Science Literature

The human race, with its apparently inexhaustible variants, never ceases to engage our attention and fascination. Explaining the diversity, however, is not at all trivial, with academic controversy never far away.
Take, for example, explanations of the origin of human pygmies. "Traditional hypotheses assume that the small body size of human pygmies is an adaptation to special challenges, such as thermoregulation, locomotion in dense forests, or endurance against starvation." Since most pygmies live in the tropics, small size is perceived as an adaptive response to avoid overheating. An alternative explanation is that most pygmies live and hunt in jungles, where small size assists moving through undergrowth to catch prey. The third option is to emphasise the potential for scarcity of food supply for hunter gatherers, where small bodies have the advantage of needing less food. These are all Darwinian 'just-so stories' which are defended by creating various virtual scenarios around the favoured causal mechanism. However, say the authors of a new study:

"None of these explanations account for the worldwide distribution of human pygmies - some pygmy-sized populations are found outside forests, and many live in cool and dry areas; furthermore, long-standing poor nutrition does not necessarily lead to pygmy size, as shown by groups who, like certain pygmies, experience frequent food shortages and yet are among the tallest populations in the world."

Last year, Robert Walker, Andrea Migliano and others showed that environmental factors appear to be affecting growth rates of small human social groups. Some tribes exhibit fast child-juvenile growth, early puberty and first reproduction, and have lower overall life spans. This has now been followed up (with Migliano as lead author) to present environmental factors as crucial for the origin of pygmies.

"We argue that human pygmy populations and adaptations evolved independently as the result of a life history tradeoff between the fertility benefits of larger body size against the costs of late growth cessation, under circumstances of significant young and adult mortality. Human pygmies do not appear to have evolved through positive selection for small stature - this was a by-product of selection for early onset of reproduction."

In other words, the short pygmy stature is a spandrel. The real factors driving miniaturisation are related to significant mortality rates in young and adult members of a tribe. A report in The Economist expresses this very clearly:

"By adding pre-existing data for African pygmies to new information they have collected about the Aeta and the Batak of the Philippines, they show that at the beginnings of their lives all these pygmy populations follow the same growth curves as taller people, including Turkana and Americans. This demonstrates that pygmyism is not a result of early malnutrition, as another hypothesis has it. At the age of about 12, however, pygmies stop growing. That is also the age at which they become sexually mature - about three years earlier than taller people." [snip]

The argument is that "a short life exerts pressure to mature early, and thus switch resources from growth to reproduction. A mathematical model used by the team confirms that, given pygmy life expectancies, their growth and reproduction patterns have indeed been optimised by natural selection. The various pygmy groups are thus the products of harsh circumstances."

Two points are worth highlighting here.
1. The temptation for Darwinists to find an adaptationist story is very strong, but proposing a plausible story is not in itself science. Most of these 'just-so' stories thrive on limited data, and more rigorous analyses reveal frequently that these imaginative scenarios are just illusions. For more on this, go here.
2. Although the terms "evolution" and "evolutionary change" are liberally used by writers, there is no support (in this case) for the Blind Watchmaker theory of evolutionary transformation. What we have here are changes in growth rates and reproduction patterns which add nothing to biological complexity. Whereas the rest of mankind exhibit reversible traits relating to size, the onset of puberty, etc, pygmies have undergone permanent genetic change. Most probably, this represents a loss of genetic variability within the population. The Economist report has the title "Darwin's Children", but this is misleading. Rather than demonstrating the branching pattern associated with Darwin's vision of life's unfolding, the pygmy tribes are telling us that harsh circumstances have forced a tradeoff with a significant genetic cost. These evidences of variation are by no means the exclusive domain of Darwinism. Journalists, by and large, appear unable to put the record straight. Consequently, it is long overdue for the community of scientists to put its own house in order about the e-word. For more recent examples of this being a significant problem, go here and here and here.

Life history trade-offs explain the evolution of human pygmies
Andrea Bamberg Migliano, Lucio Vinicius, and Marta Mirazon Lahr
Proc. Natl. Acad. Sci. USA, week of December 10, 2007 | 10.1073/pnas.0708024105

Abstract: Explanations for the evolution of human pygmies continue to be a matter of controversy, recently fuelled by the disagreements surrounding the interpretation of the fossil hominin Homo floresiensis. Traditional hypotheses assume that the small body size of human pygmies is an adaptation to special challenges, such as thermoregulation, locomotion in dense forests, or endurance against starvation. Here, we present an analysis of stature, growth, and individual fitness for a large population of Aeta and a smaller one of Batak from the Philippines and compare it with data on other pygmy groups accumulated by anthropologists for a century. The results challenge traditional explanations of human pygmy body size. We argue that human pygmy populations and adaptations evolved independently as the result of a life history tradeoff between the fertility benefits of larger body size against the costs of late growth cessation, under circumstances of significant young and adult mortality. Human pygmies do not appear to have evolved through positive selection for small stature - this was a by-product of selection for early onset of reproduction.

See also:

Walker, R., et al. Growth Rates and Life Histories in Twenty-Two Small-Scale Societies, American Journal of Human Biology, 18:295-311, 2006.

Darwin's children, The Economist, Dec 13th 2007