of fertility. Men as old as the age of 94 have been noted to father children (Diamond, 1996). The situation for human women is strikingly different. They undergo menopause, the permanent cessation of menstruation, by the time they are in their mid fifties (Lahdenpera et al., 2004). In fact, natural conception after the age of 50 is extremely rare. Menopause has become quite the evolutionary puzzle for modern scientists because natural selection favors increased reproduction, therefore increasing one’s own fitness. The essence of natural selection is that it promotes genes for traits that increase an animal’s number of descendents bearing those genes. Thus, the great question is: How could natural selection result in every female member of a species carrying genes that destroy her ability to leave more descendents? Many theories on the evolution of menopause have arisen in the past 50 years in an attempt to answer this question but they all also have key assumptions and limitations. Proponents of the Grandmother Hypothesis believe that their theory holds the greatest promise to answer this question. The Grandmother Hypothesis is meant to explain why menopause arose in human evolution, and how late life infertility could actually confer an evolutionary advantage (Williams, 1957). The human female is unique in having menopause. Apart from some mammals kept alive longer than unusual in captivity, females in almost all other species remain fertile until shortly before their death or until death (Sturdee et al.,