Mysterious DNA: two unknown ancestors found in human genome

Usagevpn.com – Researchers at the prestigious University of California, Berkeley, have made a spectacular discovery: they identified stretches of DNA in the genomes of modern humans (source in German) that come from previously unknown ancestors. The scientists talk about so-called "ghost ancestors," because nothing is yet known about these extinct human groups. Homo sapiens is today the only human species still alive.

In the distant past, however, it shared the Earth with numerous other hominins. Earlier studies had already shown that modern humans interbred with Neanderthals and the so-called Denisovans and had joint offspring. Many people still carry the genetic traces of these encounters in their DNA today.

A new study now suggests that there were at least two further, previously unknown human groups that also passed on DNA to our ancestors. One of these previously unknown ancestors interbred in Africa with the forebears of modern humans more than 50,000 years ago. This happened before the last major migration wave of Homo sapiens to Europe and Asia.

The genes inherited at that time now account for around 1% of the DNA of all humans, roughly as much as the share of Neanderthal DNA. According to the researchers' calculations, this mysterious human lineage had split from the evolutionary line leading to modern humans around 800,000 years ago, roughly at the time when Neanderthals and Denisovans also diverged from one another. "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," explains Berkeley doctoral student Yulin Zhang, one of the study's lead authors.

"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 enigmatic is the second ancestor, which the researchers describe as a "super-archaic ancestor". This human lineage had already branched off from other hominins around 1.8 million years ago.

More than 200,000 years ago, it probably interbred in Eurasia with the Denisovans, and via this detour fragments of the ancient DNA finally found their way into the genome of modern humans. "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," says Arjun Biddanda, a postdoctoral researcher at Johns Hopkins University and co-lead author of the study. "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," says Priya Moorjani, professor of molecular and cell biology at the University of California, Berkeley.

"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." Most people outside Africa today carry about 1 to 2% Neanderthal DNA in their genomes. More precisely, on average around 1 to 2% of the genome of present-day Europeans and Asians comes from Neanderthals. This is the result of interbreeding between Neanderthals and early modern humans around 50,000 to 60,000 years ago, shortly after Homo sapiens left Africa.

The inherited DNA segments still influence humans today. Some variants are associated with the immune system, skin and hair traits or adaptation to environmental conditions. Others increase the risk of certain diseases or affect the response to infections.

Population groups in Asia and Oceania also carry, in addition to Neanderthal DNA, genetic contributions from Denisovans. Corresponding remains of Denisovans have mainly been sequenced from the Eurasian region: The genome of the oldest human fossil yet discovered in Mongolia provided new insights into early human history around six years ago. Researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig and the Mongolian Academy of Sciences showed that around 25% of the DNA in the approximately 34,000-year-old remains of a woman came from West Eurasian ancestors.

The team also found in the woman's genome, as in that of a roughly 40,000-year-old individual from China, DNA traces of Denisovans, the extinct human form that lived in Asia before modern humans arrived there. It is also interesting that Neanderthals and Denisovans interbred with each other as well. From a fossil from Denisova Cave in the Altai Mountains in southern Siberia, near the borders with Kazakhstan, Mongolia and China, it is known that it belonged to a woman with a Neanderthal mother and a Denisovan father, direct evidence that the two human forms had joint offspring.

Exactly who these "ghost ancestors" and the "super-archaic ancestor" were is still unclear. However, the calculated dates of their divergence fit known human forms. The ghost ancestors could have belonged to Homo groups of the Middle Pleistocene that lived in Africa around 800,000 years ago.

The super-archaic ancestor, by contrast, could be identical with or closely related to Homo erectus, which populated Eurasia as early as about 1.8 million years ago.