Evolution: Brains and Grandchildren Drove Emergence of Menopause in Women

Two small children watching their grandmother blowing bubbles. Scientists create computer simulations to work out why menopause emerged. J. Duckworth/Hulton Archive/Getty Images

Humans are one of three known species where the female stops being able to reproduce long before her death, the other two being orcas and pilot whales. Why this is has long baffled scientists, with many evolutionary theories as to why women go through menopause emerging over the years.

One of the most prominent theories is known as the grandmother hypothesis. This says that as the cost of reproduction increases with age, resources become better spent in helping current offspring to reproduce themselves—to help rear grandchildren, for example.

Another theory, Embodied Capital Model (ECM), works on the same idea of allocating resources away from direct reproduction to aiding offspring, but with key differences. The first is that ECM relates more to cognitive resources, rather than physical. It also factors in both sexes: "As the traditional hunter-gatherer pattern of production, reproduction, and parental investment depends fundamentally on a cooperative division of labor between men and women, the ECM predicts that both aging women and men may stop producing new children to allocate resources to existing children and grand-children."

Investigating the evolutionary advantages of menopause is difficult. Studies often compare the fitness of children and grandchildren of women with varying post-menopausal lifespans. But varying conditions in which women reproduce make it hard to draw conclusions about the emergence of menopause.

Taking a different approach to the problem, Carla Aimé and colleagues at the Institute of Evolutionary Sciences of Montpellier, France, developed a set of computer simulations to work out when, how and why menopause emerges.

In the study, published in PLOS Computational Biology, the team ran various models to determine under what situations females would stop reproducing. "Testing the computer simulations in real life is difficult because all human populations today experience menopause, so we can't compare the environment of people with and without menopause to identify factors which promote this phenomenon," Aimé tells Newsweek. "This is the reason why we have chosen to do computer simulations."

In their simulations of artificial human populations, the team found menopause emerged when two factors were at play— when humans had cognitive abilities, meaning they had another "resource" available to pass on, and where they cared for grandchildren.

This model fits with the EMC, indicating that menopause emerged because it was more advantageous (from an evolutionary point of view) for women to use their cognitive abilities to help their existing children and grandchildren. Physical abilities played far less of a role, swaying away from the grandmother hypothesis.

Cognitive resources, Aimé says, have delayed benefits by allowing ageing people to survive and continue to acquire resources, even when their physical condition declines. "These surplus resources could be used for having new children or for grandchildren care," she says. "After a given age, ageing women stop reproduction to increase grandmothering. This is menopause. And it is evolutionarily advantageous as it lead to an increase in child fertility and grandchild survival, so it increases gene transmission.

"Using artificial neural networks has allowed us to model allocation decisions in a quite realistic way, to observe the emergence of menopause under some conditions, to identify these conditions (i.e. allowing grandmothering and modeling the delayed benefits of investing in neural development), and thus to bring new knowledge on the evolution of menopause."

She says the biggest limitation is that their simulation only works for a single six (female) model. For a more complex model, both sexes need to be taken into account—and this is what she and her team plans to do next.

Evolutionary biologist Kevin Arbuckle, from the University of Swansea, U.K., commented on the study. He tells Newsweek the findings are interesting and use new methods to answer an old question.

"It will be interesting to see how these develop to incorporate other key parts of adaptive hypotheses, such as including the male life history in the models, and also to consider the role that non-adaptive explanations may play in the origin of menopause (if not its extended duration) as suggested in previous work," he says.