The Little Species that Could: Boivin et al.’s "Human dispersal across diverse environments of Asia during the Upper Pleistocene"

The environmental fluctuations that characterized the Pleistocene meant there were sometimes more- and sometimes less-attractive times for our ancestors and fossil relations to have left Africa for greener pastures. In their Jan. 15, 2013 electronic release, Nicole Boivin and her co-authors leverage their [nearly] unique knowledge of Arabia and southern Asian to try out an argument that they abstract in this way:

Fossil, archaeological and genetic findings are seen to converge around a consensus view that a single population of H. sapiens exited Africa sometime around 60 thousand years ago (ka), and rapidly reached Australia by following a coastal dispersal corridor. … We argue that the fossil and archaeological records are too incomplete, the coastal route too problematic, and recent genomic evidence too incompatible for researchers not to remain fully open to other hypotheses. … Current archaeological, genetic and fossil data … appear to increasingly favour a more complex out of Africa scenario involving multiple exits, varying terrestrial routes, a sub-divided African source population, slower progress to Australia, and a degree of interbreeding with archaic varieties of Homo. (Boivin, N., et al., “Human dispersal across diverse environments of Asia during the Upper Pleistocene.” Quaternary International (2013),  

So, what can Boivin et al. tell us about our evolution that we didn’t already know? Their paper gives us a possibility, not a likelihood. It accommodates some data. It tells a story.

To the great pleasure of this reader the authors present some stunning graphics of the range of palaeoenvironments across Africa and Asia in the Pleistocene [see below]. These help to underscore their main contention that at different times during the Pleistocene, environmental conditions were changing such that anatomically modern bipedal apes would have had numerous opportunities to spread beyond Africa and into Asia. In that nutshell is, I think, the fundamental problem with Boivin et al. That’s because, rather that supporting their arguments, the archaeological record as it stands is telling us that the anatomically modern version of Homo sapiens, extant between about 195 and 40 kyr ago, was little better at adapting to new environments than an African bovid.

In the two maps shown below I’ve melded each of the four maps in the paper so as to produce a panorama of the pertinent geography at MIS 5 and 4.

MIS 5—Interglacial conditions between about 130 ka and about 71 ka. From Boivin et al. 2013.
MIS 4—Mainly glacial conditions from about 71 ka to about 60 ka. From Boivin et al. 2013.

During MIS 5, the authors point out, a continuous, hominid-friendly, environment spanned northeastern Africa, the southern Arabian peninsula, and southern Asia. In other words, Nature afforded H. sapiens a ‘way out’ of Africa during MIS 5. By contrast, in MIS 4, that same part of the globe was covered in red—i.e. not a nice place for hominids. So, no hominids got out. In so doing [IMHO] the authors paint a picture of a species that was so niche-specific as to be incapable of any kind of rapid adaptation to new environmental conditions. In none of the discussion do Boivin et al. appear to realize the apparent behavioural rigidity their model proposes as characterizing early anatomically modern H. sapiens. Indeed, their model stands in stark contrast [it seems to me] to the almost infinite behavioural plasticity that we’ve grown accustomed to hearing about whenever someone is listing the hallmark manifestation of we modern humans. Boivin et al. are begging the question “Were they human like us?”

By adopting this posture toward H. sapiens, the authors shoot themselves in the foot. Indeed, they appear to be arguing for a reduced set of ‘cultural adaptations’ prior to the time that people like us first set foot in southwestern Asia about 40 or 50 kyr ago. The more recent H. sapiens had a recognizably modern set of abilities, and it is those H. sapiens that, in a geological blink of an eye, spread across Asia and into Australia [the actual duration of the spread may have been on the order of 5,000 years]. What’s more, the behaviourally modern H. sapiens did so by changing gears whenever they encountered a novel ecosystem. Think of it. If Boivin et al.’s vision of H. sapiens’s capacities is anywhere near accurate they’d have a hard time explaining the persistence of the Inuit or the Tierra del Fuegans, both of which groups have managed to survive in, without doubt, the harshest conditions on Earth.

Overall, I’m incredibly happy to accept Boivin et al.’s portrait of the environments in marine isotope stages 4 and 5, the interglacial/glacial swing during which Homo sapiens arose in Africa, i.e. somewhere between about 190 ka and about 160 ka. From that time until about 40 to 50 kyr ago the authors aver that Homo sapiens didn’t just pop out once, but did so numerous times, following different pathways, and biomes.

