I don't stay as up-to-the-minute on human genetic evolution as I used to, so I went to check, and there it was: "Genetic evidence for archaic admixture in Africa." The interbreeding probably occurred around 35 to 40,000 years ago, with some other group that had split off from Homo sapiens around 700,000 years ago. Well goddamn. Better reading it a year and a half late than never, I guess. Two follow-up articles, which I have not read (here and here), all by separate teams, confirm that hunter-gatherers in Africa show signs of their ancestors having interbred with some archaic form of human (i.e., not Homo sapiens) tens of thousands of years ago.
Correction: the last article linked only discusses admixture among various sapiens groups in Africa, not with archaics as well.
This is analogous to the sapiens-Neanderthal and sapiens-Denisovan interbreedings, whose signs show up in the genomes of living people today. (Denisovans were closer to Neanderthals than to us, though still distinct from each other.) Those interbreedings happened in Eurasia, while the ones reported above happened within Africa.
In a round-up of some of the main implications of the first article, Dienekes Pontikos wondered:
It is certainly counterintuitive that admixture with archaics would have happened in Africa after it happened in Eurasia. . . It is difficult to believe that Homo sapiens waited 160 [thousand years] to mix with his archaic neighbors in Africa... and yet started hooking up right away with Eurasian archaics.
My hunch is that Homo sapiens was already pretty well suited to Africa, no matter which of the many diverse environments they found themselves in. So, while this mystery archaic Homo must have been locally better adapted than sapiens in some way (hence why we picked up and kept some of its genes), it wasn't a huge difference. Not so pressing of a need to get their genes.
Once sapiens left Africa, though, all bets were off -- we needed all the help we could get, and fast, by picking up whatever genes the Neanderthals had that adapted them to the strange new world that we had stumbled into. It was colder, had a different mix of infectious diseases, etc.
Hopefully I'll be able to throw together a post soon on what this all has to do with the birth of art. In the meantime, just mull over the idea that hunter-gatherers from sub-Saharan Africa aren't a 100% window into our primeval past. Some things unique to them, not shared with hunter-gatherers elsewhere in the world, could have come from the mystery archaic species in Africa. I wonder about linguistic clicks (or something related to it). Or "steatopygia," i.e. having a butt big enough that your baby can stand on it when you're upright.