Scientists studying the DNA of people alive today have uncovered signs of two previously unknown groups of ancient human ancestors, adding another layer to the increasingly complex story of human evolution.
The discovery was made using a computational method called TRACE, which allows researchers to examine the genealogical origins of individual sections of the human genome. The team applied the technique to more than 500 modern genomes representing populations from Africa, Europe and Asia.
Unlike Neanderthals and Denisovans, whose ancient DNA has been recovered and studied directly, the newly detected populations have not been identified through sequenced fossil DNA. Scientists therefore describe them as genetic “ghost” populations — ancestral groups whose existence can be inferred from genetic material passed down to later humans.
A hidden ancestor that mixed with Homo sapiens
One of the two lineages appears to have interbred with the ancestors of modern humans in Africa more than 50,000 years ago. The mixing likely occurred before Homo sapiens began their major migration out of Africa, helping explain why traces of the lineage were detected across the modern populations included in the analysis.
Researchers estimate that DNA inherited from this unidentified group makes up roughly 0.5% to 1% of the genomes of people living today.
Scientists cannot yet determine exactly who belonged to this population. However, based on when the lineage appears to have separated from other human groups, researchers suggest it may have been connected to Middle Pleistocene African populations of Homo, potentially including Homo heidelbergensis.
The finding also shows how modern genomes can preserve evidence of ancient populations even when researchers have no direct DNA samples from their remains.
An even older genetic legacy
The second lineage reaches much further back into human history. Researchers describe it as a “super-archaic” population that appears to have separated from other branches of the human family tree about 1.8 million years ago.
Rather than mixing directly with modern humans, this ancient population is believed to have interbred with Denisovans in Eurasia more than 200,000 years ago. Some of that genetic material was later passed from Denisovans into Homo sapiens, allowing DNA from a population that lived more than a million years ago to survive in people today.
Researchers have suggested that this older lineage could have been related to Homo erectus or a similar Eurasian population, although no direct genetic evidence has yet confirmed its identity.
The findings demonstrate how techniques such as TRACE may help scientists reconstruct parts of human evolutionary history that fossils alone have not revealed. Researchers also believe the method could eventually be used to detect previously unknown ancestral populations in other species.
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