Scientists have uncovered evidence that modern humans carry genetic material from two previously unknown ancient human groups, adding new complexity to the story of human evolution. Although no fossils or DNA from these extinct populations have been discovered, researchers say traces of their existence remain embedded in the genomes of people living today.
The findings come from researchers at the University of California, Berkeley, who identified previously unknown stretches of DNA in modern human genomes. The scientists refer to these extinct populations as “ghost ancestors” because there is currently no physical fossil evidence or decoded DNA directly linked to them.
Until now, scientists knew that Homo sapiens interbred with Neanderthals and Denisovans, leaving lasting genetic signatures in present-day populations. The new research suggests that at least two additional human lineages also contributed to the genetic makeup of modern humans.
According to the study, one of these unknown groups mixed with the ancestors of modern humans in Africa more than 50,000 years ago, before the major migration of Homo sapiens into Europe and Asia. Researchers estimate that DNA inherited from this ancient population now accounts for roughly one percent of the genomes of all living humans, a proportion similar to the amount of Neanderthal DNA found in many people today.
The team believes this mysterious lineage separated from the evolutionary line leading to modern humans about 800,000 years ago, around the same period when Neanderthals and Denisovans split from each other.
Lead author Yulin Zhang, a doctoral student at the University of California, Berkeley, said previous research had suggested the existence of an unidentified human lineage, but scientists were uncertain whether its genetic traces existed only in African populations or when the interbreeding occurred.
Zhang said the new analysis allowed researchers to identify and map these DNA segments, showing that the genetic contribution is present in all modern humans rather than being limited to people of African ancestry.
Researchers also identified evidence of an even older ancestor described as a “super-archaic” human lineage. They estimate this group diverged from other human ancestors around 1.8 million years ago. More than 200,000 years ago, members of this ancient population are believed to have interbred with Denisovans in Eurasia, allowing fragments of their DNA to pass indirectly into modern humans.
Arjun Biddanda, a postdoctoral researcher at Johns Hopkins University and co-lead author of the study, said the discovery points to genetic contributions from a population that lived more than a million years ago, despite the absence of any recovered DNA from that group.
Professor Priya Moorjani of the University of California, Berkeley, said advances in ancient DNA research and new genealogical techniques are changing scientists’ understanding of human evolution. Rather than resembling a simple branching family tree, she said, human history appears to be a network of repeated migrations and interbreeding among multiple human populations.
Scientists say the identities of these newly proposed ancient ancestors remain uncertain. Based on evolutionary timelines, the researchers suggest the 800,000-year-old lineage may have belonged to Middle Pleistocene human groups in Africa, while the super-archaic ancestor could have been closely related to Homo erectus, one of the earliest human species to spread across Eurasia about 1.8 million years ago.
