A month ago I posted Don’t trust an archaeologist about genetics, don’t trust a geneticist about archaeology, in response to James Fallows at At 5% Neanderthal, You Are an Outlier. Fallows has now put up a follow up, The Neanderthal Defense Committee Swings Into Action, where he links to my response post. This prompted the original archaeologist in question to reach out to me via email. I am posting the letter, with their permission, below.
By now you have read that the Clovis people may have had contemporaries. In case you didn’t know, until about ~10 years the “standard model” of the peopling of the Americas was that around ~13,000 years ago one single population crossed into the New World via Beringia, and rapidly swept north to south in ~1,000 years. These were the Clovis people, associated with a particular toolkit, and perhaps implicated in megafaunal extinctions. Today this model is no longer held to be sacrosanct, though no clear successor has emerged. It does seem likely that some sort of pre-Clovis population is presumed to exist. These data are interesting because they indicate that there may have been geographical structure in cultural forms even at this early stage in North America, implying that the original peopling of America was not homogeneous. That is, there may have been several founding groups.
I find this entirely plausible. It strikes me that a fear years ago a tendency toward rhetorical extremism occasionally found purchase when it came to the settlement of the peripheries of the human range, Oceania and the New World. Jared Diamond famously proposed that Australia and Papua may have been colonized by one pregnant woman! (presumably she then reproduced with her own son, as Gaia did with Uranus) In hindsight it seems more likely that migration of pre-modern humans was a far more organized affair, and not an ad hoc expansion in a state of anarchy, with family groups splitting off in a random fashion due to demographic pressures. Note that archaic humans did not settle Oceania and the New World, despite having a million years to do so.
Many researchers suppose that the ancestors of the First Americans sojourned in Berengia before they crossed over into the New World (their progress being blocked by glaciers). This may explain why genetic estimates of the populations’ origin tend to be deeper than the archaeological conclusions, even if you push the dates back due to Monte Verde. From the media reports archaeologists seem to be emphasizing that the original population which settled the New World may have been culturally diverse, even if they are genetically similar. Presumably this is a counterpoint to the paper which posits a “First American” group at the heart of the genomic variation of the New World groups.
If the Soviet Union was the “The Prisonhouse of Nations,” then the Caucasus region must be the refuge of the languages. Not only is this region linguistically diverse on a fine-grained scale, but there are multiple broader language families which are found nowhere else in the world. The widespread Indo-European languages are represented by Armenians, Greeks, and Iranians. The similarly expansive Altaic languages are represented by the Turkic dialects. But in addition to these well known groups which span Eurasia there are the Northwest Caucasian, Northeast Caucasian, and Kartvelian, families. These have only a local distribution despite their distinctiveness.
On the one hand we probably shouldn’t be that surprised by the prominence of small and diverse language families in this rugged region between Russia and the Near East. Mountains often serve as the last refuges of peoples and cultures being submerged elsewhere. For example, in the mountains of northern Pakistan you have the linguistic isolate of Burusho, which has no known affinity with other languages. Likely it once had relatives, but they were assimilated, leaving only this last representative isolated in its alpine fastness. The once extensive Sogdian dialects (Sodgian was once the lingua franca between Iran and China) are now only represented by Yaghnobi, which persists in an isolated river valley in Tajikistan. How the mighty have fallen! But the mountains are always the last fortresses to succumb.
But the Caucasus are peculiar for another reason: they’re so close to the “action” of history. In fact, history as we know it started relatively near the Caucasus, to the south on the Mesopotamian plain ~5,000 years ago. Therefore we have shadows and glimmers of what occurred on the south Caucasian fringe early on, such as the rise and fall of the kingdom of Urartu ~3,000 years ago. The ancient ancestors of the Georgians even show up in Greek myth, as the Colchis of Medea. And this was a busy part of the world. Hittite, Greek, Roman, and Arab, came and went. The rise of Turkic resulted in the marginalization of many of its predecessors. Some scholars even argue that the Indo-European and Semitic languages families issue from the north and south fringes of the Fertile Crescent, respectively. And it isn’t as if history has skirted the Caucasians. The Georgians faced the brunt of the Mongol armies, while the Circassians have famously been present across the greater Middle East as soldiers and slaves.
