Humanity has been on a very long journey. Anatomically modern humans – the subspecies Homo sapiens sapiens – emerged in east Africa around 200,000 years ago, spread to Eurasia, and subsequently replaced Neanderthals and Homo erectus as the dominant hominid on planet Earth. By following the path of the human genome throughout ancient history, paleontologists and anthropologists have very precisely determined our evolutionary journey out of Africa. However, some of our more recent ancestors traveled back to the cradle of our species, and a new study published in Science reveals that a wave of migration back into Africa from western Eurasia three millennia ago was up to twice as significant as previously thought.
A genuinely mysterious mass migration of humans 3,000 years ago, known as the “Eurasian backflow,” saw vast populations of humans from western Eurasia – including parts of the Arabian Peninsula – pack up their bags and move back into the Horn of Africa. This is preserved in the modern day genetic record of both eastern and southern Africans: The DNA of these ancient west Eurasians shows up in genetic studies of contemporary humans in both parts of the African continent.
This new study, led by the University of Cambridge, demonstrates that east African populations have as much as 25% Eurasian ancestry due to this mysterious migration event. Not only that, but populations from all corners of the continent, including the far western and southern regions, have 5% Eurasian genetic material in their DNA.
Remarkably, this means that a large population of humans that had previously migrated out of Africa en masse to settle in Eurasia eventually decided to turn back and return to their place of origin, spreading across a large swath of Africa in a relatively short amount of time.
“Roughly speaking, the wave of west Eurasian migration back into the Horn of Africa could have been as much as 30% of the population that already lived there,” Dr Andrea Manica, senior coauthor of the study, said in a statement. “To me, that is mind-blowing. The question is: what got them moving all of a sudden?”
A genome – the complete set of an organism’s DNA – was extracted from the skull of a 4,500-year-old man found buried face-down in a cave system in the highlands of Ethiopia. The Mota cave experienced long-term microclimate conditions that helped preserve the genetic material across the ages: a cool environment free from extreme weather and moisture that can cause DNA to degrade.
This genome was then analyzed, making it the first time an ancient human genome had been recovered and sequenced from Africa. It was revealed to contain a mixture of both east African and western Eurasian DNA; as the man pre-dated the Eurasian Backflow by 1,500 years, the team of researchers realized this meant that the backwards migration event was more significant, and occurred far earlier, than previously thought.
“Africa is a total melting pot,” said Gallego Llorente, lead author of the study. “We know that the last 3,000 years saw a complete scrambling of population genetics in Africa. So being able to get a snapshot from before these migration events occurred is a big step.”
As for why the Eurasian Backflow happened, the scientific community is decidedly unsure; it remains a tantalizing mystery yet to be solved.