PARTING GIFT

Neanderthals mated with European humans and it made their immune systems weaker

The path to humanity as we know it was long.

Over about 2 million years, early human species slowly evolved beneficial mutation after beneficial (or at least benign) mutation, shaping our bodies into how they look and work today. Simultaneously, these early humans were migrating all over the planet, and reproducing with one another.

In two papers published this week in the journal Cell, researchers from the University of Montreal in Canada and the Pasteur Institute in Paris outlined findings that this interbreeding process is what led modern humans of African and European descent to have slightly different immune systems. The earliest Homo sapiens to arrive in what is now Europe mated with Neanderthals already living there, and in doing so acquired some DNA that codes for less aggressive inflammatory immune responses to infections. The findings suggest that people of African descent are better at fighting off certain kinds of infections—and may explain why they’re more likely to have inflammatory autoimmune diseases, like lupus.

“I was expecting to see ancestry-associated differences in immune response,” Luis Barreiro, a geneticist at the University of Montreal and lead author of one of the papers, said in a statement, “but not such a clear trend towards an overall stronger response to infection among individuals of African descent.”

For Barreiro’s work, he and his team took blood samples of 80 people of African descent and 95 people of European descent. They found that when white blood cells in the samples were subjected to two types of bacterial infections, those from people with African ancestry eliminated the bacteria much more quickly than those from people of European descent. These cells also activated a stronger inflammatory response. When the researchers compared the genes of these immune cells, they found genes similar to Neanderthal DNA in European cells, but not Africans.

In the study out of the Pasteur Institute, researchers conducted a similar experiment with blood samples from 200 Dutch individuals, about half of whom had African ancestry and half of whom had European ancestry. They surrounded these white blood with a strain of the flu virus as well as the chemical markers that usually signal a viral or bacterial infection. Once again, Europeans tended to have less of an inflammatory response—especially to the flu virus—and had more genes similar to Neanderthals.

As a general rule, evolution favors those traits that are advantageous for living in certain conditions, and the researchers have a few ideas why immune responses differ between the two groups of people. For one thing, it could be that the infectious diseases in Africa were deadlier than those in Europe, Barreiro told Live Science. Homo sapiens living in Africa would need to develop stronger immune responses to combat these viruses and bacteria than Neanderthals in Europe.

That adaptation may have come with some unfavorable consequences, though. Inflammatory immune responses to some kinds of infections can actually be more harmful than beneficial, Lluis Quintana-Murci, a geneticist at the Pasteur Institute and lead author of the French study, told Nature. “Maybe the most important thing is to live in peace with the microbes.”

These studies highlight a need to investigate different populations to understand why some people are more likely to develop autoimmune diseases. Overall, women are more likely to develop these conditions than men, and women of color are two to three times more likely to do so than white women.

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