In a new twist to the tale of ancient humans and our closest relatives, African populations were revealed to share Neanderthal ancestry for the first time – dismissing the previous belief that only non-African populations carried Neanderthal genes.

Researchers from Princeton University said that the people who migrated out of Africa between 60.000 to 80.000 years ago mated with Neandertals. This led to humans returning from Africa to carry Neandertal genes that then spread throughout the continent, according to the study published in Cell journal.

Geneticist Joshua Akey and his team said the Neandertal gene variants inherited by modern Africans included genes involved in reinforcing the immune system and modifying sensitivity to ultraviolet radiation. Those genes apparently spread fast once they were introduced to African populations.

“Our work highlights how humans and Neandertals interacted for hundreds of thousands of years, with populations dispersing out of and back into Africa,” Akey says. “Remnants of Neandertal DNA survive in every modern human population studied to date.”

Akey’s team developed a statistical technique to detect ancient genetic material still present in modern DNA. The new approach detected a human journey out of Africa between 100.000 to 150.000 years ago, which led to the introduction of human genes into Neandertals through interbreeding.

The researchers looked at DNA from 2,504 present-day Africans, Europeans, and East Asians, comparing them with DNA taken by other researchers from a Neandertal fossil that had been found in Siberia and southeastern Europe. Then, they calculated the possibility of a segment of a person’s DNA to have inherited Neanderthal DNA.

Previous studies compared living people’s DNA to that of Neandertals as well as to a modern African group assumed to lack Neandertal ancestry. If those reference groups had Neandertal DNA, those earlier studies would have underestimated the Neandertal’s genetic legacy, Akey’s team said.

Neandertals were the closest evolutionary relatives of humans. They inhabited parts of Europe and Asia from more than 800.000 years ago until 40.000 years ago. Their DNA accounts for 0.5% of individual African’s genome, far more than reported before, the researchers concluded.

Meanwhile, present-day people outside Africa have three times as much Neandertal DNA as Africans do, the researchers said. The new study also identified similar proportions of Neandertal DNA in the genomes of modern Europeans and East Asians of between 1.7 and 1.8%.

“To more fully understand human genomic variation and human evolutionary history, it is imperative to comprehensively sample individuals from all regions of the world, and Africa remains one of the most understudied regions,” Akey concluded.