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A stretch of DNA linked to COVID-19 was passed down from Neanderthals 60,000 years ago, a new study said.

Scientists don’t yet know why this segment increases the risk of severe illness from the coronavirus. But the new findings, not yet published in a scientific journal, show how some clues to modern health stem from ancient history.

“This interbreeding effect that happened 60,000 years ago is still having an impact today,” said Joshua Akey, a geneticist at Princeton University not involved in the study.

This piece of the genome, which spans six genes on chromosome 3, has had a puzzling journey, the study found. The variant is common in Bangladesh, where 63% of people carry at least one copy. Across South Asia, almost one-third of people have inherited the segment.

But only 8% of Europeans carry it, and just 4% have it in East Asia. It is almost completely absent in Africa.

It’s not clear what evolutionary pattern produced this distribution over the past 60,000 years. “That’s the $10,000 question,” said Hugo Zeberg, a geneticist at the Karolinska Institute in Sweden.

Researchers are only beginning to understand COVID-19. Last month, researchers compared people in Italy and Spain who became very sick with COVID-19 to those who had mild infections. They found two places in the genome associated with a greater risk. One is on chromosome 9 and includes ABO, a gene that determines blood type. The other is the Neanderthal segment on chromosome 3.

New research has downplayed the risk of blood type. But it showed an even stronger link between the disease and the chromosome 3 segment. People who carry two copies of the variant are three times more likely to suffer from severe illness than people who do not.

Once Neanderthal DNA entered our gene pool about 60,000 years ago, it spread down through the generations. Most Neanderthal genes turned out to be harmful to modern humans and disappeared.

But some genes appear to have provided an evolutionary edge and have become quite common. In May, Zeberg, Paabo and Dr. Janet Kelso, also of the Max Planck Institute, discovered that one-third of European women have a Neanderthal hormone receptor associated with increased fertility and fewer miscarriages. Other Neanderthal genes common today help us fight viruses.

Zeberg found that the version that raises people’s risk of severe COVID-19 is the same one found in a Neanderthal who lived in Croatia 50,000 years ago.