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41uhvVKVH+L._SX331_BO1,204,203,200_ Over the past several years ancient DNA has opened a startling window onto the settlement of the European continent during the Holocene. The story is one of migration, replacement, and repeated pulsing of populations from the fringe toward the North Sea. In short, the model that farming spread to Europe predominantly through cultural diffusion is dead. Though the genetic legacy of the indigenous hunter-gatherers persists in modern Europeans, it is through their amalgamation into the populations of farmers with roots in the Near East, as well as later peoples who arrived from the Eurasian steppe. Not because they adopted the culture of the newcomers.

But while the cultural diffusionist model has been falsified, the customary alternative, demic diffusion, has not been entirely validated either. This model posits the expansion of farming as a mechanistic, mindless, process analogous to thermodynamics and the expansion and diffusion of heat. It is in many ways a “culture-free” model, as farmsteads expand in an ad hoc and uncoordinated fashion across an empty landscape. And, as L. L. Cavalli-Sforza pointed out it was actually compatible with a predominant Pleistocene period ancestry for modern Europeans, because as the wave of demographic advance proceeded it would mix with the indigenous peoples on the frontier, diluting the distinctive original genetic signal.

I won’t repeat what’s been stated a thousand times in this space. Basically, rather than a gradual movement, the DNA is suggesting that there were starts and fits, and later equilibrations. For a thousand years or so it looks as if in the region of modern Germany during the early Neolithic the farmers and hunter-gatherers remained apart, with genetic distances comparable to that between modern Northern Europeans and Han Chinese. To account for this I have presented a rough model which I termed “leapfrogging,” as farmers migrated to ecologically favored terrain, leaving much of the hinterland in the hands of Mesolithic hunter-gatherers.

It turns out that none of my ideas were original, down to the terminology! A book chapter in The Colonization of Unfamiliar Landscapes: The Archaeology of Adaptation, prefigures almost all of my thoughts. Additionally, though published in 2003 I would argue it was relatively prescient. Here is one section:

The Neolithic colonization of Europe was a complicated process that took more than 2,500 years to unfold. Analysis of calibrated radiocarbon dates reveals a series of punctuated, rapid expansions, interrupted by periods – 500 to 1,000 years long – of stasis and in-filling. The earliest colonists in Southeastern Europe sought out floodplains and lake basins that were close analogues to familiar ancestral habitats in Anatolia and optimal for their farming practices. The two-stage structure of the Early Neolithic expansion and the selectivity of initial settlement locations imply carefully planned colonizing ventures, based on detailed prior knowledge of the landscape. The second-stage farmers may have derived this vital geographic information from fishing groups that initially created settlement facilities on the Greek coast to support long distance fishing trips. These frontiersmen probably shifted opportunistically from hunting and fishing, to herding, to trading.

You can read the whole chapter, Deerslayers, pathfinders, and Icemen: origins of the European Neolithic as seen from the frontier, at Academia.edu.

 
• Category: Science • Tags: Neolithic 
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The Pith: You’re Asian. Yes, you!

A conclusion to an important paper, Nick Patterson, Priya Moorjani, Yontao Luo, Swapan Mallick, Nadin Rohland, Yiping Zhan, Teri Genschoreck, Teresa Webster, and David Reich:

In particular, we have presented evidence suggesting that the genetic history of Europe from around 5000 B.C. includes:

1. The arrival of Neolithic farmers probably from the Middle East.

2. Nearly complete replacement of the indigenous Mesolithic southern European populations by Neolithic migrants, and admixture between the Neolithic farmers and the indigenous Europeans in the north.

3. Substantial population movement into Spain occurring around the same time as the archaeologically attested Bell-Beaker phenomenon (HARRISON, 1980).

4. Subsequent mating between peoples of neighboring regions, resulting in isolation-by-distance (LAO et al., 2008; NOVEMBRE et al., 2008). This tended to smooth out population structure that existed 4,000 years ago.

Further, the populations of Sardinia and the Basque country today have been substantially less influenced by these events.

 

It’s in Genetics, Ancient Admixture in Human History. Reading through it I can see why it wasn’t published in Nature or Science: methods are of the essence. The authors review five population genetic statistics of phylogenetic and evolutionary genetic import, before moving onto the novel results. These statistics, which measure the possibility of admixture, the extent of admixture, and the date of admixture, are often presented, but nested into supplements, in previous papers by the same group. On the one hand this removes from view the engines which are driving the science. On the other hand I have always appreciated that a benefit of this injustice to the methods which make insight possible is that those without academic access can actually bite into the meat of the researcher’s mode of thought.

