Why pygmies?

Some human populations in South America, Africa, and Southeast Asia are of extraordinarily small stature: they are called pygmies.   All of them inhabit rainforests that are warm and humid.   Although they bear a common name, genetic work shows that each group has evolved independently, so it is better to speak of “the pygmy phenotype” (“phenotype” refers to any aspect of an organism that can be observed or measured).  A population is said to show the pygmy phenotype when the height of adult males averages about 160 cm (5′ 3″) or smaller. (The smallest pygmy population comprises the Efe hunter-gatherers of the Congo, where adult males and females are 143 and 136 cm tall respectively [4′ 8″ and 4′ 6″]).

A new paper in Trends in Ecology and Evolution summarizes what is known about the distribution, genetics, and evolutionary basis of the pygmy stature.  The first thing we learn is that we’re not absolutely sure if the height difference is due to genetic differences between pygmies and populations of “normal” stature.  Could it instead be due to differences in nourishment alone? The evidence is against this because pygmy populations show no clinical signs of malnutrition.  Still, other nongenetic environmental factors could be responsible.  The best way to show that the height difference is based on genes is, of course, to rear pygmy and non-pygmy infants in a common environment and show that the height difference remains; surprisingly, this has not been done (though surely there are some pygmy children brought up in different environments— the authors don’t discuss this).

Why are pygmies so short?  If the this phenotype is indeed genetically based, the obvious hypothesis is that natural selection in warm tropical environments causes humans to evolve smaller size. But why?  The authors give four hypotheses:

1.  There is a scarcity of food in the rainforest, and this selects for smaller individuals who are able to maintain their bodies with fewer calories.  The evidence for this hypothesis is mixed: pygmy populations don’t especially suffer a dearth of calories, although this may be due to their recent trading for food with other populations outside the rainforest.

2.  Living in a hot, humid environment selects for smaller bodies because smaller individuals have a higher ratio of body surface are to body volume. This allows them to lose, through sweating and heat transfer, relatively more heat than larger individuals.   The weakness of this idea is that among pygmy populations there is no correlation between body size and ambient humidity.

3.  It is easier to move through dense, tangled forest if you are small.  Bending down repeatedly while walking apparently uses quite a bit of energy.  There is only anecdotal evidence for this idea, but it may be true.

4.  If there is high mortality, then it may pay you to mature and reproduce early because otherwise you could die and leave no genes.  The authors note that infant mortality up to age 5 of  African rainforest poulations are 27-40%, about twice that of nearby populations that live in other habitats.  This idea predicts that among populations, there will be a positive correlation between life expectancy and average adult height.  This is indeed observed, providing some support for the idea.

Of course, all or some of these factors could work together.  At present, we have some fruitful ideas about why the pygmy phenotype evolved, but nothing definitive yet.  It is surprising that of the many differences in appearance between human ethnic groups– differences that involve skin color, hair configuration, facial configuration, height, body configuration, and physiology –the only one whose evolution we understand fairly well is pigmentation (see WEIT for the explanation, based on sun exposure).  The rest is mystery.  In my book I broach the idea that sexual selection may account for some of these, though it’s hard to explain the short pygmy phenotype this way (why would the target of sexual selection be correlated with humid, forested habitats?).

Note: Another population that was abnormally short were the “hobbits”: Homo floresiensis, a population (described as a species) of humans that lived on the Indonesian island of Flores. Again described in WEIT, this species is based on a single skull from one individual and an arm bone from another.  Their apparent height was about 106 cm (3′ 6″), they weighed about 50 pounds, and were the size of a modern 5-year-old child. They lived about 18,000 years ago, when H. sapiens of modern stature already lived throughout the world.  Although some think that the single tiny individual was really diseased and not a “normal” individual, it does appear from the arm bone that they really were this small.  But they had one feature not present in modern pygmy populations: very small brains, about half the size of modern human brains (the brains of pygmies scale roughly the same as short humans elsewhere).  H. floresiensis did not represent the pygmy phenotype, and were more likely an ancient population of a different species of Homo that, isolated on its island, was bypassed by the evolution of other populations into the modern human phenotype.

Here is a figure from Perry and Dominy’s paper showing the distribution of pygmy populations throughout the world (red dots) and some pictures of pygmy individuals:

Figure 1. Association of the human pygmy phenotype with tropical rainforest habitats. (a) Approximate locations of small-bodied hunter-gatherer populations discussed in this article, with mean adult male stature estimates [6], [10], [65], [78] and [79]. The smallest modern human statures (mean adult male height < 155 cm) are always associated with tropical rainforests (red circles). Some hunter-gatherer populations occupying savanna-woodlands (black circles) are also relatively small, such as the Hiwi of the Venezuelan llanos, the Hadza of Tanzania and the !Kung San of Botswana and Namibia. Precipitation data are from the Tropical Rainfall Measuring Mission (Goddard Space Flight Center, National Aeronautics and Space Administration; http://trmm.gsfc.nasa.gov). (b) Yanomamö male, Venezuela (photograph by Raymond Hames, with permission). (c) Efe male, Democratic Republic of Congo (photograph by William Wheeler, with permission from the National Anthropological Archives, Smithsonian Institution). (d) Batek male, Malaysia, with white-handed gibbon (Hylobates lar) hunted by blowdart (photograph by Kirk Endicott, with permission).

Photo and caption from Perry, G. H., and N. J. Dominy. 2009. Evolution of the human pygmy phenotype. Trends in Ecology & Evolution 24:218-225.

Research hot off the lab bench:  My friend Graham Coop at The University of California at Davis sends me this note that they may be zeroing in on the genes responsible for the pygmy phenotype:

We’ve just published a large scan for selection across various human populations: two of the of the top hundred SNPs [single nucleotide polymorphisms] whose allele frequency most differentiate Bantu populations from pygmy populations fall next to genes in the insulin growth factor signaling system (discussed on the bottom of page 6). Obviously these will require much more followup, but these seem like reasonable candidates for genomic regions habouring variation affecting height in pygmies.