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!