SHORT ON STATURE (AND LIFE): The Aeta and other pygmies have the highest mortality rates among all human populations; their small body size evolved as a life history consequence of early death.
Image: COURTESY OF RODOLPH SCHLAEPFER
-
Why Is the Penis Shaped Like That?
Why do testicles hang the way they do? Is there an adaptive function to the female orgasm? What does it feel like to want to kill yourself? Does “free will”...
Read More »
Pygmies, the most well-known group of diminutive humans, whose men on average grow to a maximum of five feet tall and their women about a half foot shorter, were thought to be endowed with their characteristic small body sizes due to poor nutrition and environmental conditions.
But the theories did not hold up, given that these populations—primarily hunter–gatherers—are found mostly in Africa but also in Southeast Asia and central South America, and thereby are exposed to varying climates and diets. Further, other populations who live under conditions of low sustenance, such as Kenya's Masai tribes, are among the world's tallest people.
So what could account for these pockets of people who grow so small?
According to University of Cambridge researchers, the key is the pygmies' life expectancy. "After going to the Philippines and interviewing the pygmies, I noticed this very distinctive feature of the population: very high mortality rates," says Andrea Migliano, a research fellow at Cambridge's Leverhulme Center for Human Evolutionary Studies and co-author of a new study published in Proceedings of the National Academy of Sciences USA. "Then, going back to life history theory, we noticed that their small body size was really linked to high mortality."
Migliano and her colleagues began their study by comparing the growth rates of two Filipino pygmy groups (the Aeta and the Batak) with data from African pygmies as well as from East African pastoralist (livestock-raising) tribes like the Masai and the lower echelon of the U.S. growth distribution (in essence, malnourished Americans). All these groups have low nutritional status but reach significantly different average height levels. The U.S. population showed the greatest growth rate, whereas both the pygmies and African pastoralists lagged behind. Although the pygmies plateaued around 13 years of age, the pastoralists kept growing, reaching their cessation point into their early twenties. Because the pygmy growth rate approximated the taller pastoralists, but had an earlier end point, the researchers concluded that their growth was not nutritionally stunted.
The group next examined the incredibly low life expectancy of different pygmy populations, ranging from roughly 16 to 24 years of age. (Pastoralists and other hunter–gatherer populations experience expectancies that are nearly one to two decades longer—a number that is still low, especially when compared with the 75- to 80-year life span expected of Americans.) Pygmies also reach their age of last reproduction a few years earlier than their taller counterparts, although there are many more pastoralist women than pygmies who reach this age at all.
Looking at fertility curves, the researchers noted Aeta appeared to reproduce on average when they were around 15 or 16 years old, which is about three years earlier than other hunter–gatherers. The tallest of these populations actually appeared to reproduce the latest. By having an early onset of reproductive abilities, the scientists say, the pygmies appear to trade off time spent growing, allowing them to continue on in the face of low life expectancy.
"Although the challenges posed by thermoregulation, locomotion in dense forests, exposure to tropical diseases, and poor nutrition do not account for the characteristics of all pygmy populations," the authors wrote, "they may jointly or partially contribute to the similarly high mortality rates in unrelated pygmy populations."
This research centered around women, but Migliano expects an analysis of males to mirror that of females, partly because the fertility of one would affect the other. Further, life history theory is anchored to the female because of the importance of reproduction as a variable. She adds that this paradigm could be used to help better understand the evolution of Homo floresiensis, the so-called "hobbit" found on the Indonesian island Flores in 2003.
See what we're tweeting about
9 Comments
Add CommentIt is dubius that reproducing first at 15 or sixteen is earyly reproduction compared to most historic populations.
Reply | Report Abuse | Link to thisHaving populations normally living to be 75 or 80 is purely a recent event due to better medicine and a higher standard of living than was at all common in the past.
The basic thought may be right but I'm not sure they proved it. I'd say they put forward an idea that might be fairly hard to prove to be a historical fact for long enough to explain the height of populations.
Interesting theory but I need a few more facts before I buy all of the assumptions asserted by these researchers. I have read a lot about Homo floresiensis and I talk about the find at my www.Floresgirl.com site but this is the first time mortality has been raised as the driving issue in the evolution of their dimunitive stature. It would be interested to compare mortality rates of other mammals that have been down-sized.
Reply | Report Abuse | Link to thisAs to lies, the greatest lies are the ones coming from men who claim to be speaking for God!
Erik John Bertel
Might this account for the apparent slower aging process of individuals who mature late sexually?
Reply | Report Abuse | Link to thisTo Fisher; do NOT confuse faith with fact!!
Reply | Report Abuse | Link to thisLiving at the margins of productive land as a survival strategy appears to have a big toll. An extreme example of the worst jobs having the lowest pay. Aside from considering the evolutionary mechanism in action, I'm saddened by the fate of these people. As for Fisher, even the dim bulb should recognize this article draws on, rather than being intended simply to support mechanisms of evolution. Scary part is how fast creationists reproduce.
Reply | Report Abuse | Link to thishey fisher , how do you know evolution is a lie??how do you know if the world is round or even flat. Have you read any of the science or are you too blinded by your mythogical based belief system to investigate the truth. Gods language is numbers. Learn math and you will be closer to god than you could ever imagine.
Reply | Report Abuse | Link to thisIf comparatively smaller body size would be related to lower life expectancy or mortality rates, this relation should also be observed among other animal species (such as dogs, for instance, to stay near home), but this is certainly not the case. I think it is rather an evolutionary adaptation to the jungle environment, where a smaller body size allows a better displacement through the thicket. This is probably also the case among jungle elephants en rhinos, for instance, which are both smaller than their counterparts from the African savannas.
Reply | Report Abuse | Link to this--
Edited by R. A. Gerrits at 12/16/2007 5:06 AM
--
Edited by R. A. Gerrits at 12/16/2007 5:08 AM
Presumably, this result should only apply to larger animals that must make a significant investment in growing to a large body size. Even domestic dogs live ~15 or so years anyhow.
Reply | Report Abuse | Link to thisAn interesting article. I have read many explorer and hunter accounts in which pygmies had described taller peoples as being "clumsy", in the forrest. There may be many hidden benifits to a smaller stature.
Reply | Report Abuse | Link to this