Ancient DNA has shown that a distantly-related “superarchaic” population interbred first with the ancestors of Neanderthals and Denisovans and later with Denisovans themselves. Other work has shown that a superarchaic population interbred with the African ancestors of all modern humans. But it is not yet clear whether these events involved the same superarchaic population. Here, we use the distribution of derived alleles among populations to evaluate hypotheses about superarchaics and their relationship to other hominins of the Pleistocene and Holocene. We find evidence for at least two distinct superarchaic populations. The one contributing to archaic Eurasian populations (Denisovans and Neanderthal-Denisovan ancestors) diverged earlier from the human lineage than did the one contributing to early moderns in Africa. These findings reveal previously unrecognized structure among hominin populations of the Pleistocene.

[–] CanadaPlus 2 points 4 hours ago* (last edited 3 hours ago)

It's worth a read just for the diagrams of possible timelines. Apparently there was at least one earlier wave of Homo Sapiens ancestors going to Europe, too, label gamma, which would have just been absorbed by the Neanderthals.

The Eurasian superarchaics would themselves have left Africa around 2 million years ago, with the very first Homo Erectus or Habilis migrants into the larger world.