All along the archaeological world had been aware of two excursions by H. sapiens out of Africa—one about 100 ka and the other at 40 to 50 ka. The authors propose that it may not have been as simple as the archaeological and record might have us believe, and that there may have been at least one more and possibly many more excursions out of Africa that are, at present, archaeologically invisible. Getting back to observations for the moment. At around 100 ka we see anatomically modern Homo sapiens at Qafzeh Cave in what’s now Israel. At that time H. sapiens was evincing the same behaviours as those of their contemporaries, the Neanderthals, which were distributed across Europe and into Central Asia. And here the authors’ idea runs out of gas, by not recognizing the possibility that, anatomical similarities notwithstanding, the two excursions could easily have been undertaken by two behaviourally very different flavours of H. sapiens. It has to be admitted that the Qafzeh H. sapiens at 100 ka was not acting as if they were people like us. They were, instead, behaviourally just like good Neanderthals—e.g. the same, Mousterian, behaviour with respect to working stone. Moreover, the Qafzeh hominids arrived in the Levant along with an African fauna, all of which appear to adhere to Boivin et al.’s model for expansion. But those earlier H. sapiens got no further, apparently, than that environment allowed. Because, by the time the environment had returned to glacial conditions, the Neaderthals were inhabiting the same geographic place that had been held by the Qafzeh hominids for at least 40,000 years previously. It appears, then, that the Neanderthals were associated with a western Asian faunal community. Thus, we have a picture of two species, acting in similar ways, each constrained by different environmental conditions. I’m sorry. That just doesn’t sound to me like they were much like us.

I’m not fond of nothing buttery. However, I see no other conclusion to be made but that anatomically modern bipedal apes at 100 ka were not cognitively similar to you and me, and thus couldn’t adapt to the transition from interglacial to glacial conditions. The authors state that many have called the dispersal of H. sapiens into the Levant around 100 kyr ago ‘failed.’ In fact they thrived for at least 40 kyr in the Levant in just the same way as they had done since arising in Africa—perhaps almost 200 kyr ago—and exploiting an almost identical environment to that which supported them in the Levant from about 100 kyr to about 60 ka. Indeed, the ‘failed’ dispersal that Boivin et al. refer to was no dispersal at all. It involved a creature bound to its ecosystem and its niche—albeit a different one—in the same way that it appears the Neanderthals were bound to theirs during their 200-or-so-thousand-year tenure.

Boivin et al. are quite correct in pointing out the equivocal nature of the genomic observations being talked about these past few years. Given the behaviour of the Qafzeh H. sapiens and that of the Neanderthals, a parsimonious reading of the data is that anatomically modern H. sapiens came in contact with, and may indeed have bred with, Neanderthals during the forty or so thousand years that H. sapiens was in the Levant. In all probability there was a frontier somewhere to the north of Qafzeh Cave where the two kinds came in contact for at least some portion of those 40 kyr. Such a scenario makes sense [to me at least] of the genetic evidence that we’re seeing in the present. Any interbreeding that may have gone on between the two species could have been a fait accompli by the time behaviourally/cognitively modern H. sapiens entered Europe. Thus, we dont have to imagine a time when people like us encountered and then mated with Neanderthals.

Further evidence that neither the Qafzeh hominids nor the Neanderthals were like us might be drawn from the observation that there was no extirpation of the Neanderthals coincident with the earlier anatomically modern H. sapiens expansion into the Levant between 100 and 60 kyr ago. In other words, for the 40 or so thousand years during which they were neighbours, there appears to have been no anthropogenic cataclysm comparable to that which many would argue was visited on the Neanderthals during the latest excursion from Africa of cognitively modern H. sapiens like you and me. If, indeed, H. sapiens spelled the doom of the Neanderthals between about 40 and 50 kyr ago, if the Qafzeh hominids were exactly like us, what explains the persistence of the Neanderthals? In the end, Boivin et al.’s narrative ignores the real likelihood that there were evolutionary changes within H. sapiens between the time they occupied Qafzeh Cave at around 100 kyr ago and about 40 kyr ago, when people like us colonized the world.

In the search for answers to the question of when hominids became human, I think the disciplines of archaeology and palaeoanthropology have some distance still to go before gaining a truly accurate accounting of recent evolution.

This piece took me much longer to put together than I had originally anticipated. Next time I’ll have something to say about the latest data on the latest Neanderthals in Spain.

