I had started this a little while ago and was hashing out the drafts when I realized Darwin’s birthday was coming up, and felt delaying it until now was appropriate. That said, I apologize in advance, because this post is simply begging for a lot more research on my part, but instead of engaging in that and coming back with something more informative, I’m plowing ahead in ignorance (like that’s something new) and posting anyway, because it’s winter ;-). Most of the questions herein, however, cannot at present be answered even if I did the research, because paleontologists haven’t answered them yet either. It’s the nature of the field.
So, let’s get into it. The history of humankind is convoluted: the Hominini (that eventually became modern Homo sapiens, us,) split off from the Paninini (which eventually became chimpanzees and bonobos) somewhere around seven million years ago – more or less kinda, and this is a topic all its own that I’m not going to tackle right now. Anyway, since that split, there have been numerous distinct species on the Hominini side, branching out in many directions, Australopithecus afarensis and Paranthropus boisei and so on, but the finds are sporadic enough and the distinctions large enough that we have no way to determine our direct ancestry. It’s not like it matters all that much; the progression to an upright, more mobile, and especially hunting and tool-using species is clear and more than enough evidence to indicate a close family tree.
Right now we’re going to talk about something much more recent, which are the various species that existed within the past half-million years. Not quite two hundred years ago we found the remains of a species that would come to be known as Neanderthals, with a silent ‘H’ – “Neander-tall” (it’s from German origins.) Most people are aware that Neanderthals existed alongside Homo sapiens up until, oh, somewhere between 50,000 and 30,000 years ago, but no longer exist. When I was growing up, the nomenclature for these ‘cave men’ in early Europe were Neanderthals and Cro-Magnon, which were/are our direct ancestors. The term “Cro-Magnon” has largely fallen into disuse in favor of simply Homo sapiens, and the adoption of Homo neanderthalensis as the counterpart. There was wide speculation as to why H. neanderthalensis doesn’t exist anymore, initially being either unable to outcompete H. sapiens or actively killed off by the same.
But then the real twist. Mitochondrial DNA can sometimes be extracted from remains that aren’t too old, including Neanderthals, and sequencing this genome (the portions allowed by mtDNA anyway) showed an intriguing find: that a certain percentage of people of European descent today have Neanderthal DNA traces, meaning the two species were interbreeding within Europe. Those of direct African descent show no such traces, indicating that it occurred after at least the first emigration.
First pause: We really don’t know exactly when those ancestors (some of them) left Africa, and what route they took in expansion, and how many times it occurred. The fossil record is too sporadic and at times in apparent disagreement. What we do know is that Neanderthals are only found in Europe and just barely into Asia – or at least so far. And H. sapiens appears to have developed within Africa somewhere in the vicinity of 300,000 years ago, after the first emigrations (those that became Neanderthals, for one) and emigrated later.
Now a little history of paleontology. Neanderthals were recognized as distinct from Cro-Magnons/H. sapiens because of their differing morphology, their body shape: Neanderthals were a little shorter, stockier, and with differently-shaped skulls and jaws (and larger brains.) But those differences, while semi-consistent, were not anywhere near as extreme as the differences between modern gorillas and orangutans and chimpanzees; they’re minor enough, it has been said, that if a Neanderthal appeared in a crowd today we wouldn’t be alarmed, those traits falling close to or within the normal genetic variations of H. sapiens.
Which might be telling, because the two species did interbreed, so some of that variation might actually be Neanderthal genes. And this is where I started researching (I didn’t say I didn’t do any, just not enough yet,) because the definition, every definition, of how to distinguish species contains their inability to interbreed. By that token, they were not separate species, but perhaps only subspecies. And in some circles this is reflected, though it’s not universal: there is a proposed scientific classification of Homo sapiens neanderthalensis and Homo sapiens sapiens (modern humans.) This, to me, makes the most sense, because we can’t call them separate species if they could interbreed, but as I said, there remains a lot of the literature that still refers to them as separate, even while recognizing the interbreeding bit. Proposing that modern humans are a separate species now from our ancestors 50,000 years ago still doesn’t make the species distinction between Neanderthals and Cro-Magnon viable.
The differing morphology is evidence of long-term splits between populations, as is the difference in genetic makeup, and this fits with the proposed timelines of emigration from Africa: up to a little over 2 million years ago for the first wave, likely Homo heidelbergensis (the common ancestor of H. Neanderthalensis and H. sapiens,) but this is still questionable because, at that age, we’re back to working with morphology, and the thin remains from those times aren’t specific enough to differentiate all that clearly. Species change slowly, without distinct lines to cross, so the whole process of applying labels to them is strictly a conceit of modern humans. Meanwhile, populations of H. heidelbergensis still remained in Africa too, and eventually gave rise to H. sapiens, that began migrating out about 300,000 years ago. And despite this vast separation in time, could still interbreed with the first émigrés.
Added to this mix we have the Denisovans, a recently-discovered offshoot in Asia, and the only one we know solely by their genetic code; the very sparse bone fragments we have still yielded enough for mtDNA testing, and revealed a broad split from other remains. And evidence of interbreeding, too, since up to 4% of modern Asians bear Denisovan DNA traces. The Denisovans have not yet received a proposed Homo distinction, largely because we have no idea what they looked like; one of the older requirements for such classifications, still retained, is that we have a ‘Type’ fossil, a representative example of the distinctive difference from other species (or ‘species’.)
Just for the sake of it, I’ll mention Flores Man, Homo floresiensis, recently discovered only in a small spot in Indonesia. Those remains are 50,000 years old, around the time that H. neanderthalensis was dying out and H. sapiens spreading out beyond Eurasia, but nothing like H. sapiens at all, being less than half the mass and bearing some archaic traits. There is no evidence for how long ago this offshoot reached Indonesia, but it is suggested that they’re a remnant of that first emigration up to 2 million years ago, and I have found nothing yet that indicates we have genetic information on them.
And thus I remain a little confused, because again, the interbreeding means they’re not distinct species and should be all absorbed into one, though perhaps with subspecies distinctions as noted above – H. floresiensis may yet fit the bill for “no interbreeding,” and this might only be due to its isolation. But to all evidence, we’re all descendants of H. heidelbergensis and only have racial/regional differences. I really need to find a paleontologist who will address these questions for me.