Pygmies, life-history, and maturation

After writing in an earlier post about Rushton’s & Lynn’s theories of black precociousness, it occurred to me to write about Pygmies’ unusual maturation pattern.

Unlike many other groups for whom there is a suspicion of different genetic potentials for height, but no absolute evidence, for Pygmy’s the matter has been pretty confidently settled. Here’s a map of pygmy world distribution. The black circles are groups that are not technically pygmy’s, but are also fairly short (notice their close geographical proximity).

From: "Evolution of the human pygmy phenotype" (2008)

[Note: there is conflicting evidence on whether pygmy and nearby shorter peoples cranial capacity is lower than surrounding African groups (1,2)]

The evidence for a genetic cause to the pygmy height:

“…three lines of evidence suggest that this phenotype is determined principally by genetic, rather than environmental, factors. First, … genetic disruptions of the growth hormone (GH) and insulin-like growth factor I (IGF1) pathway are likely to have etiological roles … Second, the childhood growth rates of some rainforest hunter-gatherer populations are surprisingly fast… for at least some populations, small adult body sizes are a reflection of relatively slow growth in adolescence rather than childhood… which is inconsistent with a simple model of stunted growth from poor nutrition. Indeed, other populations that also endure frequent episodes of nutritional stress still achieve adult heights that are greater than those of rainforest hunter-gatherers… Third, the offspring of Efe mothers and Lese (agriculturalist) fathers have statures intermediate to those of the two parental populations…”
– “Evolution of the human pygmy phenotype” (2008)

[Update: a new study looking at the genomes of Pygmies and their Bantu neighbours found that, among Pygmy, having more Bantu ancestry was correlated to being taller.]

It’s also worth noting that the geographical distribution of this phenotype, across genetically diverse populations, makes it highly unlikely that genetic drift could be a factor. So then what drives the pygmy phenotype.

Various explanations have been provided: thermoregulatory, improved mobility (reduced metabolic costs), dietary limitations, and most recently, a life-history proposal of high death rates in young-adulthood (source).

Correlation between adult height and mortality in short-populations. (Evolution of the human pygmy phenotype, 2008)

The idea lying behind the life history account is that the high death rate amongst the young promotes early truncation of growth to permit earlier reproduction. A recent archaeological study of a Khoisan (not pygmy, but also small) skeletal record, however, does not support the thesis of increased juvenile mortality (source). However, they also did not find an early cessation of growth, which the earlier (Migliano, 2007) study did find, among actual pygmies. Likewise, the Migliano study did find a higher juvenile (and adult) death rate among the (contemporary) pygmy’s. So while the Khoisan study contradicts what one would expect, there is clearly differences between the two populations studied. Not surprising since they inhabit pretty different environments as well.

[In any case, fortunately for the Pygmy’s, height preferences in a mate may not be universal.]