In the rainforests of Africa, being small is so helpful that it evolved twice in separate groups.
Luis Barreiro of the University of Montreal in Quebec, Canada, and his colleagues identified 16 regions of the genome associated with short stature in the Batwa pygmies of Uganda. They then compared these regions in 169 Batwa and 74 Baka pygmies from West Africa.
“In both groups, there was greater variation in those regions associated with being short, but no overlap between them,” says Barreiro. This suggests they evolved their stature independently instead of inheriting the same “pygmy genes” from a common ancestor.
Pygmyism clearly offered an evolutionary advantage, says Barreiro. He thinks it might be because pygmies tend to live almost exclusively in the dense rainforest. “If you’re short, you find it much easier because you’re bending much less to duck under branches,” says Barreiro. His team followed pygmies in forests in the Philippines and Uganda. “The shorter ones bend over less, sweat less and thereby expend less energy.”
Or it could be that rainforests are so dangerous that people need to reach sexual maturity earlier to be sure of reproducing, says Oscar Lao of Erasmus University Medical Center in Rotterdam, the Netherlands. That would cause them to stop growing earlier, reducing average height.
Journal reference: PNAS, DOI: 10.1073/pnas.1402875111
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