As anatomically modern humans (AMHs) migrated out of Africa and around the rest of the world, they met and interbred with at least four extinct hominin species.
Traces of genetic input from past interbreeding events are embedded in the genomes of modern-day human populations, and serve as a powerful tool to map the location of these introgression events as well as to infer past human migration routes.
“These archaic groups were widespread and genetically diverse, and they survive in each of us. Their story is an integral part of how we came to be.” said Dr. João Teixeira, co-author of the paper published.
The most surprising finding is that of the 4 hominin species modern humans have interbred with, only Neanderthals and Denisovans are already known, the other two hominin groups remain unnamed and have only been detected as traces of DNA.
The main Neanderthal interbreeding event likely occurred soon after the ancestors of modern-day non-African populations left Africa, 55 to 50 thousand years ago, based on the size of Neandertal DNA fragments preserved in the genome of an early AMH specimen. The ancestral population of modern humans then appears to have split as it moved across Asia with one pulse dispersing north into mainland Asia, where it met and mixed with a Denisovan group.
The interbreeding events with other extinct hominin groups remain poorly understood, and are potentially far more complex.
"The Island Southeast Asia region was clearly occupied by several archaic human groups, probably living in relative isolation from each other for hundreds of thousands of years before the ancestors of modern humans arrived.” says Dr Teixeria.
Current genetic evidence suggest that AMH interbred with extinct hominin 1 (EH1) in South Asia, and potentially with extinct hominin 2 (EH2) near Flores in Indonesia. The timing also makes it look like the arrival of modern humans was followed quickly by the demise of these archaic groups in each area.