A little while ago I ran across an interesting, if rather sad, item in the Physorg.com daily newsletter, having to do with the small stature of pygmies. Previous notions had been that having such wee bodies better adapted them to food shortages, or to moving about in dense forests, but neither of these explanations has held up well.
Now a group of researchers has come up with a likelier hypothesis: pygmies live very brief lives, and therefore must rush to reproduce.
Because of their short life expectancies, the researchers speculate that pygmies have had to shift their reproductive years forward. The average life expectancy at birth for different pygmy populations ranges from just 16 years to 24 years. Very few pygmy women reach the end of their reproductive period, as only a small percentage survive past age 40.
In order to compensate for the lack of older reproductive women, natural selection has shifted the reproductive period forward. The fertility peak of age at first reproduction in the Aeta is around 15 years old, which reduces generation time and compensates for their short lifespan.
In order to make this fertility shift, pygmies must reach full maturity faster than longer-lived human populations. For this reason, many pygmies stop growing at about age 12, several years earlier than other humans. Their childhood growth rate isn’t any more or less rapid than the growth rate of other (traditional) humans; pygmy youth are roughly the same size as non-pygmy youth. (This is the opposite of what is observed in cases of nutritionally induced stunting, where humans delay growth but achieve adult body size later). Instead of experiencing the “teenage growth spurt,’ pygmies’ growth is simply truncated.
Average life expectancy at birth is just 16 to 24 years. I realized that all of our days are numbered ((Woody Allen has wondered why they are numbered rather than lettered, but that’s beside the point here.)), and that life is short, but this seems cruelly brief; barely enough time upon life’s stage to get any strutting or fretting done at all.
Anyway, once you’ve blinked back a tear for these abbreviated aboriginals, you will probably ask yourself, as I did: “Well, that’s an interesting explanation of their size, but then what makes them die so young in the first place?” According to the article:
Still, the life history hypothesis leaves a few unanswered questions. For one, what originally caused the extremely high mortality rates among pygmies? The researchers suspect that the traditional hypotheses of environment, nutrition, thermoregulation, and other challenges may jointly or partially contribute to the high mortality rates observed in a wide variety of pygmy populations. In that case, the traditional explanations may be indirect causes of pygmies’ short stature, although the chain of effects would be much more complex than originally thought.
Learn more here.
2 Comments
Is there any reason to believe that pygmy lifespans are all that much shorter than any others in aboriginal, pre-historic, tribal societies? Given the time frame of evolution, that’s the relevant contrast–and while I have only the vaguest of notions about the numbers, my impression is that none of us (or our ancestors, rather) were all that long-lived s few thousand yeaars back. But I could be wrong–often am. Or do we even have any relaible idea of average lifespan that far back?
I think the point is that pygmy lifespans are short compared to other current populations that live in the same way, and that the height and the lifespan appear to be correlated.