Men's tendency to choose younger mates means that fertility may became pointless for older women, evolutionary geneticists from Canada's McMaster University revealed in a recent study. BBC News reports that this trend in our evolutionary biology may have eventually lead to women developing menopause.
It has long puzzled researchers as to why humans are the only species where females cannot reproduce throughout their entire lives. Previous theories suggest that women may lose their fertility at an age where they might not live to watch their children grow, known as the "grandmother effect," with menopause therefore seen as a biological means to prevent older women from reproducing.
However, the latest new theory suggests that it may instead be the lack of reproduction in older women that has given rise to menopause, and it is men's preferential choice of younger women as partners that may be to blame. The research team from McMaster University used computer modelling to test the "preferential mating" desires of men.
"There is evidence in human history; there was always a preference for younger women," Professor Rama Singh, an evolutionary geneticist who led the study, said to the BBC. He said that by choosing younger women as mates, men were "stacking the odds" against continued fertility in women. He also stressed that this is not a current social problem, but the result of human development that began thousands of years ago.
However, Singh said that the extended longevity of human life expectancy, in addition to women having children later on, could potentially alter the timing of menopause in today's society.
"The social system is changing," he said. "There are women who are starting families later, because of education or a career," and suggested that women who have later menopause could pass on this gene to their daughters "with the possibility of menopausal age being delayed" over time.
"The authors argue that the menopause exists in humans because males have a strong preference for younger females," said Dr Maxwell Burton-Chellew, an evolutionary biologist in the department of zoology at the University of Oxford, who challenged the theory.
"However, this is probably the wrong way round - the human male preference for younger females is likely to be because older females are less fertile," he said to BBC News. "I think it makes more sense to see the human male preference for younger females largely as an evolved response to the menopause, and to assume that ancestral males would have been wise to mate with any females that could produce offspring."
Burton-Chellew added that evolutionarily speaking, older females faced the choice of having children that may not reach adulthood before your own death, or "stop reproducing and instead focus on helping your younger relatives reproduce."
