Mechanisms of development and body size

I'm just doing some background reading about the body size of pygmies (for both obvious and not-so-obvious reasons) and I thought it worth making a note of this quote, from last year's paper by Andrea Migliano, Lucio Vinicius, and Marta Lahr:

Finally, the data presented here show that pygmy body size evolved through earlier cessation of growth, being therefore the result of changes in late rather than early stages of growth. This explains why brain growth, which is completed years before the onset of adolescence (28), is not affected in human pygmies (29). Therefore, if Homo floresiensis is a dwarfed form of Homo erectus, as proposed in ref. 29, the evolution of small body size on Flores could be understood as the life history consequence of ecological conditions in islands, such as increased extrinsic mortality rate and reduced resource availability (30); however, its small brain size and low encephalisation require the postulation of different adaptive mechanisms affecting earlier stages of development.

That's the concluding paragraph of what is a very nicely-done study of mortality and fertility in pygmy populations. It came out the during the acceleration press flurry in December, so I wasn't able to write it up at the time. It's certainly worth doing so, though.

The paper proposes that pygmy human populations are small because of a life history tradeoff. A "tradeoff" is the idea that a phenotypic change in either direction may have advantages and disadvantages, and selection may arrive at different optima in different populations.

In the case of life history and body size, both growing longer (and larger) and maturing faster (and smaller) have possible payoffs. Growing longer may have a fertility payoff, as larger size facilitates larger infants and shorter birth intervals. But maturing faster has a direct payoff of shortening the generation length -- all other things equal, an individual improves her fitness by reproducing younger.

So either younger or older maturation may enhance fitness, in some circumstances. Which will work in any given population depends on other factors -- in particular, the mortality pattern. If individuals have a high risk of death in early adulthood, delaying reproduction will be a bad strategy. In short, individuals should reproduce at 16 (or earlier) if there is a fair chance they will be dead by 25 or 30.

Naturally, everyone would rather live longer. But assuming that people can't control when they die, the only way to insure their fitness is to reproduce earlier.

This hypothesis, presented by Migliano et al., is about the proximate mechanism of evolution. The authors seem content to rely on traditional hypotheses about locomotion, nutrition, and thermoregulation to explain the ultimate causes of small body size -- "ultimate" in the sense that these may be the environmental causes of high mortality:

If our hypothesis is correct, the causes of the extremely high mortality rates among human pygmies need to be explained. It is here that the traditional hypotheses explaining the small body size of pygmies may prove useful. 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, as pointed out by Diamond (5), they may jointly or partially contribute to the similarly high mortality rates in unrelated pygmy populations. We argue that the small body size of African and Asian pygmy populations evolved independently as a case of evolutionary convergence, resulting from a life history tradeoff between the fertility benefits of larger body size and the costs of late growth cessation under the circumstance of significant young and adult mortality.

The demographic data presented in the paper are sobering -- particularly the low survivorship values for pygmy populations across late childhood and early adulthood. However, I wonder how much of the early adult mortality in the pygmy demographic data is attributable to new pathogens. These are certainly important today, but they would not have been during most the time that small body size was being selected in these groups. On the other hand, ancient endemic pathogens and parasites also may contribute to those mortality numbers, and these might well have occurred at higher intensities in forest peoples across their histories.

References:

De Souza R. 2006. Body size and growth: The significance of chronic malnutrition among the Casiguran Agta. Ann Hum Biol 33:604-619. doi:10.1080/03014460601062759

Migliano AB, Vinicius L, Lahr MM. 2007. Life history trade-offs explain the evolution of human pygmies. Proc Nat Acad Sci USA 104:20216-20219. doi:10.1073/pnas.0708024105