Scarce oxygen, cold temperatures, and intense ultraviolet radiation make the Andes a tough place to live. How did humans adapt to such heights? A new study of ancient and modern DNA suggests in some South American highlanders, the answer includes changes to their heart muscles. The same study found that ancient highlanders adapted to digest starch more easily as they came to rely on potatoes for food, and that they most likely split from their lowland brethren some 8750 years ago. But those conclusions have been questioned by scientists who say the comparison population is simply too distant to reveal anything specific about highland life.
To find out how ancient Andeans adapted to living at more than 2500 meters, John Lindo, a population geneticist at Emory University in Atlanta, sequenced seven genomes from people who lived near Lake Titicaca in the Peruvian Andes from 6800 years ago to about 1800 years ago. The team then compared those genomes to genetic data from two modern populations: the high-living Aymara of Bolivia, and the Huilliche-Pehuenche, who live on the lowland coast of southern Chile.
Another high-dwelling folk—the people of the Tibetan plateau—have genetic variations that reduce hemoglobin levels in their blood and make their bodies extremely efficient at using oxygen. So Lindo and his colleagues scanned the ancient South American genomes for signs of similar adaptations. They didn’t find what they were looking for, but they did see signs of selection on a gene called DST, related to cardiovascular health and heart muscle development, they report today in Science Advances. That, says Mark Aldenderfer, an archaeologist at the University of California (UC), Merced, and a co-author of the new study, “suggests a very different process by which ancient Andean people adapted to high elevation life.”
An even stronger sign of natural selection turned up in genes related to starch digestion. Because the starchy potato was domesticated in the Andes and quickly became a dietary staple, such an adaptation makes sense, Lindo says. By measuring the number of random genetic differences that accumulated steadily over time between the highland and lowland populations, Lindo’s team estimates the genetic split between those peoples likely happened about 8750 years ago, a date that fits with archaeological data.
But some geneticists question whether these differences are really related to highland living. The coastal Huilliche-Pehuenche live so far south of the Andes that their genomes aren’t a meaningful comparison to the ancient and modern highland groups, says Lars Fehren-Schmitz, a biological anthropologist at UC Santa Cruz. He thinks the genetic variations used to estimate when the groups split are not evidence of people moving to the mountains, but likely a remnant of differences already carried by various populations as they settled South America. “It’s like comparing apples and pears,” agrees Bastien Llamas, a geneticist at The University of Adelaide in Australia. The only way to see which genetic differences are true high-altitude adaptations, they say, would be to compare ancient Andeans to ancient inhabitants of the nearby coasts of Peru or northern Chile. Lindo agrees that those would be better comparisons—if data about them were available.
One big difference between the Huilliche-Pehuenche and the modern highlanders is how their ancestors were affected by European contact and colonization. By modeling past population bottlenecks, Lindo calculated that the indigenous highland population fell by 27% after European contact. But a shocking 97% of the Huilliche-Pehuenche’s ancestors died, possibly because of extensive wars with colonists that lasted well into the 19th century.
Finally, the team found one specific sign of European contact in modern highlanders’ genomes: an immune receptor that kicks into gear in response to the smallpox vaccine. Smallpox epidemics were documented in the Andes, especially in the early days of European contact, and the marker drives home the fact that modern Andeans descend from the survivors, Lindo says. “It’s a signal that human evolution continues on,” Aldenderfer says.