Monday, February 27, 2006

Life outside of kids II: Great grandma

This is the second one of about four posts dealing with the grandmother effect. Menopause, and the relatively extended life in human women after the cessation of reproduction, distinguishes humans from their primate relatives and possibly earlier hominins. The "grandmother effect" proposes that postreproductive women can nevertheless contribute to the propagation of their genes, specifically by helping their juvenile grandchildren reach adulthood. The first post talked about searching for the grandmother effect in other species. This post will talk about the evidence for the grandmother effect in humans and some speculation about the evolutionary mechanics.

It should be pointed out right away that the grandmother effect really does refer only to post-menopausal women. Men survive for similar lengths of time, but retain their fertility. This concept is only invoked to explain how the survival of the human species might have been enhanced by the presence of vigorous, non-reproductive women.

Strong evidence for grandmothers' contributions to survival of their grandkids came in a 2004 demographic study of 19th century Finnish and Canadian villages. In new families with a surviving mother-in-law, the newlyweds had their first child sooner, had more closely spaced kids, and those kids made it to adulthood with greater frequency. The Finnish records were detailed enough to show that the grandma's impact was greatest during the early post-weaning years, and might correspond to feeding and helping with young kids. So it's pretty clear that, in modern human societies, a grandma is a big help.

Comparative biology and the fossil record both suggest that "senior citizens" are a bigger proportion of human than non-human primate demographics. Among modern-day primates, the ratio of lifespan to age at maturity is relatively constant.Humans are at the far end of this relation, being very long-lived (thus with lots of surviving seniors) and very slow to grow up (both physically and cognitively). This special place on the curve is also true of hunter-gatherer societies.
A similar picture comes from the fossil record. By looking how worn-out the teeth are, especially the late-erupting 3rd molars (wisdom teeth), you can classify fossil jaws as belonging to a young adult or a "senior." Scientists who scored many fossils with this approach found the percentage of seniors keeps increasing in the hominin line, reaching a peak in upper paleolithic modern humans. It's important to keep in mind, though, that this analysis does not give the gender of the senior.

Putting it all together, the grandmother effect concept suggests that as human ancestors started taking longer to develop, it became advantageous for somone else with free hands to help. The increased number of humans surviving into old age would be a combination of better success of the human lifestyle and, perhaps, active selection for a vigorous grandma.

Another, gender neutral, factor which might have contributed to human longevity would be the complexity of the human lifestyle and the value of accumulated wisdom. This whole issue gets very interesting in the case of the Neanderthals, who probably grew up very fast but who had reasonable numbers of elders, And who might have cared for their elderly into their dotage. Neanderthal grandmas might have had a much less direct impact on the survival of the fast-growing grandkids, but the wisdom of the ages would still have been valuable.

UPDATEs: The Oscars and human evolution?

Also, I had this bookmarked but didn't get to it: the idea (from fossilized teeth)that Neanderthals grew up faster than H. Sapiens is disputed.

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