An ancestor without a name
This is not a place you can visit, but a discovery you can carry within your own cells. In 2020, geneticists Arun Durvasula and Sriram Sankararaman from UCLA published a landmark study in the journal Science Advances. They analyzed 405 genomes from four West African populations, including the Yoruba people of Ibadan, Nigeria, and the Mende of Sierra Leone. Using novel statistical methods, they found that between 2% and 19% of the genetic ancestry in these groups comes from an archaic, unknown hominin population.
This "ghost lineage" is not Neanderthal or Denisovan. In fact, the analysis suggests this group split from the ancestors of modern humans and Neanderthals between 360,000 and 1.02 million years ago. The interbreeding that introduced this ancient DNA into the Homo sapiens gene pool likely occurred in the last 124,000 years. The researchers identified this lineage without any physical fossils or ancient DNA. Instead, they developed a computational technique to spot unusual sections of modern DNA that did not match any known human or archaic reference genomes, revealing an ancestor known only by its genetic echo.
The genetic echo
The existence of this ghost lineage complicates human evolution. It shows that while the ancestors of Europeans and Asians were interbreeding with Neanderthals and Denisovans, a similarly complex web of interactions was happening within Africa. The term "ghost" is used precisely because no physical remains have ever been linked to this population. The warm, humid climate of West Africa makes the preservation of ancient bones and DNA extremely difficult.
The DNA inherited from this archaic group is random code; some of it appears to have been beneficial. The analysis by Durvasula and Sankararaman identified specific genes from the ghost lineage that appear at unusually high frequencies in modern West African populations, evidence of positive selection. These genes are involved in functions like tumor suppression, hormone regulation, and male reproduction. This process, called adaptive introgression, shows that interbreeding with other hominins provided our ancestors with useful genetic tools to adapt to new environments and challenges. The study of the Yoruba and Mende people revealed approximately 500 megabases of archaic sequence in their genomes.