"If the misery of the poor be caused not by the laws of nature, but by our institutions, great is our sin."
- Charles Darwin

Jul 18, 2007

Life Lessons

How the grandmother hypothesis hits the nail on the head.

Navaho grandmother with her granddaughter in Monument Valley.

The geniuses over at the Discovery Institute have now taken to denying evolution by denying that studies showing evolution in action have any validity (because they deny evolution, see?). All they really demonstrate is their lack of understanding for what they oppose a priori.

Responding to an article in Scientific American, Bruce Chapman (President of the Discovery Institute and former Deputy Asst. to Ronald Reagan) impugns the work of respected biologist Virpi Lummaa and asserts that:

when all you’ve got is a theoretical hammer, every study is a nail. Once again, we have biologists desperately seeking relevance and self-worth.
In point of fact, Dr. Lummaa has conducted numerous highly regarded studies verifying the role of evolutionary theory in human populations. Her 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.

One of the most fascinating areas of research in life history theory is the question of reproductive senescence. Why do women go through menopause? Chimpanzees and bonobos, our closest evolutionary relatives, are reproductive throughout their lifespans but human women spend the last third of their lives infertile. Why?

One hypothesis 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.

Another hypothesis is 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 (note to the Discovery Institute: 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 be pervasive. 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 won'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.
So to answer Mr. Chapman about his seeming lack of interest in genuine discovery, this research is indeed evolutionary biology because it makes specific predictions based on evolutionary theory that are subject to disproof. A good carpenter will construct a good house and Dr. Lummaa is among the best. Perhaps if the Discovery Institute put its time and resources into generating testable predictions for its own hypotheses rather than merely criticizing without understanding they wouldn't be so bitter about the poor shelter they've constructed. Its abundantly clear in this case that, not only did their own hammer miss the nail, they smashed the dickens out of their thumb.

Go see Dr. Lummaa's response to this post click here.

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