From Ancient DNA, a Clearer Picture of Europeans Today

Carl Zimmer, New York Times, October 30, 2014

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Up till now, scientists have learned about evolution on the Continent mostly by looking at living Europeans. But advances in biotechnology have made it possible to begin extracting entire DNA from the bones of ancestors who lived thousands of years ago. Their genomes are like time machines, allowing scientists to see bits of European history playing out over thousands of years.

Recently David Reich, a geneticist at Harvard Medical School, and his colleagues analyzed the genomes of nine ancient Europeans. Eight belonged to hunter-gatherers who lived about 8,000 years ago, seven in what is now Sweden and one in Luxembourg. The ninth came from a farmer who lived 7,000 years ago in present-day Germany.

The scientists compared these genomes with those of living Europeans. As they reported last month in Nature, the study revealed something scientists never knew: Europeans today have genes from three very different populations.

The oldest of these populations were the first Europeans, who appear to have lived as hunter-gatherers. The second were farmers who expanded into Europe about 8,500 years ago from the Near East.

But most living Europeans also carry genes from a third population, which appears to have arrived more recently. Dr. Reich and his colleagues found the closest match in DNA taken from a 24,000-year-old individual in Siberia, suggesting that the third wave of immigrants hailed from north Eurasia. The ancient Europeans that the scientists studied did not share this North Eurasian DNA. They concluded that this third wave must have moved into Europe after 7,000 years ago.

Last week, another team of scientists reported data from an even bigger haul of ancient European genomes–13, all told. While Dr. Reich and his colleagues studied ancient Europeans separated by hundreds of miles, these scientists focused on just one region in Central Europe called the Great Hungarian Plain.

The people whose genomes the scientists retrieved lived on the plain at various times between 7,700 years ago and 2,800 years ago.

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The oldest genomes retrieved from human remains in the area–one from a man and one from a woman–date back to the dawn of agriculture on the plain. The woman’s DNA showed that she belonged to the ancient farming population documented by Dr. Reich and his colleagues.

The man, however, did not have the genes of a farmer. He belonged to the oldest population of hunter-gatherers.

“The archaeological information isn’t enough to say whether he was married to a local farmer,” said Ron Pinhasi, an archaeologist at University College Dublin and a co-author of the new study. It may even be that the man’s skull was a trophy of some sort, Dr. Pinhasi added.

Archaeologists have found that early farming culture didn’t change drastically for the next 3,700 years. But about 4,000 years ago, the Bronze Age arrived. People started using bronze tools, trading over longer networks and moving into fortified towns.

Dr. Pinhasi and his colleagues found that the era also brought a sudden shift in human DNA. A new population arrived on the Great Hungarian Plain, and Dr. Reich believes he knows who they were: the northern Eurasians.

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At the start of the Bronze Age, life settled down on the plain for a thousand years. But then came the Iron Age, bringing another shift in culture–and genes.

People began traveling across the plain by horse-drawn chariots and wagons, and the genomes from 2,800 years ago show that the people of the Bronze Age had begun to be supplanted by a new Iron Age population. These are the people most closely related to living Hungarians.

In the new study, Dr. Pinhasi and his colleagues also surveyed individual genes known to have changed over the course of European history.

Today, for example, people in Hungary tend to have light skin and light brown hair, and half of them carry a mutation that lets them digest milk as adults. It took thousands of years for the genes for these traits to appear on the Great Hungarian Plain, the scientists found.

The hunter-gatherer that lived 7,700 years ago, for example, probably had black hair and dark skin, along with blue eyes. His genes suggest that he also probably couldn’t digest milk–not surprising, since he came from a population that didn’t raise livestock.

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It is not until 6,400 years ago that the scientists find the first genetic evidence on the Great Hungarian Plain for light brown hair. And the milk mutation appeared even later, just 3,100 years ago.

It is possible that these new genes and others were brought to the plain by successive waves of immigrants. But natural selection probably played a role in making these genes pervasive.

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