An analysis of the whole genome sequences of hundreds of modern-day West Africans, along with those of ancient Neanderthals and Denisovans, points to the existence of a “ghost” species that interbred with Homo sapiens before dying out, researchers at the University of California, Los Angeles, report yesterday (February 12) in Science Advances.
The finding further complicates an evolutionary history in which different hominin species diverged, only to—in some cases—meet up and swap genes with each other hundreds of thousands of years later. Modern humans have already been found to have interbred with Neanderthals and Denisovans.
In the new study, Arun Durvasula and Sriram Sankararaman analyzed the whole-genome sequences of people from four populations living in Nigeria, Sierra Leone, and the Gambia, as well as those from Neanderthal and Denisovan fossils. They found patterns in the genomes of two of the modern groups, Yoruba and Mende, that indicated some of their DNA sequences originated in a species other than those analyzed.
Whatever that species was, “They seem to have made a pretty substantial impact on the genomes of the present day individuals we studied,” Sankararaman tells The Guardian. “They account for 2% to 19% of their genetic ancestry.” He and Durvasula also found hints of the archaic species’ genes in a population in Kenya, and in individuals of Han Chinese and northern and western European ancestry.
Sankararaman and Durvasula estimate that the ghost species split between 1 million and 360,000 years ago from the lineage that produced modern humans and Neanderthals, and that members of that species interbred with Homo sapiens sometime in the past 124,000 years.
“It’s an exciting moment because these studies open a window showing us that there is much more than we thought to learn about our ancestors. But actually knowing who those ancestors were, how they interacted, and where they existed is going to take fieldwork to find their fossil and archaeological remains,” John Hawks, an anthropologist at the University of Wisconsin-Madison who was not involved in the study, tells The Guardian.
The New York Times notes that one candidate for the ghost species is a hominin for which a skull was discovered in 2011 in Nigeria. But researchers have not yet succeeded in extracting and sequencing DNA from that fossil, or from others found in Africa that are thought to be from now-extinct hominins.