Most of us think that we were never meant to live on for very long after the menopause and that we are only doing so thanks to modern medicine and science. But that is a mistake. Women evolved to have a menopause, and therefore the postmenopausal years must have offered the early humans a survival benefit.
The most popular theory to explain this is the “grandmother hypothesis”. There are also a few other theories, such as the “prudent mother -” the “absent father-” and the “patriarch hypothesis”. None of these theories is good enough on its own to explain why the menopause evolved, but they complement each other and the real reason is probably a combination of them.
Not just a by-product of a longer life
The menopause cannot be just a by-product of a longer life, because why would the reproductive system stop while the rest of the body is happy to carry on? Many women live for half of their adult life in the menopause without any health problems.
Previously, life expectancy was so much shorter because most people died in their childhood. It is therefore a very misleading statistic. Women who were lucky enough to survive the early years could live long enough to become postmenopausal, and studies of contemporary hunter-gatherer groups show that even without modern medicine 10% of the individuals survive until over 70.
It is true that today more women live to old age than ever before, but this does not mean that our lives are artificially extended.
The grandmother hypothesis
The idea is that women stopped making babies to help raising their grandchildren. If a mother was busy looking after her youngest child, the older siblings could lose out unless a grandmother took care of them. This allowed younger women to have more children.
In modern hunter-gatherer groups, only half of the food comes from hunting. Fruit, leafs and root gathering is thus very important. It requires knowledge and skills which a child is unlikely to have, but it can easily be done by a grandmother.
This system made a long childhood much safer, and gave youngsters also the opportunity to learn and experiment before they had to look after themselves. As experience and knowledge were early human’s main advantages over other animals, it must have helped them to survive.
Some researchers however, maintain that to improve the survival of the group, helping out cannot compensate for having more children and that there must have been additional reasons for the menopause to be an advantage.
The prudent mother hypothesis
As every mother knows, being pregnant, giving birth and looking after a baby is physically very hard. Women who did so too often died early, especially as there was no modern technology or medicine, and without a mother a small child would not survive. As the risk of dying increases with age, it was safer for older women to stop making babies and to concentrate on the survival of the existing ones.
Being a parent is much harder for a woman than for a man, and men could therefore continue to reproduce.
The importance of the father
Men died often earlier than women. A mother could get a new husband, but maybe he would care less for the children she had with a previous man than for his own.
Without a man to provide food and shelter for the family, the situation was even worse.
The safest option was thus to prevent women from making further babies.
The patriarch hypothesis
Success at hunting became slowly more and more dependent on skills, experience and the quality of tools or weapons. Older men could be more efficient hunters than younger ones and, as women of all times prefer mates who bring home a lot of food, their status increased.
Successful older men could have more women, father more children and pass their longevity genes on. Both sexes slowly increased their life span. However, as women have a limited number of oocytes in their ovaries, their reproductive life could not increase and they became postmenopausal as they grew older.
There are two conditions to this theory:
1) Selection did not favour a larger supply or a more careful use of the oocytes.
2) Longevity genes are not situated on the Y chromosome.
As yet, these conditions are not proven and future research and discussions will further clarify more aspects of the nature and evolution of human menopause.
References & Further reading:
M A Cant, R A Johnstone. Reproductive conflict and the separation of reproductive generations in humans. Proc Natl Acad Sci USA 2008 April 8; 105 (14): 5332-5336
K Hawkes, J F O’Connell, N G Blurton Jones et al. Grandmothering, menopause, and the evolution of human life stories. PNAS 1998 February 3; 95(3): 1336-1339
R U Johnstone, M A Cant. The evolution of menopause in cetaceans and humans: the role of demography. Proc Biol Sci 2010 December 22; 277(1701): 3765-3771
B X Kuhle. An evolutionary perspective on the origin and ontogeny of menopause. Maturitas 2007 Aug 20; 57(4):329-337
M Lock. Menopause: lessons from anthropology. Psychosomatic Medicine 1998; 60 (4): 410-
419
F Marlowe. The patriarch hypothesis. Human Nature 2000; 11(1): 27-42
D J Penn, K R Smith. Differential fitness costs of reproduction between the sexes. Proc Natl Acad Sci USA 2007 January 9; 104 (2): 553-558
D P Shanley, T B Kirkwood. Evolution of the human menopause. Bioessays 2001 March 23(3): 282-287
D P Shanley, R Sear, R Mace. Testing evolutionary theories of menopause. Proc Biol Sci 2007 December 7; 274(1628): 2943-2949
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