New findings suggest modern North Africans carry genetic traces from Neanderthals, modern humanity's closest known extinct relatives.

Although modern humans are the only surviving members of the human lineage, others once roamed the Earth, including the Neanderthals. Genetic analysis of these extinct lineages' fossils has revealed they once interbred with our ancestors, with recent estimates suggesting that Neanderthal DNA made up 1 percent to 4 percent of modern Eurasian genomes. Although this sex apparently only rarely produced offspring, this mixing was enough to endow some people with the robust immune systems they enjoy today.

The Neanderthal genome revealed that people outside Africa share more genetic mutations with Neanderthalsthan Africans do. One possible explanation is that modern humans interbred with Neanderthals mostly after the modern lineage began appearing outside Africa at least 100,000 years ago. Another, more complex scenario is that an African group ancestral to both Neanderthals and certain modern human populations genetically split from other Africans beginning about 230,000 years ago. This group then stayed genetically distinct until it eventually left Africa.

To shed light on why Neanderthals appear most closely related to people outside Africa, scientists analyzed North Africans. Some researchers had suggested these groups were the sources of the out-of-Africa migrations that ultimately spread humans around the globe.

The researchers focused on 780,000 genetic variants in 125 people representing seven different North African locations. They found North Africans had dramatically more genetic variants linked with Neanderthals than sub-Saharan Africans did. The level of genetic variants that North Africans share with Neanderthals is on par with that seen in modern Eurasians.