One of the most fascinating areas of research in evolutionary studies is the question of reproductive senescence. Why do women go through menopause? Chimpanzees and bonobos, our closest evolutionary relatives who we share 99% of our DNA with, are reproductive throughout their lifespans but human women can spend the last third of their lives infertile. Why?
Biologist Virpi Lummaa, whose recent work on evolutionary theory and birth control I wrote about earlier, has conducted numerous highly regarded studies verifying the role of natural selection in human populations. According to her research, menopause may in fact have helped our ancestors survive. However, the reasons for this may be a surprise.
One hypothesis to explain menopause is that humans didn't live as long as we do now with modern medicine and that, in our natural environment (i.e. the African savanna 100,000 years ago), women never would have lived long enough to reach menopause. It's true that modern indigenous people (many of whom live in environments similar to that of our distant African ancestors) have an average lifespan between 35-40. But the high infant mortality in these populations are skewing that statistic to make it seem that adults are dying younger than they are. Most modern hunter-gatherers, if they make it through childhood, will reach a ripe old age. So that can't be the reason for menopause.
Lummaa's work has looked at a key prediction of evolutionary theory, namely, that an individual's fitness will always be a trade-off between reproductive and somatic investment. In other words, the fitness differences (and by fitness we're talking reproductive success) of an individual that has numerous offspring at an early age versus focusing instead on their somatic interests (physical health and growth) in order to reproduce at a later time.
This is what is known as Life History Theory. Many species go through what is called an r-selection strategy and put all of their investment in as many offspring as possible, many of whom will never reach sexual maturity. Others take a K-selection approach and produce fewer offspring but help ensure that each one grows to reproduce. What's more, these strategies are not fixed but will frequently shift depending on available resources.
Utilizing life history theory, Lummaa suggests that reproductive senescence is actually an adaptation, and a fitness enhancing adaptation at that. So how exactly would cutting off 1/3 of an individual's reproductive potential result in higher reproductive success? Therein lies the grandmother hypothesis and a key contribution of Dr. Lummaa's work.
The grandmother hypothesis suggests that humans have "given up" their reproductive potential in later years in order to invest in the children they already have as well as their grandchildren. Naturally, this is an unconscious, biological adaptation that emerges over many generations and is not the result of individual decision-making. For such a hypothesis to be confirmed it would have to be demonstrated that children are significantly more likely to survive when a grandmother is present than when she isn't.
Dr. Lummaa has done just that in her study published in the journal Nature, demonstrating that children are 12% more likely to survive to adulthood when they have a grandmother's support than when they don't. That may not seem like a lot, but consider all of the descendants from that surviving 12%, each carrying the trait for reproductive senescence, and you can see how it wouldn't take long for the trait to become fixed in a population. Furthermore, one of the key innovations of her study was her choice of sample set. By using Finnish records dating from the 18th and 19th centuries she could ensure that any modern health benefits wouldn't influence the results and would therefore accurately pinpoint the grandmother's role.
In concluding their study Dr. Lummaa and her colleagues state:
[O]ur results lend strong support for the hypothesis that prolonged female post-reproductive lifespan is adaptive, to our knowledge revealing for the first time the substantial fitness benefits that females accrue by living beyond reproductive age.
Grandmothers just may have played a key role in the success of the human species. Given this, don't you think it's time you gave yours a call?
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