Researchers Identify Two 'Ghost' Ancestor Lineages in Modern Human DNA

Discovery of 'Ghost' Lineages

Researchers led by the University of California, Berkeley, have identified genetic evidence of two previously unknown archaic human lineages, referred to as "ghost ancestors," within the genomes of modern humans. These populations are termed "ghosts" because, unlike Neanderthals and Denisovans, no prehistoric physical remains have been recovered to provide a direct DNA sample for comparison.

To uncover these traces, the research team developed a new computational framework called TRACE (TRacking Archaic Contributions via ARG Estimation). This tool allowed scientists to identify stretches of DNA in modern humans that originated from these extinct groups.

Divergence and Interbreeding Timelines

The two identified lineages diverged from the human family tree at significantly different points in history. One ghost lineage is estimated to have separated from the ancestors of modern humans approximately 800,000 years ago. This timing is close to when Neanderthals and Denisovans also split from one another. This specific lineage likely integrated into human genes around 300,000 years ago through introgression, the process of transferring genes between different species.

A second, more ancient lineage is estimated to have diverged over 1.7 million years ago in Eurasia. While one lineage is associated with interbreeding in Africa more than 50,000 years ago, the other is linked to the Eurasian continent.

Genetic Impact and Composition

DNA inherited from one of these ghost lineages constitutes approximately 1% of the modern human genome. This proportion is similar to the amount of DNA modern humans inherited from Neanderthals.

According to the researchers, the ghost population that diverged 800,000 years ago exchanged genes with modern humans at an earlier point than the encounters between modern humans, Neanderthals, and Denisovans that occurred outside of Africa.

Challenges in Species Identification

Despite the genetic evidence, researchers cannot yet definitively name the specific fossil species these genetic signatures represent. The estimated divergence times overlap with several known hominin groups from the Middle Pleistocene and earlier periods.

Scientists stated that genetic evidence alone is insufficient to confidently assign either ghost lineage to a particular known fossil species. However, the findings indicate that the human family tree was more interconnected than a simple sequence of populations replacing one another, with ancient groups repeatedly moving and exchanging genes.