Scientists reported on Wednesday that they had discovered evidence of an extinct branch of humans whose ancestors split from our own a million years ago. The evidence of these humans was not a fossil. Instead, the researchers found pieces of their DNA in the genomes of living people from West Africa.

Arun Durvasula and Sriram Sankararaman, two geneticists at the University of California, Los Angeles, described this so-called ghost archaic population in the journal Science Advances. Their discovery may shed light on human genetic diversity in Africa, which has been hard to chart until now because the fossil record is sparse.

The new study builds on a decade of research into ancient DNA extracted from human fossils. In 2010, a team of researchers published the first genome of a Neanderthal. Later, they found DNA from fossils in a Siberian cave called Denisova. That genetic material belonged to a second lineage of humans, called Denisovans, who proved to be closely related to Neanderthals.

The ancestors of Neanderthals and Denisovans split from our shared ancestor about 600,000 years ago, quite likely in Africa. They expanded into Eurasia, where the Neanderthals moved west while the Denisovans moved east. By roughly 40,000 years ago, both populations became extinct.