Ancient DNA shows Stone Age humans evolved quickly as they took up farming
Stonehenge at the winter solstice. People in western Eurasia underwent evolutionary changes as they adopted farming as a way of life. (AP/Matt Dunham)
Prehistoric people who adopted farming as a way of life underwent evolutionary changes to adapt to their new lifestyle, a dramatic example of natural selection operating on the human species in the relatively recent past.
That's one of the conclusions of a new study of the genomes of 230 individuals who lived thousands of years ago and whose bones have been recovered from Western Eurasia — a broad area that includes what is now Turkey, the Russian Steppe and Europe.
The research, published Monday in the journal Nature, identified 12 specific genetic mutations that corresponded to the rise of agriculture and the migration of people into new regions. They include the ability to digest milk and metabolize fats. The mutations also favored greater height at maturity, lighter skin and lighter eye color in northern populations. There are also genetic markers that appear to be connected to resistance against such diseases as leprosy and tuberculosis.
The new genetic analysis also provides an answer to the question of how agriculture arrived in Europe. There have been two competing scenarios. One is that agricultural people — farmers — arrived as migrants, replacing indigenous populations. The other is the practices of farming were transmitted culturally, a contagion of innovation known to anthropologists as "cultural diffusion."
The new research strongly supports the first scenario, showing that the people who began farming in Europe, starting about 8,500 years ago, were closely connected to a population of farmers in Anatolia, a region that largely overlaps with modern-day Turkey.
“It is a migration. It’s a movement of people. The farmers in Europe from Germany and Spain are genetically almost identical to the farmers from Turkey," said Iain Mathieson, a geneticist at Harvard Medical School and the lead author of the new report.
Modern human beings spent many tens of thousands of years as hunters and gatherers. But at the end of the last Ice Age, as temperatures stabilized, people in Mesopotamia and the Levant — the Fertile Crescent — began planting crops and domesticating animals as livestock. The farmers and their new way of life spread to other parts of Eurasia. Farming allowed greater population density, but it was a difficult way of life that at first led to poor nutrition and zoonotic diseases associated with living in close quarters with domesticated animals.
“It's a change in the food people are eating. It’s a change in social organization. People are living in much bigger communities. People are living in much closer proximity to animals," Mathieson said.
That was a technological revolution that had genetic repercussions. Natural selection functions as a filter, favoring people with certain genetic mutations that allow them to more easily reach maturity and have children who are themselves advantaged. Thus, around 4,000 years ago, according to the new study, Europeans begun showing a genetic change associated with lactase persistence — the ability to digest milk into adulthood.
That such evolutionary changes have been taking place in the relatively recent past is not a surprise. Indeed, scientists have modeled many of these genetic adaptions simply by looking at people alive today and comparing their genomes. But this new work is more of a direct look at the prehistoric evolutionary processes as they were happening.
“It's taking ancient DNA to actually go back in the past," said Rasmus Nielsen, an evolutionary biologist at the University of California at Berkeley. He was not part of the team that published the new findings. "The paper is able to verify many of the predictions that have been done in the past 20 years from looking at modern populations. In some sense we have this scientific time machine," he said.
One possible implication of this research is that the popular "Paleo Diet," which embraces foods available to Stone Age people and avoids the dairy products and grains that came along only in the last 10,000 years, ignores the recent evolutionary changes in the human species. But Mathieson did not take a stance on this latest food fad.
"I don't think we can really speak to this," he said. "We show that people were able to adapt genetically to an agricultural diet, but it's rather an open question how well they adapted."