Modern humans carry genetic material inherited from two previously unknown ancient human populations, according to a new study that provides further evidence of widespread interbreeding during human evolution.
In the study, published Thursday in the journal Science, researchers at the University of California, Berkeley developed a computational technique called TRACE to reconstruct ancient genealogical relationships using hundreds of present-day human genomes without requiring DNA recovered from fossils.
The study identified a "ghost" lineage that appears to have interbred with modern humans in Africa more than 50,000 years ago, before the most recent major migration of Homo sapiens out of the continent.
Members of the lineage diverged from modern human ancestors around 800,000 years ago. Their DNA now accounts for approximately 0.5% to 1% of each modern human genome and was detected in both African and non-African populations.
"We were actually able to find and map genomic locations in modern humans that are from this ghost lineage and show that this ghost ancestry is in all modern humans, not only in Africans," researcher Yulin Zhang said.
The team also found evidence of a much older "super-archaic" lineage descended from a population that separated from other human ancestors around 1.8 million years ago.
That group likely interbred with Denisovans in Eurasia more than 200,000 years ago. Some of its genetic material was later passed to modern humans when Denisovans interbred with Homo sapiens.
Researchers said the identities of the two populations remain unknown, although their estimated timelines overlap with those of Middle Pleistocene Homo groups in Africa and Homo erectus in Eurasia.
The findings suggest human evolution resembled a complex network of migration and interbreeding rather than a simple branching tree, according to UC Berkeley researcher Priya Moorjani.