In the age of 500,000 SNP studies of genetic variation across dozens of populations obviously we’re a bit beyond lists of ABO blood frequencies. There’s no real way that a conventional human is going to be able to discern patterns of correlated allele frequency variations which point to between population genetic differences on this scale of marker density. So you rely on techniques which extract the general patterns out of the data, and present them to you in a human-comprehensible format. But, there’s an unfortunate tendency for humans to imbue the products of technique with a particular authority which they always should not have.
The History and Geography of Human Genes is arguably the most important historical genetics work of the past generation. It has surely influenced many within the field of genetics, and because of its voluminous elegant visual displays of genetic data it is also a primary source for those outside of genetics to make sense of phylogenetic relations between human populations. And yet one aspect of this great work which never caught on was the utilization of “synthetic maps” to visualize components of genetic variation between populations. This may have been fortuitous, a few years ago a paper was published, Interpreting principal components analyses of spatial population genetic variation, which suggested that the gradients you see on the map above may be artifacts:
Nearly 30 years ago, Cavalli-Sforza et al. pioneered the use of principal component analysis (PCA) in population genetics and used PCA to produce maps summarizing human genetic variation across continental regions. They interpreted gradient and wave patterns in these maps as signatures of specific migration events. These interpretations have been controversial, but influential, and the use of PCA has become widespread in analysis of population genetics data. However, the behavior of PCA for genetic data showing continuous spatial variation, such as might exist within human continental groups, has been less well characterized. Here, we find that gradients and waves observed in Cavalli-Sforza et al.’s maps resemble sinusoidal mathematical artifacts that arise generally when PCA is applied to spatial data, implying that the patterns do not necessarily reflect specific migration events. Our findings aid interpretation of PCA results and suggest how PCA can help correct for continuous population structure in association studies.
A paper earlier this year took the earlier work further and used a series of simulations to show how the nature of the gradients varied. In light of recent preoccupations the results are of interest. Principal Component Analysis under Population Genetic Models of Range Expansion and Admixture:
A new paper in The New Journal of Physics shows that a relatively simple mathematical model can explain the rate of expansion of agriculture across Europe, Anisotropic dispersion, space competition and the slowdown of the Neolithic transition:
The front speed of the Neolithic (farmer) spread in Europe decreased as it reached Northern latitudes, where the Mesolithic (hunter-gatherer) population density was higher. Here, we describe a reaction–diffusion model with (i) an anisotropic dispersion kernel depending on the Mesolithic population density gradient and (ii) a modified population growth equation. Both effects are related to the space available for the Neolithic population. The model is able to explain the slowdown of the Neolithic front as observed from archaeological data
The paper is open access, so if you want more of this:
Just click through above. Rather, I am curious more about their nice visualization of the archaeological data:
When it comes to the synthesis of genetics and history we live an age of no definitive answers. L. L. Cavalli-Sforza’s Great Human Diasporas would come in for a major rewrite at this point. One of the areas which has been roiled the most within the past ten years has been the origin and propagation of the agricultural lifestyle across the European continent between 10,000-6,000 years before the present (starting in Europe’s southeast fringe a few thousand years after the origination of the Neolithic lifestyle in the Levant, and finally pushing into the southern Scandinavian peninsula only ~6,000 years ago). The reasons for this particular debate about the origin of the European are manifold. First, most scholars are of European ancestry, and some of the debates have roots going back a century. So a natural interest exists based on normal human biases. Second, when it comes to genetics the climate of Europe is ideal for the preservation and extraction of ancient DNA. Third, there are relatively clear and distinct theoretical models which can be tested by the data, whether to verify or refute.
