Tens of thousands of years ago, modern humans may have interbred with a yet-to-be-identified related species. We know this because traces of this ancient interbreeding have been transmitted to the genomes of modern-day west African populations, according to a recent study.

Human evolution hasn’t followed a straight line but rather a series of divergent branches. Some of our ancestor’s lineages went extinct entirely, while others branched further.

It’s remarkable to imagine that at one point in time Homo sapiens shared this planet with at least four other lineages: Neanderthals, Denisovans, Homo floresiensis, and perhaps with Homo luzonensis (the latter still awaiting confirmation).

Now, a new study suggests that there may be a fifth lineage of humans. We haven’t found their fossils, but their genes are still very much etched in the genomes of certain west African populations. In some cases, up to a fifth of their DNA comes from these mysterious missing relatives.

The discovery was announced by a team of researchers led by Sriram Sankararaman, a computational biologist at the University of California, Los Angeles, in a new study published in Science Advances.

Sankararaman and colleagues sequenced the genomes of 405 individuals from four West African populations: two from Nigeria, one from Sierra Leone, and one from Gambia.

By employing statistical techniques, the researchers looked for markers of interbreeding that may have occurred in the distant past — turns out it did.

When they compared the African genomes with DNA from Neanderthals and Denisovans, the researchers found a huge chunk of DNA that came from an unidentified group of archaic humans. Their impact was highly substantial, ranging from 2% to 19% of the populations’ genetic ancestry.

“We don’t have a clear identity for this archaic group,” Sankararaman told NPR. “That’s why we use the term ‘ghost.’ It doesn’t seem to be particularly closely related to the groups from which we have genome sequences from.”

These ‘ghost’ people seem to have split from the common ancestor shared with Neanderthals and modern humans between 360,000 and one million years ago. According to the molecular genetic data, as many as 20,000 individuals belonging to this group of archaic humans interbred with the ancestors of modern West African humans at some point in the last 124,000 years.

We don’t know how these human relatives looked like or how they lived, but their genes paint a compelling story of complex interactions with our ancestors. In the process, our own story has suddenly become richer.

There is still much to learn. Next, the researchers plan to focus on some of these genes in order to tease out what they do. It’s very possible that many of these genes were kept across the generations because they provide an adaptive value to West African populations. What would prove to be a gold mine is finding fossils belonging to this mysterious extinct human relatives. Alas, that’s highly challenging since Africa’s climate is not kind on fossil preservation — but one can never know!