Tuesday, May 15, 2012

Digging up the past (single species hypothesis revisited): Part II

In yesterday's post, we offered a brief rundown of the competing arguments re. the history of human expansion from Africa.  Briefly, the rival multiregional (MR) continuity, and OutOfAfrica (OOA) replacement ideas became iconified as two competing (food-fighting) schools of thought, and so it was for decades.  Either Homo erectus forms expanded across and also out of Africa, their descendants dispersing over many generations gradually to colonize the rest of the Old World in the Pleistocene 2 million years ago, and evolved gradually into modern humans, or modern humans evolved in East Africa and expanded into the rest of Africa and the Old World, into contact with other humans already living wherever they wandered into, and somehow driving them to extinction.  The idea that the two may have interbred was prurient and juicy, but seemed dismissed by the fossil DNA data.  People, and certainly anthropologists, like to polarize into rival camps and are not particularly good at nuance.  So we had the dispute, fruit and vegetables flew back and forth, but the kiddie cafeteria seemed to settle down when the OOA hypothesis felt in a position to declare victory.

But the issues are heating up again, triggered this time by a typically melodramatic series of articles in a typically melodramatic May 3d issue of Nature. Further Neanderthal genome sequence, from a few specimens, compared to sequence from some contemporary 'anatomically modern' fossils, all roughly 30-40,000 years old, and then more recent data from a Siberian fossil group known as Denisovans, have muddied the waters.  They show, in various technical ways, that there seemingly was interbreeding among these groups: basically, there are many nucleotide variants found in humans that differ from chimpanzees, and seem to represent new mutations having arisen in our lineage.  But other variants, found in the Neanderthal and Denisovan sequences but not in chimpanzees, are also found in humans.  They seem to represent ancient variants later introduced (by interbreeding) into the our modern human lineage--you and me!

The new interpretation, which of course could be only as reliable as last year's newspaper, is that a small percent, currently estimated at about 2.5%, of our DNA is due to admixture between the Neanderthal/Denisovan/modern denizens of the Eurasian past. 

Now there are the expected crow-calls being heard on the blogosphere, claiming vindication if not victory for the Michigan, single-species or MR school (at least, that if there had been many early in human evolution, there was only one by the time of Neanderthals)!   Get your lunch boxes ready, folks, as it is time yet again to hurl what you don't like in your lunch at somebody else.

Please hold your fire!
But let's try to sort credible evidence from religion here.  In truth, if the evidence is being reliably interpreted, there does seem to have been some mixing and mingling (and mating) among lineages of our ancestors who had been long-separated since their common ancestry in Africa.  This clearly shows that a strict multiple species view was wrong.  But much hinges not on incisive insight but on definition--of what a 'species' is and the criteria by which species are declared.

We've posted before on the whole species question, which is well-known to be a vague area with various definitions.  The idea, in a nutshell, is that organisms diverge from common ancestry to an extent that, eventually, they cannot interbreed.   The presumption is that this is for genetic reasons.  But others would define species as populations that do not interbreed.  Be they on different continents or in different levels of a forest, if they don't mate they're different species, and we need not worry particularly about whether technically they could interbreed.  If they don't do it, then eventually the assumption (which was Darwin's as well) is that they will lose that ability even had they the lingering desire.

In this case, assuming the evidence and interpretation that's current, clearly the Neanderthals and modern OOAers diverged in morphology and this probably occurred in geographically distant areas, which is what one would expect of adaptative as well as chance genetic change, because isolated individuals don't mate and hence blend their various characteristics.

In the morphological sense, there were differences and the OOA view is correct.  Even if we agree that the differences were major enough for species designation, by the usual definition these were different species at the time ancestors expanded out of Africa 100,000 years ago.  So much for the single species view.

Well, not really!
And yet, the fact of inter-mating, again if interpretations today are accurate, shows that in a technical sense the groups were not different species by the mating-incompatibility definition.  So, as my alma mater goes, Hail, Hail to Michigan, Champions of the West!

We must note that some anthropologists still argue that they see comparable trends of modernization in Asian fossils that resemble those in Africa, in support of their MR view, and that they didn't need the DNA evidence to make this point. And many would argue that in any practical sense, such as related to cognitive abilities and so on, that some of these inter-breeders like the Neanderthals, were not inferior to 'modern' humans--they were all 'human', just not the same as each other.

Splitting hairs
We should stop all of this, which largely amounts to hair-splitting, and try to be accurate scientists instead of tribal warriors.  Regional divergence arose, just as there is regional divergence among humans today: In many obvious ways Africans don't look like Asians, and this is manifestly clear in terms of geographic genetic variation, too.  Except in the narrow technical definition, there were different species.

But the OOAers have no reason to cheer.  That's because interbreeding shows that genetic species differences of the classical sort hadn't arisen.  In that sense, the MR view is correct. But there is no place for crowing, because in the do-not-mix definitional sense, there were separate species.

Since there is no single answer to the amount or nature of genetic difference required to prevent fertile breeding,  and since what we do know is that that can vary hugely from example to example, and even the species definition itself is somewhat arbitrary, the dispute is largely a fake one, and the news-splash it regularly receives is either ignorance,  marketing, or just sport.

What remains interesting is the question I raised last time: the way in which the more modern creatures expanded out of Africa,  and met and interacted with their less modern-looking brethren.   And why the modern-lookers did, the evidence still seems to suggest, drive the others out of existence.  That is the really important question, and it's far more interesting than the DNA-based food fight.

(Thanks to Holly for helping me fix some of the phrasing of this and the previous post.)


occamseraser said...

At the risk of being both redundant and pedantic, the SSH was NOT just about modern human origins and the fate of Neandertals (and Denisovans!). It was about gracile and robust australopiths too. It was about all Homo-anity being a single interbreeding species for the last 2 million years.

Your points above are well-taken -- and well-made as usual -- but for others to focus now almost exclusively on aDNA and modest levels of gene flow in the last 50-100 Kya as the core of the SSH is simply convenient, revisionistic history. And other pop genetic models might explain these tiny percentages (as frequently discussed on other blogs). As far as I can tell, another wolverine named Fred Smith got it right -- OOA with modest degrees of assimilation.

Ken Weiss said...

Yes, Holly pointed that out to me and actually I put in some obviously rather unclear caveats in this and the earlier post.

I agree about revisionism, but changing the goalposts is how science always seems to work so that nobody can ever be forced to admit an error!

I remember well that the SSH was at that time largely about Australopithecines and Paranthropines (whatever their proper names are these days). But the same was true of Neaderthals, who were referred to as 'them' and all sorts of explanations were available to show that they were different (even if they had bigger brains).

Coon did say in the '30s I think, that a Neanderthal was pretty much just like a Penn State Football player compared to the rest of us.

But part of my reason for writing these posts is the sense that SSHers are going to claim an Aha! moment from the aDNA when that is a goalpost changing revisionism in the sense you say.