SA announces new posts on the Subversive Archaeologist‘s facebook page (mirrored on Rob Gargett’s news feed), on Robert H. Gargett‘s page, Rob Gargett‘s twitter account, and his Google+ page. A few of you have already signed up to receive email when I post. Others have subscribed to the blog’s RSS feeds. You can also become a ‘member’ of the blog through Google Friend Connect. Thank you for your continued patronage. You’re the reason I do this.

I Have A *cough* Bone To Pick with the Neanderthal Interbreeding Advocates: The 1000-Genome African Sample is Seriously Inadequate!

Since the Very Serious Anthropologists and Geneticists don’t seem to get it, let me try another way. Those estimable scientists believe that they have more-or-less irrefutable evidence that people like you and I, upon entering Europe around 40,000 years ago encountered the Neanderthals and, in true human fashion saw no reason to avoid having sex with them. As you read on, you’ll discover why it is that the 1000-genome project and its [always provisional] conclusions are inherently flawed–their choice of African samples simply can’t satisfy the purpose for which the project has intended it. Moreover, the absence of anything resembling a representative sample of the ancestral genetic diversity in Africa renders their conclusions equivocal, at best, and at worst, null and void.

First let me remind you that, even without the recently acquired ability to sequence entire genomes, it’s intuitively obvious that the modern human and Neanderthal genomes would share a large number of genes of all kinds by virtue of a common ancestry reaching all the way back to the first DNA molecule. In like fashion, we share a slightly smaller number of genes with gorillas, chimpanzees, and bonobos. Less still with gibbons, siamangs, and orangutans. And it’s equally logical that we share fewer DNA segments with the other mammals, fewer with the fish, and so on. I can say this even without knowledge of a single genome.

A Neanderthal genome has been published. Likewise a Denisovan. And fine-grained comparisons have been made between those observations and those on about two-and-a-half thousand people alive today. As yet, we have no clue as to the genome of Homo erectus / H. ergaster. Svante Pääbo and colleagues, and biological anthropologist John Hawks, among many others, want to persuade us that their data reveal interbreeding between people like us and the Neanderthals.

At times over the past year I’ve made great fun of the notion. That’s primarily because, in my view there’s no evidence that the Neanderthals could communicate in the way I am with you at this moment. Most anthropologists would probably concede that absence of language in the Neanderthal species would pretty much rule out what has historically been called human culture, and thus would likely render any modern human impression of the Neanderthals on a par with our impressions of gorillas and chimps. If, and I must [alas] emphasize, if the Neanderthals and their contemporaries were bereft of language, it would be highly unlikely that you or I or an Aurignacian or even a present-day member of the American Republican Party would think that it was a high-percentage move to mate with one. My guess is that it would be tantamount to the tongue-in-cheek proverbial shepherd and his fearful sheep.

[Admittedly, animal behaviorists and primatologists who study living anthropoid primates will tell you that most of them evince something like a ‘culture,’ in that they learn through species-specific social behaviour–i.e. learning. Some primatologists will also want to tell you that the African great apes are capable of language. To date, however, not a single chimp, bonobo, or gorilla have come forward with evidence that they are capable of even one Nth of what you and I can can accomplish with our minds.]

In undertaking anew my effort to scrutinize the data underlying claims for interbreeding, I’m undaunted by the reality–that most of us would have to go back to school to understand the even-more-sophisticated analyses of the genome data acquired by the ultra-sophisticated techniques upon which the 1000-genome project folks and the Neanderthals-R-Us crowd base their claims for interbreeding between the Neanderthals and modern people like you and me. Notwithstanding my staggering naiveté with regard to genomics, I still think it’s possible to focus the spotlight on a methodological problem in the prodigious dataset thus far compiled. That problem resides in the choice of populations sampled.

At its heart, my complaint about the 1000-genome sample is simple, but fundamental. It’s so fundamental, and I’m at such a loss to explain how the project leaders allowed it to be the case, that when I imagine the interbreeding advocates coming face to face with my argument I can’t help but recall the image of cartoon character Wile E. Coyote the instant he looks down and realizes that he’s no longer standing on solid ground. If the 1000-genome project-affiliated biological anthropologists were to ‘look down’ at their data for a moment they’d prolly realize that they weren’t, any longer, on a solid theoretical footing, and will ultimately stand or fall on the implications of their flawed data.

