One of Mother Nature's greatest mysteries may have been unravelled thanks to the work of a Cornwall-based scientist.
Andy Russell, working as part of an international team, believes the menopause is a way of making women focus on their grandchildren's wellbeing.
The so-called "change," which women undergo in their mid-50s when they generally can no longer have children, is a true rarity in the animal kingdom.
The study co-authored by Dr Russell, of Exeter University, suggests that it evolved, in part, to prevent competition between a mother and her new daughter-in-law.
Dr Russell, who works at Penryn's Tremough campus, said the menopause was something of an evolutionary enigma.
"We are so used to the fact that all women will experience menopause, that we forget it is seriously bizarre," he said.
"Evolutionary theory expects animals to reproduce throughout their lifespan and this is exactly what happens in almost every animal known, including human men. So why are women so different?
"Our study shows for the first time that the answer could lie in the relationship between a mother-in-law and daughter-in-law."
Researchers analysed data which showed that a grandmother having a baby later in life and at the same time as her daughter-in-law resulted in the newborns of each being 50 per cent less likely to survive to adulthood.
Dr Russell said their discovery could add weight to the theory that the menopause evolved to allow women to focus on their grandchildren. Traditionally, this role included providing food for the family and protecting young children from accidents and disease.
The topic has rarely been analysed, because it requires detailed data on the reproductive success of several generations of women, with knowledge on who lived with whom and when.
However, for the first time scientists were able to analyse records stretching back 200 years from church registers of pre-industrial Finland.
Researchers looked at information on birth and death rates from 1700 to 1900, before the advent of modern contraception or healthcare.
They found that women had more grandchildren if they stopped reproducing around the age of 50. The research team believes this was partly because of reduced competition between the older woman and her daughter-in-law and partly because of the support she could offer her grandchildren.
A child born to families with a mother-in-law and daughter-in-law reproducing simultaneously was twice as likely to die before reaching the age of 15. However, this was not the case in the instances when a mother and daughter had babies at the same time.
Dr Russell said this suggests that related women breed co-operatively and unrelated women do not.
Different theories have been put forward for the evolution of the menopause in humans, including the idea that it evolved to protect older women against the danger of dying during childbirth. However, the study found that under two per cent of the pre-industrial Finns died in childbirth in their mid-40s and such risks of dying in childbirth are similarly low in hunter-gatherers today.