I have already reviewed earlier work in three previous posts, European man perhaps a Middle Eastern farmer, European man perhaps not a Middle Eastern farmer, and Völkerwanderung back with a vengeance. Instead of rehashing everything I’ll take it as a given that you’ve read or skimmed those posts. Rather, let’s move on to a new paper in PLoS Biology, Ancient DNA from European Early Neolithic Farmers Reveals Their Near Eastern Affinities:
The past ten years has obviously been very active in the area of human genomics, but in the domain of South Asian genetic relationships in a world wide context it has seen veritable revolutions and counter-revolutions. The final outlines are still to be determined. In the mid-1990s the conventional wisdom was that South Asians were a branch of a broader West Eurasian cluster of peoples, albeit more distant from the core Middle Eastern-North-African-European-Caucasian clade. The older physical anthropological literature would have asserted that South Asians were predominantly Caucasoid, but with a Australoid element admixed in at varying proportions as a function of geography and caste. To put it more concretely, and I think accurately, a large degree of South Asian physical variety can be defined along the spectrum between A. R. Rahman and Nawaz Sharif. The regional and caste truisms are only correlations. Subrahmanyan Chandrasekhar was a Tamil Brahmin, but experienced anti-black racism in the United States. I think that is reasonable in light of his appearance.
This rough & ready mainstream understanding, supporting by classical genetic markers, was overturned in the early years of the 21st century. One line of thought argued that South Asians were much more distinctive from the broader Western Eurasian cluster of peoples. Representative of this body of work is a paper like The genetic heritage of the earliest settlers persists both in Indian tribal and caste populations. These researchers tended to start with the female lineages, mtDNA, and then supplement that with Y lineages, the paternal descent. A separate line of evidence, generally drawn from Y chromosomal results, indicated that there were deep connections between the people of India and those of Central Eurasia, in particular via the R1a haplogroup. Additionally, one aspect of the first set of results which was very surprising was that it actually placed South Asians closer to East, not West, Eurasians. But by the end of the aughts the uniparental studies had been supplemented by a range of results produced from SNP-chips, which looked at hundreds of thousands of genetic variants. These studies seemed to support the older view of South Asians being closer to West Eurasians than East Eurasians. Finally last year a paper came out which posited that almost all South Asian populations were actually an ancient stabilized hybrid between two groups, a European-like population, “Ancient North Indians” (ANI), and another group which is no longer present in unadmixed form, “Ancient South Indians” (ASI), of whom the Andaman Islanders are distant relatives. Though there was a slight bias toward ANI as a whole, the fraction of ASI increased as one went southeast, and down the caste ladder. The distinctive “South Asian” ancestral group in other words then may actually be conceived of as a compound of these two elements; an admixture of the native substrate against a European-like genetic background.
Last week there was an article in The Washington Post that caught my eye, Scientists find evidence discrediting theory Amazon was virtually unlivable. The headline flattens a complex and roiling debate within academia. A generation ago the forests of the Amazon basin were seen as a pristine climax ecosystem. In the 1990s and 2000s that view started shifting, with the maximalist revisionisms recounted in Charles C. Mann’s 1491: New Revelations of the Americas Before Columbus. The general thesis is that the much of the Amazon was a managed ecosystem run for the benefit of a large population, but that the social systems collapsed under the weight of European diseases introduced after 1492. Naturally there are still some holdouts from the older paradigm who are deeply skeptical of the emerging consensus. From the article:
The number of scientists who disagree has diminished, but influential critics remain, none more so than Betty J. Meggers, director of Latin American archaeology at the Smithsonian Institution. She said the new theories are based more on wishful thinking than science.
“I’m sorry to say that archaeologists like to produce sensational refutation of previous theories,” said Meggers, whose 1971 book, “Amazonia: Man and Culture in a Counterfeit Paradise,” holds that the region is unfit for large-scale habitation. “You know, this is how you get your promotions.”
One aspect of human demographic expansions seems to be the fact that we often model them as a constant diffusion process, when in reality there were likely pulses (economic historians can conceive of this as the periodic gaps between land and labor factor inputs). I don’t know much about the human movements prior to H. sapiens sapiens, and from what I can gather the fossil remains are too sparse to be too wedded to a specific model, but it seems clear that anatomically modern human expansion occurred through a series of rapid outward sweeps which would periodically reach a “natural barrier.” Modern humans reached the Solomon Islands ~30,000 years ago, after which there was stasis for ~25,000 years. Only with the Austronesian expansion did humanity push past the Solomons. And this was no baby-step, ultimately the Austronesians went as far as the Hawaiian islands and Easter Island.