Scientists at the University of California, Berkeley, have announced a significant discovery, identifying segments of DNA in modern human genomes that originate from two distinct, previously unknown ancestral groups. These extinct human populations have been dubbed "ghost ancestors" due to the lack of prior knowledge about them.
While Homo sapiens stands as the sole surviving human species today, Earth was once home to numerous other hominins. Earlier research had already established that modern humans interbred with Neanderthals and Denisovans, leaving genetic imprints still present in many people's DNA. This new study suggests the existence of at least two more human groups that contributed DNA to our ancestors.
According to the researchers' calculations, one of these mysterious human lineages diverged from the evolutionary path leading to modern humans approximately 800,000 years ago. This timeframe roughly coincides with the split between Neanderthals and Denisovans.
Yulin Zhang, a doctoral student at Berkeley and a co-lead author of the study, explained, "Earlier studies had already hypothesised that there is a so-called ghost lineage, meaning genetic traces of unknown extinct human lineages in the genomes of modern humans. However, it was previously unclear whether this unknown lineage occurs only in Africans and when this admixture happened. We have now been able to identify and map the relevant DNA segments in the genomes of modern humans and show that this ghost lineage is found in all people living today, not just in Africans."
Even more ancient is the second ancestor, which the researchers have termed a "super-archaic ancestor." This lineage branched off from other hominins around 1.8 million years ago. More than 200,000 years ago, it likely interbred with Denisovans in Eurasia, and through this indirect route, fragments of its ancient DNA eventually entered the modern human genome.
Arjun Biddanda, a postdoctoral researcher at Johns Hopkins University and another co-lead author, highlighted the significance of this finding: "The evidence for this super-archaic ancestor is particularly exciting because it reveals genetic contributions from a human lineage that lived more than a million years ago, even though no DNA from this population has yet been decoded."
Priya Moorjani, a professor of molecular and cell biology at the University of California, Berkeley, emphasized the evolving understanding of human ancestry. "Thanks to ancient DNA from Neanderthals and Denisovans and new genealogical methods, we are increasingly realising that human populations have repeatedly mixed with one another in the course of evolution," she stated. "Human evolution therefore resembles less a branching family tree and more a complex network in which different populations were linked by repeated migrations and admixture."
Today, most individuals outside of Africa carry approximately 1 to 2% Neanderthal DNA in their genomes, with Europeans and Asians, on average, inheriting this percentage. This genetic contribution resulted from interbreeding between Neanderthals and early modern humans around 50,000 to 60,000 years ago, shortly after Homo sapiens migrated out of Africa.
These inherited DNA segments continue to influence modern humans, with some variants linked to the immune system, skin and hair traits, or adaptation to environmental conditions. Other variants can increase the risk of certain diseases or affect the body's response to infections.
Beyond Neanderthal DNA, population groups in Asia and Oceania also possess genetic contributions from Denisovans. Denisovan remains, primarily sequenced from the Eurasian region, offer further insights into this ancient interbreeding.
For instance, the genome of a 34,000-year-old female fossil discovered in Mongolia, analyzed by researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig and the Mongolian Academy of Sciences, revealed that about 25% of her DNA came from West Eurasian ancestors. This woman's genome, along with that of a roughly 40,000-year-old individual from China, also contained DNA traces of Denisovans, an extinct human form that inhabited Asia before the arrival of modern humans.
Intriguingly, Neanderthals and Denisovans themselves interbred. A fossil found in Denisova Cave in the Altai Mountains of southern Siberia, near the borders of Kazakhstan, Mongolia, and China, belonged to a woman with a Neanderthal mother and a Denisovan father, providing direct evidence of joint offspring between the two groups.
