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.

Aha!
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.)

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

This and tomorrow's post are stimulated by the appearance of a series of papers on human evolution that have appeared, with the usual hullaballoo, in the May 3 issue of Nature.   They are good papers, summarizing current views and recent data, and while they don't present much if any new information, they package recent results into a show-piece issue that looks good on newsstands.  And they have the blogosphere buzzing, so we thought we'd comment on the issues which, as usual, are being somewhat overblown.

Schools of thought schools
When I was a graduate student (long, long ago), there were major divisions among prominent physical anthropologists.  Some -- probably the bulk of the profession -- saw different fossils that seemed to be contemporary as being samples of representatives of different species.

Others applied population thinking and held the view that human ancestors were a single variable species, fossils were never exact contemporaries or had been living in different places in Africa (or, for later fossils, in Eurasia).  To them, variation included sexual dimorphism and ordinary geographic variation such as we see among humans today.  In this view, there had always been only one species of humans or our direct ancestors.

The two views represent widespread differences between 'lumpers' and 'splitters' in the field of systematics--naming and classifying organisms. Lumpers see fewer species and focus on continuity and variation, while splitters tend to favor more categories when they see differences among specimens. We won't get into that bigger difference of view, because it's not in our expertise.

Multiregional hypothesis

The single-species hypothesis saw the attempt by paleoanthropologists to assign species names to every specimen they found as being a kind of naming-credit gambit, or else a subtle reluctance to accept anything brute-like as being in our noble ancestry.  They all represented extinct side branches: they just weren't human enough to be our relatives!

A more serious argument for the single-species view was that possession of culture was our defining trait and signs of it could be seen even in ancient sites.  It was our 'ecological niche', and by the 'competitive exclusion principle' of population ecology at the time, only one species could occupy a given niche: other interlopers would be driven out, and to extinction. The single-species view could be called the Michigan school of paleoanthropology, because it was the view at the University of Michigan (where I was studying) by major protagonists Loring Brace and Milford Wolpoff.  The view was also influenced by the sophisticated population thinking of geneticist Frank Livingstone, and that was how I was trained.

Making the story plausible

Migration out of Africa

But there was a problem!  Human-ancestor-like fossils were found, from about a million years ago in Africa and in Eurasia.  But modern-looking human fossils we can generically call Homo sapiens were found in Africa around 200,000 years ago, and only in Eurasia around 100,000 or fewer years ago.  This satisfied the multiple-species people, whose Out of Africa (OOA) replacement hypothesis held that the modern guys dispersed from Africa in an expanding gradually, generation by generation in a front of local hunter-gathering groups, that somehow exterminated the dolts already living in Eurasia. How they managed to achieve this displacement was the subject of much speculation.

The single-species response became known as the regional-continuity or multi-regional (MR) view.  It held that via gene flow of people choosing mates from neighboring populations, as primates and humans do, and/or similar selective forces worldwide, our ancestors developed modern-looking anatomy (and brains) all over our area of habitation--Africa and Eurasia:  we had always been, and remained, one species.

Earlier fossils may more clearly show species differences, but by the time of the Neanderthals and other fossil specimens, such distinctions were less clear, and more debatable, but that didn't answer the overall continuity argument.

But then along came genetic data from living populations in the mid 1970s, in the form of frequencies of a modest number of gene variants sampled in Africa, Europe, Asia.  If after settlement these continental populations had been totally isolated, one could estimate how long it would take for the observed differences to accumulate, and the answer was only around 100,000 years.  That seemed clearly to show the replacement hypothesis to be true.  One possibility, that I myself (with population geneticist Takeo Maruyama) investigated was whether, if there had not been complete isolation among continents, the mating between adjacent populations that must have happened, would have gradually spread advantageous 'modern' genes across the whole human-occupied range, keeping us a single species: the genetic time estimates could be wrong if the complete-isolation assumption were wrong because there had been gene flow (which, again, we know always occurs), the true ancestral time could be closer to 1,000,000 but the estimated time only 100,000.  So single species ideas might be true. 

Then along came the ability to extract DNA sequences from fossils of up to around 40,000 years old.  Those data, too, seemed to suggest the recent split-time, and more sophisticated modern genomic data show that current Eurasian variation represents a subset of current African variation.  That means that all our ancestors must have arisen in Africa, and some subgroup of them were the people who expanded gradually out of Africa.  The fossil DNA raised the question of 'admixture', or mating between Neanderthals and more advanced modern-looking hominids, but the data initially provided little if any evidence for that.

The  MR hypothesis seemed as dead as those unfortunate descendants. Textbooks changed:  only OOA was a legitimate hypothesis.  Michigan school:  Bah, humbug!

The more interesting question raised by OOA
But if the replacement hypothesis triumphed, it left by far the most interesting question: how could some hunter-gatherers, small dispersed bands, have expanded mile by mile, over many generations, out of arid, tropical East Africa (which is what the genetic evidence seems to indicate), and killed off Homo erectus?  The latter had occupied all sorts of ecosystems very different from Africa for hundreds of thousands of years, from hot to humid to temperate or even frigid....and whose skeletal anatomy seemed to many (especially to Michiganders) to have been modernizing in ways similar to what was going on in Africa.  And how did one isolated region of Africa evolve our modern form, leaving the rest of the continent behind, because OOA also implied that the moderns expanded across Africa and did in the Homo erectus that were there as well?

One could invoke (or more accurately, invent) various ecological and behavioral explanations for the extermination, but while superficial and rather untrammeled speculation was rife, the question was never substantially answered, or in many ways, not even seriously addressed. Indeed, the Michiganders themselves seemed to have retreated, tactically, saying that no, they never really said only one species!  Like Emily Litella of the old SNL shows: "Never mind."

But something interesting has happened, that we'll talk about tomorrow.  We will have some new humbug to discuss!