A Batwa woman and her child, pygmies who live in the rainforest, shown here in Bwindi Impenetrable Forest National Park, Uganda.
Credit: George Perry
Pygmy traits independently evolved many times among different peoples around the world, because shorter heights may have helped them live in rainforests, researchers say.
The small body sizes known as pygmy traits are seen worldwide, limited to peoples who traditionally hunted and gathered food in tropical rainforests, such as in Central Africa and Southeast Asia. These small statures apparently developed independently in these populations, an example of convergent evolution, much as fish and dolphins both evolved streamlined bodies to better swim in their watery worlds.
Scientists have suggested that small body size might confer a number of evolutionary benefits for life in rainforests. For instance, while tropical rainforests are the most complex and diverse ecosystems on land, home to half of all living species on the planet, "there is actually not that much food for humans," said lead study author George Perry, an anthropologist at Pennsylvania State University in University Park. Small body sizes, therefore, may have evolved because they require fewer calories. [The Awa: Faces of a Threatened Tribe (Photos)]
In addition, taller individuals have more difficulty moving through the dense vegetation of tropical rainforests. Moreover, these forests are hot, and their humidity makes it difficult for sweat to evaporate and cool people down. Since small bodies generate less heat during activity, they could survive more easily.
However, it remained uncertain whether pygmy traits actually confer evolutionary advantages in tropical rainforests. To find out, Perry and his colleagues analyzed the genomes of 169 pygmy members of the Batwa people, rainforest hunter-gatherers from Uganda in east central Africa.
When the researchers compared Batwa DNA with the genomes of 61 members of the neighboring agricultural Bakiga population, they found the Batwa had genetic variations linked to stature and growth hormone. Batwa stature was also linked with how much Bakiga ancestry the individuals had — the less Bakiga ancestry they had, the more likely Batwa had pygmy statures — suggesting a genetic component to Batwa stature, rather than an environmental influence.
Additionally, the researchers discovered that the Batwa genetic variations linked to stature and growth hormone evolved quickly compared with the normal rate of mutation in the human genome. This suggests these genetic variations are somehow adaptive, proving beneficial to the Batwa.
The scientists also investigated the genomes of 74 pygmies from Baka rainforest hunter-gatherers who lived in Cameroon and Gabon in west central Africa, comparing this DNA with genomes from 73 members of the pygmies' agricultural neighbors. Researchers found pygmy traits apparently had different genetic roots in the Baka pygmies than in the Batwa pygmies, suggesting convergent evolution.
Perry cautioned that the number of Batwa individuals studied would probably not meet the standard or minimum for genetic analyses of traits that depend on many genes, such as height. However, "in the entire Batwa population, there are not as many individuals as are in many of these studies," Perry said. The scientists say the fact that the genetic variations they analyzed are linked with growth hormones and stature in other groups suggests at least some of these mutations underlie pygmy traits.
In the future, Perry and his colleagues plan to investigate other ways besides stature in which rainforest hunter-gatherer populations have adapted to their environment. "In addition, I would like to expand these studies to rainforest hunter-gatherer populations in Southeast Asia, in order to study the extent to which any such adaptations like height have also evolved convergently across continents," Perry said.
The scientists detailed their findings online Aug. 18 in the journal Proceedings of the National Academy of Sciences.