Homo has been spreading out of Africa since long before the evolution of “true” or “modern” humans. But what exactly is a “true” human? What is a species?
Once it was simple. God made the species, which were unchangeable. Then the naturalists got into it, and the head-scratching began. The recognition that species could actually go extinct made more problems yet. Which modern species were they most like? Were they even “new” species, or variants of modern ones? Remember that the first “natural histories” included some very odd beasts from travelers’ tales, some of which were probably at fourth and fifth hand.
Comparisons within the human family tree have always been particularly fraught. Quite aside from the fact that many still refuse to accept the evolution of human beings, every paleontologist wants to be remembered as the discoverer of a new species. But it seems likely that Homo habilus, Homo erectus (who left Africa and included the subspecies Neanderthal and Denisova) and Homo Sapiens were valid species in that it is unlikely that an early Homo Habilis could have interbred with a late Homo erectus – though DNA is providing some surprises.
Even a relatively few years ago, when Jean Auel’s first book was published, the idea that Homo Sapiens, the upright and noble cave artists, could have interbred with brutish Neanderthals was laughed at by many anthropologists. Physically impossible! Any such rare hybrids would have been sterile, like mules!
Then DNA sequencing from bone fragments became possible. DNA of two variants of Homo Erectus, Neanderthal and Denisovian, was successfully sequenced. Bits of Neanderthal and Denisovan DNA were found in every human population except those of pure sub-Saharan African descent.
Love or war? We don’t know and most likely never will, but probably both. Obviously our DNA was still compatible. It is quite possible that the “extinction” of the Neanderthals by Homo Sapiens was more of a genetic swamping. We even know what some of the Neanderthal genes we retained were – part of our modern immune system. Makes good sense: the Africans would be wide open to cold-adapted parasites and diseases, while the Neanderthals had adapted to them over a couple of hundred thousand years.
We know far less about the Denisovans, though since I turned out to have a whopping 3% Denisovan, I’m going to be following their story with considerable interest.