The Evolution of Menopause

The "Grandmother Hypothesis" is confirmed in a new study


Grandmothers from Gambia are crucial to infant survival.

In my earlier post Life Lessons I discussed the grandmother hypothesis as an explanation for human reproductive senescence, or menopause. A problem arises in understanding why women forgo one-third (and sometimes as much as one-half) of their reproductive lives, a condition unique in the natural world. What selection pressure(s) could result in this unique human adaptation? One cause may have been that, since childbirth is risky in humans, menopause allowed older women to survive longer and better raise their existing children. Another possibility is commonly known as the grandmother hypothesis and argues that women who stopped ovulating in their golden years were freed from the costs of reproduction and were better able to invest in their existing children and grandchildren (thus helping to ensure that more individuals with their menopause inducing genes thrived and had children themselves).

One recurring problem with understanding the evolution of menopause is in how to test it. Unlike fruit flies or bacteria, humans are notoriously slow where reproduction is concerned. Finding a large enough data set to determine which scenario provides the best explanation is therefore difficult. However, in the early edition of Proceedings of the Royal Academy: Biological Sciences (subscription required), Daryl P. Shanley and colleagues utilized a remarkably complete and instructive data set from Gambia. The birth and death records date from 1950 to 1975 and include 5,500 people. Furthermore, since the records predate the existence of a medical clinic they offer a window into a world without the benefits of modern health care. What the data reveal is that children were significantly more likely to survive to adulthood if they had a grandmother’s assistance.

As reviewed in New Scientist:

The data revealed that a child was over 10 times less likely to survive if its mother died before it was 2 years old, but that children between one and two had twice the chance of surviving if their maternal grandmother was still alive. No other relatives had any effect.

Furthermore, by applying a mathematical model to the data to predict the optimum age of menopause (that which would provide women with the most time to assist dependent offspring) the researchers confirmed that menopause should be found just where it currently is.

If the menopause happened later, the population grew more slowly, or even declined. The optimum age for menopause was – as in real life – 50 years.

In our modern world grandmothers may spoil us with presents or candy only on special occasions. But accumulating evidence strongly suggests that grandmothers have been crucial to our very survival for much of human existence. So while women may take a direct fitness hit by reducing the total number of children they can have in their lifetimes, their sum fitness gain (through inclusive fitness) is ultimately higher as the result of menopause. Natural selection offers many strange, seemingly counter-intuitive adaptive strategies such as this (for another example see my post Brooding Angelmakers). However, such empirical explanations reveal the sublime beauty of the evolutionary dance and help us to appreciate the causative factors influencing who we are today.

Do men undergo reproductive senescence? For the answer see my post The Potent Fear of Male Menopause.

Reference:

Daryl P. Shanley, Rebecca Sear, Ruth Mace, Thomas B.L. Kirkwood (2007). Testing evolutionary theories of menopause. Proceedings of the Royal Society: Biological Sciences. 10.1098/rspb.2007.1028