The shortcoming that I see lies in what they term “the African” populations, and the conclusion that there’s a distinct difference between the degree of genetic material shared amongst Neanderthals, modern Europeans, and modern Asians and the degree of genetic material shared between the Neanderthals and present-day African populations. It’s assumed [or presumed] that a few hundred African genomes are at least as representative of the genetic diversity of Africa–as a whole–as are those of Europeans and Asians in the sample. As you’ll see, this doesn’t accord with the facts.

The table below shows the African and African-related portion of the 1000-genome project’s data. You can see that the present-day African groups represented are the Yoruba, Luhya, Gambia, Mende, and Esan.  I’ll come back to the American genomes in a moment.

The 1000-genome project, in attempting to characterize the range of variability in modern Africans, has sampled a very few groups from the west coast of central Africa and one from inland East Africa. That’s a piddling sample given that the African continent gave rise to the species and presumably comprises the greatest range of variation of any similar segment of the species elsewhere in the world. But that’s not all. So restricted is their sample that they might just as well have called their African sample the Bantu sample. Indeed, 80% of the Africans sampled [thus far] in the project can trace their ancestry to a Bantu homeland in west Central Africa near the borders of Nigeria and Cameroon. The remaining 20%–the Mende sample–derives from Sierra Leone, just to the north of the Bantu homeland, and linguistically closely related to Bantu. The historical picture of the linguistic and, no doubt, genetic distribution of the the Africans sampled for the 1000-genome project resembles most closely that of the Indo-European speakers, which, around the same time–about 4,000 years ago–spread their language and culture, and their genes, from a point in southwestern Asia across Europe.

The map at left depicts the relative time-depth and geographic extent of the Bantu expansion I’ve just mentioned, which appears to have occurred in two stages, and which profoundly changed the demographics of Africa south of the Sahara Desert beginning about 4,000 years ago. The problem this presents to the 1000-genome project is enormous. While maintaining that they’ve got a representative sample of Africa-wide populations, they’ve very likely drawn from a single, widespread population that has a relatively small degree of genetic diversity due to its single, small region of origin and its rapid expansion. [The likelihood that a relatively homogeneous genome spread and mingled with the ancestral groups in their path may well be the reason that some of the 1000-genome data are seen to suggest a population ‘bottleneck’ in Africa at some time in the past.]

Let’s face it, at a minimum these African results should be approached with caution. I mentioned that the project also sampled Americans of African ancestry. Unfortunately for the project these samples are more of the same. That’s because the African slave trade grew up in coastal regions, as you can see in the map of the diaspora that is shown below, the predominant origin for Africans destined for the New World was the same west coastal strip from which the 1000-genome project’s indigenous African samples are drawn. As you can easily see, all those purple arrows aiming at the Americas originate on Africa’s west coast, the southern portion of which is, as mentioned above, Bantu territory. And, even though some Africans from further north did make it to the Americas, as can be seen by reference to the pie chart inset in the map below, 52% came from Bantu speaking populations–Angola and the Bight of Biafra. Another 23% came from the Bight of Benin, which is the northern region most proximal to the Bantu homeland, and which therefore might be expected to evince a substantial genetic similarity to that of the Bantu peoples who were uprooted and transported to the Americas.

Taken together, the 1000-genome project’s African sample may in fact be sampling a very narrow portion of the original, ancestral African, genetic diversity. Unfortunately for the project, when you add up all of the Bantu-related genomes, almost the entire sample representing Africa could be seen to derive from a single, geographically circumscribed ancestral population in western Africa. I can’t even begin to calculate the effect this would have on the claims made so far for African input into the parts of the genomes shared between modern humans outside of Africa and those darned Neanderthals. At a minimum, the proponents of the interbreeding hypothesis might want to examine their methods more closely, so as to redress this glaring shortcoming of their sampling strategy.

The fundamental question posed by the 1000-genome project was what proportion of ancestral African populations share any part of their genome with those of the Neanderthals? I’d have to say that the jury’s still out, regardless of what the project mavens would have us believe.

SA announces new posts on the Subversive Archaeologist‘s facebook page (mirrored on Rob Gargett’s news feed), on Robert H. Gargett‘s page, Rob Gargett‘s twitter account, and his Google+ page. A few of you have already signed up to receive email when I post. Others have subscribed to the blog’s RSS feeds. You can also become a ‘member’ of the blog through Google Friend Connect. Thank you for your continued patronage. You’re the reason I do this.