A schematic of a DNA molecule. (Public Domain image from Wikimedia commons.)
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.
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I think the information is delightful. It says to me we’re all the same species, since Cro-magnons weren’t horses and Neanderthals or Denisovans, donkeys. The offspring of Cro-magnons and Neanderthals and Denisovans had viable young (young that could reproduce, unlike mules from horses and donkeys). Having viable young after mating is fundamental to the definition of species. I carry 2.9% Neanderthal DNA and 4% Denisovan. I keep chuckling that if scientists can come up with DNA for Homo erectus and I carry 3.1% of that DNA, then 10% of me would be early man! I agree with your premise that Cro-magnon swamped the “others.” When I think of where are the Neanderthals and Denisovans today, however, I can point to myself and grin. “I am they!”
Strictly speaking, I think Neanderthal and Denisovan are considered to be subspecies of Homo Erectus.
You’re right! Many teach that Homo erectus is a progenitor. But upon what is that view based? Where is the evidence? How has it been ruled out that they did not coexist with others? I’m asking rhetorically and am not at all posing a challenge here. I am not an anthropologist, but I do understand the nitty gritty of the scientific method. I just have not seen enough science for me to conclude validity of today’s initial premises, let alone theories. And that’s not because I didn’t look and look hard. Seriously, I’ll be fascinated if ever Homo erectus DNA is found, and we can see what that implies. If it makes it clear that Neanderthals and Denisovans are subspecies of Home erectus, I’ll be more accepting. It could as easily throw a boomerang in there. Who’d have thought today’s people carry Neanderthal and Denisovan DNA 20 years ago? I’m patient. I do split hairs sometimes. When it comes to science as contributing fact, I’d rather wait when I’m unconvinced to see what the future unfolds than to accept what I feel needs more foundation. So far, I am persuaded by evidence that Cro-magnons, Neanderthals, and Denisovans intermingled, mated, and had viable offspring. By definition that should make them the same species.
Just take a bus, in any city of Europe, North America, South America and Asia. You will see several people with one or more Neanderthal features (Low, flat, elongated skull, or barrel-shaped rib cage, or large round finger tips or longer torso with shorter legs or a projecting mid-face or lack of chin and even some people have brow ridges); so does not matter if our neanderthal DNA is just 2% or 5%, they left their imprint in our bodies.