Short people known as pygmies are scattered across equatorial Africa, where they speak various languages, inhabit different types of forests, and hunt and gather food in diverse ways. Despite their cultural variety, a new study shows that the pygmies of Western Central Africa descended from an ancestral population that survived intact until 2800 years ago when farmers invaded the pygmies' territory and split them apart.
The origins of pygmies have long been a mystery. Researchers have debated whether African pygmies inherited their height from a common ancestor they shared long ago or whether shortness evolved independently in each tribe because it was advantageous for life in the forest. For instance, getting enough calories to grow taller might have been more challenging than in more open terrain. Pygmies grow up just like other modern humans until they become teenagers, when they fail to undergo a final adolescent growth spurt.
Although humans have lived in the forests of Western Central Africa for at least 30,000 years, there are no fossils to show whether the ancestral population was short to begin with--or whether the trait evolved more recently in different groups. Previous DNA studies haven't resolved the question.
In the largest study of Western Central Africans to date, anthropological geneticist Paul Verdu of the Musee de l'Homme (Museum of Man) in Paris and colleagues analyzed DNA from nine pygmy groups and 12 neighboring groups of people who were of normal height. The researchers report in this week's issue of Current Biology that although the pygmies have a lot of genetic diversity, they probably can trace their ancestry to the same population that could have lived as recently as 2800 years ago, says Verdu.
In the most likely scenario, a small group of short people split off from nonpygmy populations between 50,000 and 90,000 years ago. The founding group of pygmy ancestors was fairly cohesive, with tribes interbreeding until 2800 years ago. At that point, taller Bantu-speaking farmers probably swept across central Africa and pushed them apart. Once the pygmy groups split, they stopped interbreeding. As a result, each group evolved separately. Even today, they seldom know of each other's existence, says Verdu.
The study also detected an unusual pattern: More DNA flows from nonpygmy neighbors into pygmy populations than the other way around. This is curious because pygmy women tend to marry nonpygmy men and move to their homes, not vice versa. But these marriages often fail because of discrimination against low-status pygmy wives, Verdu says, and the pygmy women return to their pygmy groups with children who have DNA from their taller fathers.
The data that the pygmies shared a recent ancestor, however, are convincing, because "they genotyped an impressive number" of pygmies from Western Central Africa, says molecular anthropologist Sarah Tishkoff of the University of Pennsylvania. Now, says Tishkoff, the next step is to figure out if East African pygmies are also descended from the same ancestral population.