Granny off the hook in menopause mystery

Geneticists appear to have failed in their latest attempt to indict human grandmothers in the court of life for indirectly visiting menopause upon their daughters.

Dr Alan Rogers, a population geneticist at the University of Utah, in Salt Lake City, told the XIX International Congress of Genetics that his team's comparative study of female chimpanzees and humans had revealed a deep flaw in the so-called 'grandmother hypothesis' of human menopause.

The GM hypothesis is a variant of an earlier theory that at some point in the game of life, where the object is to leave as many offspring as possible to carry one's genes, it is better to stop baby-making and concentrate on raising one's existing offspring to ensure they survive to reproductive age. By doing so, mothers enhance their own daughters' fertility and reproductive success.

Natural selection found a solution by favouring genetic mechanisms that, around mid-life, terminate female fertility: menopause.

The amended theory suggests that grandmothers have played an important role in the process by assisting their daughters to raise their own grandchildren.

Most female mammals, including chimps, don't undergo menopause -- it is almost unique to human females, and the payoff for their abbreviated reproductive life is that natural selection has been culling out mutations that could shorten the human life span, ensuring that grannies are around to help their daughters raise their granddaughters.

Some complicated mathematics are involved in testing the 'granny' hypothesis, and Rogers' team has confirmed certain aspects of it: human females mature more slowly and become fertile much later than female chimps, and continue to produce offspring for at least 10 years longer. Mortality, not menopause, limits female chimp fertility.

Rogers says the flaw in the theory is that chimp mothers are less important to their daughters' reproductive success than are human mothers, so any benefit from grandmothering is marginal, at best, partly because chimp grandmothers are shorter-lived than their human counterparts.

Human evolutionists have long assumed that the early Australopithecus ancestors of modern humans had a life history like that of chimps -- they were relatively short-lived, and grandmothers didn't really contribute to increasing their daughters' reproductive success.

So how did natural selection conspire to produce a long-lived species in which the female undergoes menopause, from a relatively short-lived species that does not have menopause?

Rogers suspects a resolution of the mystery of menopause may lie in the fine detail of how pre-menopausal mothers influence the survival of their young children. Until then, granny remains innocent.