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African rainforest hunter-gatherers are among the smallest humans on the planet. Adult men rarely exceed 1.5 meters tall, about a quarter-meter shorter than the global average. Now, the largest ever genetic analysis of this group may have fingered the gene responsible—and settled a mystery that has vexed scientists for decades.
Once called “Pygmies” by outsiders, African rainforest hunter-gatherers live in densely forested environments across Central Africa. Their way of life includes gathering wild fruits and vegetables, fishing, and hunting monkeys and antelope. Their most striking physical characteristic is their relatively short stature (The name “pygmy” is derived from the ancient Greek word for “dwarf.”)
Some anthropologists have speculated that the group’s small body size gave them an advantage in Africa’s spectacularly hot, humid rainforests. Put simply, there’s less body to cool down, be it by sweating or other means. But other scientists say their stature may be just an accident. People in African rainforests have long battled numerous infectious diseases—including hepatitis B and C—and the genes this group evolved to help protect them have been linked to reduced levels of growth hormones.
To settle the debate, researchers at the Pasteur Institute in Paris consulted existing DNA databases built from blood or saliva samples given by nearly 300 African rainforest hunter-gatherers from Cameroon, Gabon, and Uganda, as well as nearly 300 people from neighboring agriculturalist groups who live outside the rainforests. The researchers report that the donors gave their informed consent to use their DNA in these studies.
They ran the data through a computer algorithm that calculates whether it’s more likely that particular snippets of DNA arose by pure chance or through natural selection. What they found surpassed their expectations: All the hunter-gatherer populations showed a strong signal of selection within a short stretch of DNA on chromosome 8, and all the agriculturalists lacked this signal. This genetic region helps regulate a gene called TRPS1, which plays an important role in skeletal development. That suggests natural selection specifically favored short stature in this group, the authors say.
Further analysis revealed additional strong signals of natural selection in genes unrelated to height that code for proteins thought to protect against various types of viruses. As with TRPS1, this signal was pronounced in the hunter-gatherers but not in the agriculturalists, the scientists report this month in Current Biology.
The finding further confirms that short stature and enhanced protection against viruses were both critical adaptations to rainforest living for African rainforest hunter-gatherers, says Pasteur Institute geneticist and study author Lluís Quintana-Murci. He hopes the work could one day help researchers develop more effective medicine for this population.
The study presents a solid argument that height and viral disease resistance were separate, independent targets of evolution, says Thomas Kraft, an anthropologist at the University of California, Santa Barbara, who studies hunter-gatherers. He says he’d like future studies to delve into whether these evolutionary pressures are still at work in modern hunter-gatherer societies.
Pontus Skoglund, a population geneticist at the Francis Crick Institute in London, adds that the results make him want to know more about when and where these unique genetic adaptations first arose. “There’s not so many people with a hunter-gatherer lifestyle in the world today,” he says, “and this can perhaps tell us about important processes that happened in the past.”
Source: Science Mag