Traces of two "ghost lineages" of extinct human populations lie hidden within our genome, a new study reveals.

One of these ghost lineages left its DNA scattered in all present-day people. The other, a "super-archaic" lineage, dates back nearly 2 million years, researchers reported Thursday (July 30) in the journal Science.

But scientists have suspected that modern humans may have interbred with other now-extinct human groups that have not been identified yet. However, the scarcity of ancient DNA samples makes it challenging to identify what these unidentified groups, often called ghost lineages, might have contributed to our species.