Out of Africa – and back again: When did humans return to Africa?

Human ancestors first emerged on the scene in Africa before spreading out across Eurasia and eventually into the Americas. But that trek out of Africa wasn't a one-way trip. 

And humans might have been migrating back into Africa remarkably early, according to clues found in the mitochondrial DNA of a 35,000-year-old skull discovered in a cave in Romania. 

This paleolithic woman was an early member of a lineage that today appears mainly in northern Africa, according to a paper published Thursday in the journal Scientific Reports. And finding an older version of this genetic lineage, called haplogroup U6, in Europe could confirm that a back-migration carried this genetic signature into Africa.

"Now we find a paleolithic migration, a back-migration, from Eurasia to north Africa," study co-author Mihai Netea of Radboud University Medical Center in The Netherlands, tells The Christian Science Monitor. "We demonstrated that the origin of U6 is in western Eurasia and it migrated."

"U6 is found in a relatively large proportion of the population in north Africa," he explains. But "this is much older, much more basal, so we demonstrate that the origin of these populations from north Africa were basically from western Eurasia."

This back-migration wasn't a complete surprise, says Cosimo Posth, a PhD candidate in archaeogenetics at Tübingen University in Germany, who was not part of this new study. When the U6 haplogroup was spotted in the mitochondrial DNA of people living in northern and western Africa today that is almost absent everywhere else, some scientists proposed that a back-migration had carried these genetic markers into Africa. 

Finding an older version of this lineage outside of Africa would confirm that. Mr. Posth and colleagues reported a basal version of haplogroup U6 in a different skull from the same site, Peștera Muierii, in a paper published earlier this month.

"This actually suggested that this haplogroup originated somewhere outside of Africa and then migrated back into Africa during the paleolithic time," Posth says. And this new paper "is a confirmation of those previous studies."

The researchers aren't sure when exactly the U6 haplogroup first migrated into Africa, as the archeological DNA record between the Romanian individuals and modern-day people is spotty. 

"What is sure is that north African populations with modern U6 originate from a population that had the same paleolithic U6 as PM1" (the individual examined in the new paper)," Dr. Netea says. 

But that doesn't mean PM1, which is short for Peștera Muierii 1, is a direct ancestor of the populations in northern Africa today, he cautions. She could have been a member of a group that migrated into Europe from the Middle East as other groups of the same haplogroup spread back into Africa. 

If the former story is the case, then the migration back to Africa would have happened more recently than 35,000 years ago. But in the latter scenario, the back-migration could have occurred at the same time as the modern human expansion into Europe some 45,000 years ago.

"It's really challenging to say [the timing of this back-migration] because we do not have ancient data from Africa," Posth says. "The only ancient genome that we have from Africa is much more recent, from 45,000 years ago from Ethiopia." And that individual suggests a much later, try 3,000 years ago, migration back into Africa. Sequencing older DNA would be "the final confirmation" of this earlier, paleolithic migration, Posth says.

Either way, this back-migration suggests "the prehistoric world was much more interconnected than we believed," Netea says.

Initially, Netea and his colleagues dug into the mitochondrial DNA of PM1 with other questions in mind. 

The team was particularly curious about the DNA of this paleolithic woman because her "skull had morphological indication of mixture between modern humans and possibly a more ancient human species, like Neanderthal," Netea says. But "the mitochondrial DNA that we identify is purely of modern human origin."

That doesn't mean she has no archaic human ancestry, he says. Mitochondrial DNA is passed just from mother to child, so perhaps sequencing the nuclear genome, which is inherited from both parents, will help fill in the gaps in that story.

Furthermore, "we were interested to understand the relationship of this mitochondrial DNA with the current populations living in Europe," Netea says. The team expected a U haplogroup, but they were looking for the U5 group that has already been spotted in Europeans. "Identifying a U6 group, which is especially present in north Africa, was a surprise," he says.

But it was a "very exciting finding" to spot evidence of the back-migration.

"Our history is basically the way that different populations migrate," Netea says. "What we are is the history of these migrations. Understanding what is happening is crucial to understanding who we are."