Many scientists have contended that humans have evolved very little over the past 10,000 years.
A few hundred generations was just a blink of the evolutionary eye, it seemed. Besides, our cultural evolution — our technology, agriculture and the rest — must have overwhelmed our biological evolution by now.
A vast study, published last month in Nature, suggests the opposite. Examining DNA from 15,836 ancient human remains, scientists found 479 genetic variants that appeared to have been favoured by natural selection in just the past 10,000 years.
The researchers also concluded that thousands of additional genetic variants have probably experienced natural selection. Before the new study, scientists had identified only a few dozen variants.
“There are so many that it’s hard to wrap one’s mind around them,” said David Reich, a geneticist at Harvard Medical School, US, and an author of the study.
He and his colleagues found that a mutation that is a major risk factor for celiac disease, for example, appeared just 4,000 years ago, meaning the condition may be younger than the Egyptian pyramids.
The mutation became ever more common. Today, an estimated 80 million people worldwide have celiac disease, in which the immune system attacks gluten and damages the intestines.
The steady rise of the mutation came about through natural selection, the scientists argue. For some reason, people with the mutation had more descendants than people without it — even though it put them at risk of an autoimmune disorder.
Other findings are even more puzzling. The researchers found that genetic variants that raise the odds of a smoking habit have been getting steadily rarer in Europe for the past 10,000 years.
Something is working against those variants — but it can’t be the harm from smoking. Europeans have been smoking tobacco for only about 460 years.
The scientists can’t see from their research so far what forces might be making these variants more or less common. “My short answer is, I don’t know,” said Ali Akbari, senior staff scientist at Harvard and an author of the study.
The study is making a stir among evolutionary biologists. Experts generally agreed that at least some of the genetic variants Akbari, Reich and their colleagues had found would turn out to be have been influenced by natural selection.
“The scale of what Reich’s research team has accomplished with more than 15,000 genomes of ancient people is just astounding,” said John Hawks, a palaeoanthropologist at the University of Wisconsin, US.
But some were cautious about the full scale of natural selection the team is claiming to have found. Sasha Gusev, a geneticist at Harvard Medical School who was not involved in the new study, has qualms about their statistical methods.
“The findings require a leap of faith,” he said.
In the past few years, scientists have managed to find a few cases of recent human evolution. When societies in Africa, Asia and Europe domesticated cattle and other livestock, for example, they began to drink milk and consume other dairy products.
Amutation then spread that helped people digest these foods in adulthood. It may have helped them survive famines in the Bronze Age.
In the early 2000s, Reich and other researchers pioneered a new way to reconstruct human evolution — they began extracting DNA from ancient bones. In 2015, scientists in Reich’s laboratory carried out a survey of ancient DNA in search of natural selection.
They found 12 variants that had been subject to natural selection, including a variant that can help people digest milk as adults. Subsequent studies raised the total to only 21 variants.
Reich’s team went on to amass thousands of new samples of ancient DNA, while Akbari developed new methods to analyse them. He was shocked to find so many new variants had been evolving in the past 10,000 years.
“It took about two years to convince myself this was real,” Akbari said.
Akbari and his colleagues found thousands of additional genetic variants that showed smaller — but still striking — increases over the past 10,000 years. Even if just half of those variants were actually being affected by natural selection, that would mean about 3,800 were subject to evolutionary change.
“That’s where the evidence starts to get weaker,” said Graham Coop, a geneticist at the University of California, Davis, US. He believes it’s possible that many of those variants increased in the human population thanks to changes that had nothing to do whatsoever with natural selection.
In another line of research, the scientists looked at traits in living people that are modestly influenced by dozens or hundreds of genes. These traits include everything from cigarette smoking to diabetes to the years that people spend in school.
Of the 563 traits he scientists studied, 44 showed signs of natural selection in related genes, a process known as polygenic selection.
Variants linked to a risk for type 2 diabetes have gotten rarer, for example. So have variants linked to a wider waist and a high percentage of body fat.
Farming might be responsible for those trends. Hunter-gatherers benefited from turning food to fat as a way to survive times of scarcity. But this strategy may have become harmful when farmers ate a steady diet of wheat and other carbohydrate-rich food.
It’s much harder to make sense of changes in other traits. The scientists concluded that natural selection had favoured genetic variants linked to the number of years people spend in school.
Currently, scientists are debating what sort of biology allows genes to influence how long people stay in school, which itself is a modern institution. “What it meant to the past was a very different thing,” Reich said.
NYTNS