How come women in their 50s stop having babies? And could it be that the increasing number of older mothers is evidence that evolution is working in reverse and menopause is now coming later rather than sooner?
Research from Britain's Sheffield University published this month bolsters the theory that menopause evolved so that older women could focus on being grandmothers rather than having babies forced to compete for finite resources with the offspring of their daughters and daughters-in-law.
"It becomes in the mother's interest not to reproduce," said Australian evolutionary ecologist Rob Brooks in response to the research.
"Genes that underpin retiring from reproduction and looking after your grandchildren tend to be favoured because that conflict is suppressing that late-life reproductive success."
Brooks, from Sydney's University of New South Wales, was commenting on research led by Dr Virpi Lummaa that found that women had more grandchildren if they stopped reproducing around the age of 50.
Lummaa's team trawled through church records of 653 women born in Finland between 1702 and 1823 who had 4,703 offspring and 9,164 grandchildren.
They also found that a child born to families with a mother-in-law and daughter-in-law reproducing at the same time was twice as likely to die before reaching the age of 15 than when a mother and daughter who were breeding simultaneously.
"If there's only a finite amount of food then it either goes into one body or another body," Professor Brooks said.
In pre-industrial Finland, as everywhere else at the time, finding enough to eat was a struggle and competition sparked conflict, Brooks said.
"Pregnancy lasts nine months and in any agricultural society there's always a couple of months of hunger before the harvest, so most pregnancies would have overlapped with those couple of months."
Brooks said conflict within the family was the missing ingredient in our understanding of menopause. Conflict with younger women forces mothers-in-law into menopause and to become full-time grandmothers.
"Arguments and homicides and all those things that we think of as conflict are extreme expressions of competition," he said. "In this case it's mostly during gestation that the two mothers would be competing for food and there might not be enough food for both of them to have a baby."
Brooks speculated that the increasing prevalence of older mothers nowadays points to an evolutionary reversal with menopause being delayed to accord with a plentiful supply of food and less conflict.
"I imagine that that's probably happening right now - just the fact that there are lots of older mothers."