Making humans modern:
I guess it’s official by now. (Chris Stringer What makes a modern human NATURE vol. 485 no. 7396 May 2, 2012 page 33) Humans are descended mostly from what we have been calling “modern humans” or Homo sapiens, most of us (those not from sub-Saharan Africa in the past 60,000 years) also have a significant contribution from the Neanderthal people and a fairly small proportion, pretty much confined to New Guinea and Australia, have an even larger proportion from a group called the Densovans. Oh, yes. Sub-Saharans also have a significant contribution from an unknown group that was not Neanderthal nor Denisovan. So there is much to talk about when the question of human biodiversity comes up even before one engages in subtleties.
Human biodiversity is not of great interest to me. I see none through my window on the world. And as long as nobody else ever looks out of my window, I feel no great obligation to spend a lot of time looking out of theirs. But what has been learned is kind of fun.
The Neanderthal (I’ve mentioned my spelling preference) were widespread, successful and well adapted in Europe, Asia and the Middle East when Homo sapiens came out of Africa and the two encountered each other and formed a sort of hybrid population first in the Holy Land. The first question is, “What? Are these not different species? They can’t have interbred.” Well they did, so there.
But there is a bit more to it. After the interbreeding all subsequent descendants, at least all living ones, have had absolutely the same ratio of Neanderthal to Sapiens genes. That’s a clue. Sure, there had to be poor fertility between them, successful crossbreeding had to be very rare. In fact throughout Europe and Asia, where the two species lived side by side in substantial numbers, there cannot have been a successful cross because that would have produced an inhomogeneity in the ratio as one looked across the world. So presumably there was one an only one crossbreeding. After that, the band stayed together long enough for the genes to be mixed thoroughly, so everybody had the same ratio.
At this point, one is reminded of cosmology. The cosmologists attribute the homogeneity of the universe to something called “inflation” with the universe expanding faster than the speed of light. The universe was expanding so fast that it was impossible to reach equilibrium. The rapid expansion caused the local equilibrium to be stretched out to something like the size of the universe we know. Here we have the same effect for the opposite reason. It was the lack of expansion that caused the observable homogeneity. When I think about this, I get a bit queasy because, this isn’t making as much sense as I wish it did.
The offspring of the cross may have been at a significant disadvantage. But if there is one things humans do well it’s take care of babies and children. That can’t be a recent innovation. So the children survived. It would have been advantageous to have had the initial community rather small, of course, because we all know that the small gene pool has the greater fertility in the long run. How small? The proportion of Neanderthal genes is about 2.5%. That’s one part in 40. So at one time everybody outside Africa who was going to have descendants survive to this day was a group of 39 Sapiens and 1 Neanderthal.
If that isn’t an odd claim, take the contribution of the Denisovans. Again we must assume that what to all appearances was a different species interbred with humans once and only once. The contribution of the Denisovans, in the lands where it is now found, is 5%. Community size 20. Cool?
Then there is that unknown ancestor, I mean ancestor of unknown species, for the sub-Saharans. That contribution is 2%. Band size 50. Of course this does not take into account any post-zygotic infertility that may have persisted for more than a generation, so that by rights should be taken into account. Still, it’s nice to be able to shake the numbers and get answers to questions.
Here’s a thought. If there were three crossbreeding events we know of, surely there were more. What happened to them? Well maybe a small band can accommodate a single outsider and survive. But maybe if there were several interlopers they introduced enough genetic diversity (or epigenetic diversity) so that the band was no longer viable. That is a mathematical question. So far as I know there is only one person on earth who has the tools to investigate it, and that would in all modesty be moi. Maybe some day I shall, but it is not a critical question; there is no hurry. For the foreseeable I shall press on with what is indeed urgent.
So that’s where we come from. But what should we call ourselves? I would tend to stay with current terminology because I hate change, particularly when it comes to words. But people have an odd way of talking sometimes. For instance, given the distribution of the non-Sapiens human genes, it is said in the article that some people have more archaic genes (archaic meaning genes from species that are not alive today) than others. If the cosmological anti-parallel was a problem, this one really gets me. ALL of our genes come from archaic humans, except a trifling number that have slipped into the gene pool for certain traits. I mean that’s what evolution means, right? We descended from that-which-was-not-us?
But suppose some purist said no, even if it was rare, the occasional successful interbreeding, that is enough to make us all one species. What species would that be? Sorry, it can’t be Sapiens. It has to be the last common ancestor of Sapiens, Donisovans and Neanderthals. The name is Homo heidelbergensis, the Heidelberg man in the old way of speaking. Oh dear. Oh that’s a problem.
You see, Heidelbergensis may not have originated in Africa. The last time I saw “The Student Prince” Heidelberg was in Germany. And of course there is enormous pressure to think that “humans evolved in Africa.” That’s not just political correctness. It’s scripture. Take a look at Hosea 1:1. “Israel,” which context suggests identifies a people prior to the life of Jacob (another name of his is Israel), is called from Africa to the Holy Land. Weird, is it not?
I’m reminded of a night in a Veterans Administration hospital when I was called to see a patient in the wee small hours. The chart was two inches thick. He looked as bright as a daisy. I was wondering just how I was going to throw him out; I had never actually done that nor did I ever, but I was working up to it. The first page of the chart was a letter to the Director of the VA from a United States senator demanding that the patient be treated in a way that satisfied the patient. I thought, “I ain’t goin’ up agin’ no US senator.” I gave the patient everything he wanted. Well I’m happy to go up against scripture, but seeing a multi thousand year old document saying the same thing as one of the most recent and unexpected revelations of science gives me the willies. That’s before you drag the New Testament into it. And if I’m spooked, I dare say others are, too.
But they can always say it’s just political correctness.
You know I used to think humans evolved in England, because there was this skull and jaw called the Piltdown man. He was shown to be a hoax long before political correctness could demand a change in terminology. Ever since then I have personally thought that humans probably evolved in Africa. There’s a good reason for that. Africa is huge. It’s bigger than Asia. In terms of human existence, the Sahara has not always been a desert. It probably was as lush as Florida at one time; it’s about the same latitude. On the other hand in terms of human existence, the northern parts of Asia have been really cold. So just for starters, given the large size of habitable Africa it would have been a really good candidate.
So maybe the Heidelbergs originated in Africa. It remains a good bet. But quite possibly not. Perhaps some day we shall know.
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