Multiple sclerosis is more common among white Europeans. Researchers studying ancient DNA may have figured out why.

• Researchers compared DNA from hundreds of ancient teeth and bones to DNA from modern humans.
• They wanted to find out how genes linked to diseases like multiple sclerosis originated and spread.

Genes that may have once helped ancient herders fight infectious parasites could contribute to autoimmune diseases today, like multiple sclerosis.

Multiple sclerosis is a progressive disease that affects the brain and spinal cord and can lead to a range of symptoms, including paralysis. While it's not hereditary (environment also plays a role in its development), susceptibility is related to certain gene variants.

Researchers at the University of Copenhagen, University of Cambridge, and other institutions started searching for clues as to why certain diseases like MS are more common in some populations compared with others.

For example, rates of MS are twice as high in the northwest of Europe, including in Scandinavia, compared to southern Europe.

They analyzed teeth and bones from Europe and Western Asia, adding to an ancient DNA database of about 1,600 genomes. Then they compared the genetic information of over 400,000 modern people, mainly white Europeans, from the UK Biobank.

They published their results in a series of papers in the peer-reviewed journal Nature. One study connects UK populations with a higher genetic risk of MS with a group of Eurasian herders who may have benefited from an increased ability to ward off some infections.

"One way I think about this is that there are trade-offs," Rasmus Nielsen, a University of California, Berkeley, professor who was involved in the research, said during a press conference.

Genes were likely passed on in a certain environment because they offered protection against some infectious diseases and carriers were more likely to survive.

Today, those genes can also make people susceptible to autoimmune diseases. As the environment changes, the balance of the advantages and disadvantages changes, as well, Nielsen said.

"It gives us sort of an insight into why these diseases exist today," he said.

The legacy of herding ancestors

Around 5,000 years ago, the Yamnaya people, herders from the Eurasian steppe, arrived in Europe and moved north.

Yamnayan genes show up prominently in many Scandinavian populations, the BBC reported in 2015. Northern Europeans are also more susceptible to MS than in southern Europe, where Yamnayan ancestry is less common.

The Yamnaya herders would have close contact with their sheep, goats, and cows — and the accompanying parasites. Zoonoses are diseases animals pass to humans, including parasitic infections.

Many of these parasitic infections have mostly disappeared in the Western world, coauthor and immunologist Dr. Astrid Iversen said during the press conference. Diets have changed, many people live further from livestock, and hygiene has improved.

But humans' immune systems haven't caught up. They're still primed to create the pro-inflammatory responses, even without the parasites to fight.

As a result, "our immune system is a bit unbalanced," Iversen said.

For some people, this inflammatory response can go overboard and start attacking the body's own cells, as with MS, Iversen said.

Right now, doctors treat MS with steroids that suppress the immune system. By better understanding where the disease comes from, researchers hope they can find new and more targeted treatments.

"Rather than just knocking it out, we should just try to find out in greater detail how it's unbalanced, and then try to recalibrate it," Lars Fugger, an expert on multiple sclerosis at the University of Oxford who was involved in the study, told The New York Times.