The Denisovans and Neanderthals Left a Mark on Our DNA.. What Did Researchers Discover?

- The human genetic code may hold clues capable of solving the mystery of human ancestors that have no clear presence in the fossil record, yet their traces remain in the genes of people living today. Scientists had discovered years ago that Homo sapiens interbred with human relatives that later went extinct, including Neanderthals and Denisovans.

However, identifying some other lineages has remained more difficult, as they, unlike Neanderthals and Denisovans from whom DNA was extracted from ancient remains, resemble "genetic ghosts" with no direct samples of their DNA available for comparison with signals found in the genome of modern humans. Although previous studies have detected remnants of these mysterious lineages, they have not been able to determine when interbreeding occurred or how long ago they separated from the ancestors of contemporary humans.

Now, researchers have developed a new method for analyzing the genomes of modern humans and have uncovered evidence of the existence of two previously unknown lineages. They relied on tracing the DNA of living humans throughout history to reconstruct missing parts of the human family tree.

It appears that one of the lineages interbred directly with Homo sapiens in Africa more than 50,000 years ago, while the other belongs to a much older group that transferred some of its DNA to modern humans indirectly through Denisovans. The results were published in the journal Science on July 30th.

The new method allows scientists to reconstruct genetic maps that reveal the source of different parts of DNA and how they have been passed down through generations, providing a glimpse into branches of our evolutionary past that were previously unknown. Priya Morgani, a co-author of the study and an assistant professor at the University of California, Berkeley, stated that one of the most fundamental human questions is: Why are we here?

These genetic maps allow us to look at the past in a way that was not possible before. The search for ghost lineages began when modern humans left Africa in a major migration about 50,000 years ago, which later led to their interbreeding with Neanderthals and Denisovans in Eurasia.

However, the recently discovered lineages add other chapters to this story. Researchers suggest that one of the unknown ancestors interbred with humans in Africa before that migration.

Estimates indicate that the DNA inherited from this lineage constitutes about 0.5% to 1% of the genomes of present-day humans, a percentage that is close to the amount of Neanderthal DNA found in many contemporary humans. The second lineage is much older, and researchers describe it as a "super ancient" lineage because it branched off from the human family tree about 1.8 million years ago.

It appears that members of this lineage interbred with Denisovans in Eurasia more than 200,000 years ago, and then part of their DNA later transferred to modern humans through interbreeding with Denisovans. To uncover these ancestors, researchers developed a new computational method called "TRACE," which searches the genomes of contemporary humans for traces left by long-extinct human groups.

Morgani explains that the human genome resembles a mosaic made up of small pieces of DNA inherited from a large number of ancestors. Therefore, each region of the genome has a different family tree; some are relatively recent, while others extend deep into the past.

The scientists tested the technique on more than 500 genomes of contemporary individuals from Africa, Europe, and Asia. They found traces of the most recent ghost lineage in all the groups they studied, indicating that it transferred its genes to the ancestors of humans before the great migration from Africa.

Researchers are still unable to identify the human groups to which the two lineages belong, but they have narrowed down the possibilities based on the history of their separation from the human family tree. The timing of the most recent lineage's separation aligns with human groups that lived in Africa during the Middle Pleistocene, opening the possibility of its connection to Homo heidelbergensis, which lived in Africa and Europe between about 700,000 and 200,000 years ago.

As for the much older lineage, researchers believe it may have been related to Homo erectus in Eurasia. So far, scientists have not been able to extract analyzable DNA from either of these two groups.

Researcher Argon Bidanda stated that the discovery of the extremely ancient lineage is exciting, particularly because it reveals genetic contributions from a human lineage that lived over a million years ago despite the absence of sequenced DNA from it. Paleoanthropologist John Hawks believes that the differences we focus on today among humans, such as skin color or facial features, are very small compared to the differences among our ancient ancestors.

However, those different groups were able to interact, interbreed, and coexist. Scientists hope that the TRACE technique will reveal more hidden lineages in the future, not only in humans but also in other animal species.

However, the best potential evidence remains the DNA extracted directly from ancient fossils. Until such evidence is found, the traces hidden within the genes of contemporary humans remain the closest window through which scientists can view these lost ancestors.

Two people living today in different parts of the world may carry the same piece of DNA inherited from a mysterious ancestor who lived hundreds of thousands of years ago, possibly more than a million years ago.