The Neanderthal contribution to modern humans includes variants in genes involved in immune function — specifically in genes called Toll-like receptors, which are part of the innate immune system’s first-line defense against infection.

These Neanderthal-derived immune variants are present in modern populations at high frequencies, which suggests they were advantageous to modern humans who encountered novel pathogens in the environments they expanded into — environments that Neanderthals had inhabited and adapted to for hundreds of thousands of years.

Other Neanderthal genetic contributions affect hair and skin characteristics — specifically, some of the variants that determine hair texture and skin pigmentation in modern populations have Neanderthal origins.

The Neanderthal variants may have been adaptive for the light levels and temperatures that modern humans encountered as they expanded into higher latitudes.