You could argue that life is all about survival and
the contribution to the evolution of species through sexual reproduction. Human
reproduction can be very mystifying as our reproductive potential has evolved
to terminate early. At an average age of 38, women start to become rapidly less
fertile only to permanently lose the ability to reproduce some 10 years later
during menopause.This decline seems uncanny from an evolutionary point
of view. Fertility remains until close to the end of the lives of other
long-lived animals like elephants as they continue breeding until their 60’s.
In contrast, a human woman loses her child-bearing potential years or decades
before she passes away. Even in hunter-gatherer
societies that lack access to technology and modern medicine, women who pass
through menopause are expected to live well into their sixties.
Figure 1. Native Tribes of South America. Anonymous (2015).
Figure 1. Native Tribes of South America. Anonymous (2015).
A new model has been proposed by Michael Cant (University of Exeter) and Rufus Johnstone (University of Cambridge), in 2008, to explain the origins of menopause. They suggest that reproductive conflicts between successive generations of women is reduced due to the help of fertility loss.
Figure 2. Four Generations of Women. A.Prucha (2015)
To resolve this conundrum, a few theories have been put forward; with one of the most popular being the “grandmother hypothesis”, which proposes that post-reproductive survival is favoured by kin selection as relatives are helped by older females with reproduction (Johnstone & Cant. 2010). With some studies to back this up, this basic idea it’s clearly not the whole story. Some analyses of hunter-gatherer populations have found that the potential benefits of having more children yourself is not outweighed by the indirect advantages of helping your family. Alone, women continuing to live past menopause is explained by the grandmother hypothesis, however what isn’t explained is why women have evolved to go through it in the first place.
References:
Anonymous. (2015) Native Tribes of South America, http://www.angelhills.org/Pages/satribes.aspx;
Retrieved 20/04/2015.
Johnstone, R.A., & Cant, M.A. (2008).
Reproductive conflict and the separation of reproductive generations in humans,
105(14):5332-5336. Doi:10.1073/;pnas.0711911105.
Johnstone, R.A., & Cant, M.A.
(2010). The evolution of menopause in cetaceans and humans: the role of
demography, 277(1701):3765-3771. Doi: 10.1098/rspb.2010.0988.
Very interesting. I’m also curious why this evolved in the first place, particularly as you are always more closely related to your own offspring than your grand-offspring (Hamilton’s rule), therefore your inclusive fitness is always likely to be higher if you invest in your own offspring. The question I have is, is there any evidence that reproductive conflict between successive generations is actually high in humans?
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