Ancient DNA helps explain why northern Europeans have a higher risk of multiple sclerosis than other ancestries: It’s a genetic legacy of horseback-riding cattle herders who swept into the region about 5,000 years ago.

The findings come from a huge project to compare modern DNA with that culled from ancient humans’ teeth and bones — allowing scientists to trace both prehistoric migration and disease-linked genes that tagged along.

When a Bronze Age people called the Yamnaya moved from the steppes of what are now Ukraine and Russia into northwestern Europe, they carried gene variants that today are known to increase people’s risk of multiple sclerosis, researchers reported Wednesday.

Yet the Yamnaya flourished, widely spreading those variants. Those genes probably also protected the nomadic herders from infections carried by their cattle and sheep, concluded the research published in the journal Nature.