Scientists at the prestigious University of California, Berkeley, have made a remarkable discovery: they have identified sections of DNA in the genomes of modern humans that originated from previously unknown ancestors. Researchers refer to them as “ghost ancestors” because these extinct human groups have so far remained completely unknown, Euronews reports.

Today, Homo sapiens is the only surviving human species. However, in the distant past, it shared the Earth with numerous other hominin groups.

Previous studies have already shown that modern humans interbred with Neanderthals and so-called Denisovans and had offspring with them. Genetic traces of these encounters are still present in the genomes of many people. Now, a new study suggests that there were at least two additional, previously unknown human groups that also passed some of their DNA on to our ancestors.

One of these previously unknown ancestors interbred in Africa with the ancestors of modern humans more than 50,000 years ago. This occurred before the last major migration of Homo sapiens into Europe and Asia. The genes inherited at that time now account for about 1% of the genome of all humans—roughly the same proportion as Neanderthal DNA.

According to the researchers’ calculations, this mysterious human lineage diverged from the lineage leading to modern humans approximately 800,000 years ago—around the same time that the ancestors of Neanderthals and Denisovans split from one another.

“Previous studies had already suggested the existence of what is known as a ghost lineage of ancestry—that is, genetic traces of unknown, extinct human lineages in the genomes of modern populations,” explains Yulin Zhang, a graduate student at the University of California, Berkeley, and one of the study’s first authors. “However, it was previously unclear whether this unknown ancestry was found only in Africans and when the admixture occurred. We were able to identify and map the corresponding DNA segments in the genomes of modern humans and show that this ghost lineage is present in all living humans, not just Africans.”

The second ancestor is even more mysterious. The researchers call it a “super-archaic ancestor.” Its human lineage split from other hominins approximately 1.8 million years ago. More than 200,000 years ago, it most likely interbred with Denisovans in Eurasia, and through this indirect route, fragments of ancient DNA eventually entered the genome of modern humans.

“The evidence for the existence of this super-archaic ancestor is particularly interesting because it reveals a genetic contribution from a human lineage that lived more than a million years ago, even though not a single DNA molecule has yet been recovered from this population,” explains Arjun Biddanda, a postdoctoral researcher at Johns Hopkins University and a co-author of the study.

“Thanks to ancient Neanderthal and Denisovan DNA, as well as new genealogical methods, we are increasingly seeing that human populations repeatedly mixed with one another throughout evolution,” says Priya Moorjani, a professor of molecular and cell biology at the University of California, Berkeley. “Human evolution therefore resembles not so much a branching family tree as a complex network in which different populations were connected through repeated migrations and admixture.”

Today, most people living outside Africa have about 1–2% Neanderthal DNA in their genomes. More precisely, on average, around 1–2% of the genomes of modern Europeans and Asians are inherited from Neanderthals. This resulted from interbreeding between Neanderthals and early modern humans approximately 50,000–60,000 years ago, shortly after Homo sapiens left Africa.

Some of these genetic variants are associated with immune defenses, skin and hair characteristics, or adaptation to environmental conditions. Others increase the risk of certain diseases or affect the body’s response to infections.

In addition to Neanderthal DNA, populations in Asia and Oceania also carry genetic fragments inherited from Denisovans. The remains of Denisovans have been studied primarily through discoveries made across Eurasia.

The genome of the oldest human fossil discovered in Mongolia to date opened new chapters in our understanding of early human history about six years ago. Scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig and the Mongolian Academy of Sciences found that approximately 25% of the DNA in the remains of a woman who lived around 34,000 years ago came from West Eurasian ancestors. The team also detected traces of Denisovan DNA in her genome, as well as in the genome of a person who lived around 40,000 years ago in China. Denisovans were an extinct human group that lived in Asia before the arrival of modern humans.

Interestingly, Neanderthals and Denisovans also interbred with one another. One fossil from Denisova Cave in the Altai Mountains of southern Siberia, near the borders with Kazakhstan, Mongolia and China, belonged to a woman whose mother was a Neanderthal and whose father was a Denisovan. This provides direct evidence that the two human groups had offspring together.

Exactly who these “ghost ancestors” and the “super-archaic ancestor” were remains unclear. Nevertheless, the estimated dates of their divergence correspond well with known human groups. The ghost ancestors may have belonged to Middle Pleistocene Homo populations that lived in Africa around 800,000 years ago. The super-archaic ancestor, meanwhile, was most likely Homo erectus or a closely related group, as this species spread into Eurasia approximately 1.8 million years ago.