The result is that non-African modern humans today carry approximately one to four percent of Neanderthal ancestry in their genomes.

Specific Neanderthal-derived variants have been identified that appear to have been adaptive — variants affecting immune system function, metabolism, and other traits that may have helped early modern humans in the environments they were entering for the first time.

One of the Neanderthal-derived genes that has received particular attention is a variant associated with enhanced immune response to certain pathogens.

Neanderthals had lived in Europe and western Asia for hundreds of thousands of years, adapting to the local disease environment during that time.

When modern humans arrived and interbred with them, some of those adaptations transferred — a genetic inheritance that may have been genuinely useful to people entering an unfamiliar biological environment.