Life history trade-offs explain the evolution of human pygmies

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.

Fig. 1.

Growth curves for different human populations. Pygmy women [Aeta (black triangles), n = 214; Biaka (13) (gray triangles), n = 157; and Agta (14) (open triangles), n = 83] grow at rates bellow the 5th U.S. percentile (intermediate solid line), and their growth curves level off early at ≈13 years old, reaching an adult body size equivalent to the percentile 0.01 (lower solid line). African Pastoralists (16) [Massai (open squares) and Turkana (black squares), n = 228], also grow at slow rates compared with U.S. subjects but extend their growth trajectories and achieve final height equivalent to the U.S. 50th percentile (solid upper line).

Fig. 2.

Age-specific survivorship (a) and age-specific fertility (b) for different human pygmies. Pygmy populations: Eastern African Pygmies (3) (gray triangles, dashed line), Western African Pygmies (1) (gray triangles, solid lines), Aeta (3) (black triangles, solid line), Batak (3) (black circles, solid line), and Agta (21) (open triangles, solid lines). Non-pygmy populations: !Kung (19) (gray square, solid line), Ache (18) (cross, dashed line), Turkana (20) (black square, dashed line), and chimpanzees (open circles, solid lines).

Fig. 3.

Fitness as a function of size achieved at first reproduction. (Left) estimated relative fitness (r) as a function of age at first birth for Aeta (solid line). The histogram shows observed frequencies of first birth in Aeta women; the dashed vertical line shows the predicted fitness peak (age at first birth = 15 years). (Right) Models (adapted from refs. and 25) of relative fitness as a function of age and size at first birth for Ache, !Kung (18), and Gambian rural women (25).