I did read through the methods. Twice. I’ve encountered all the statistics before, and I’ve read how they were generated, but I’ll be honest and admit that I haven’t internalized them. That has to end now, because the authors have finally released a software package which implements the statistics, ADMIXTOOLS. I plan to use it in the near future, and it is generally best if you understand the underlying mechanisms of a software package if you are at the bleeding end of analytics. I will review the technical points in more detail in future posts, more for my own edification than yours. But for the moment I’ll be a bit more cursory. Four of the tests use comparisons of allele frequencies along explicit phylogenetic trees. That’s so general as to be uninformative as a description, but I think it’s accurate to the best of my knowledge. In the basics the tests are seeing if a model fits the data (as opposed to TreeMix, which finds the best model out of a range to fit the data). The last method, rolloff, infers the timing of an admixture event based upon the decay of linkage disequilibrium. In short, admixture between two very distinct populations has the concrete result of producing striking genomic correlations. Over time these correlations dissipate due to recombination. The magnitude of dissipation can allow one to gauge the time in the past when the original admixture occurred.

Let’s look at some results. To the left is a section of a table which illustrates the most significant 3-population test scores in the HGDP. The authors checked all the various combinations, and these came out at the top as likely admixtures (i.e., the two sources produce particular patterns in the target). Please remember that these triads should not be taken literally. The Uygur are not descended from Japanese and Italians. Rather, they are descended from populations with genetic affinities to these two sources. Precisely, the Uygurs are descended from Northeast Asian Turks, who assimilated an Indo-European speaking substratum. Most of the results are rather obvious and explicable. Several Middle Eastern populations are known to have Sub-Saharan African admixture, and this is shows up in the results. Others may be more confusing because of the obscurity of the populations, but the Burusho clearly have ancient East Asian ancestry on clustering algorithms, so their presence is not surprising to me. Similarly, the Russians in the HGDP data set have an ‘eastern’ affinity (or at least some do), either due to Finno-Ugric or Turkic ancestry (Tatars regularly assimilated into a Russian ethnic identity as the Tsars expanded their domains).

Some of the other results are more confusing, but one can still find a historical explanation. I have seen evidence that some of the Cambodian samples may have old Indian admixture, though it is not entirely clear to me. But that could explain why there is a signature of West Eurasian admixture into this population (though one wonders why the donor was not Baloch or Pathan.). The Xibo and Tu are Northeast Asian groups, on the border between China proper and the great Eurasian interior. West Eurasian admixture into these groups is not unexpected. West Eurasians are historically attested among the mercenaries and soldiers who arrived on the North China plain after the collapse of the Han dynasty, down to the Alans who served under Kublai Khan. Some of Mongolian and Turkic peoples have individuals who are attested as having characteristics more typical of Europeans (e.g., red hair), so it is likely that this admixture was relatively old and widespread, well before the era of the Pax Mongolica.

There is a minor dissonant note in these results above. The authors used rolloff and inferred an admixture of ~800 years before the present. This is far lower than earlier estimates, which were >2,000 years before the present. First, I have to say that I was mildly skeptical of the higher value reported earlier. From what little I know the roiling of Turco-Mongol peoples which reordered the Inner Asian landscape did not really establish itself beyond the Chinese fringe at this time. Recall that Central Asia was the domain of the Iranians from prehistory down to the Islamic age (the full transition of Central Asia from Persianate to Turkic has not completed itself to this date, though it has progressed over the centuries since 1000 A.D.). Is it creditable that the Turkic hordes were shut on the other side of the Pamirs for ~1,000 years? Perhaps. But it should warrant skepticism, and openness to the lower values proffered here. The technical reason that the authors consider is that STRUCTURE based inferences may overestimate admixture when reference populations are not appropriate. And yet the authors still concede that 800 years is simply difficult to credit when one consults the historical literature. Strangely though it does align with the date of the Mongol ascendancy, during which time the Uygurs served as civil servants in the barbarian empire (Mongol script derives from the old Uygur script). I managed to dig up a cave painting of Uygurs from this period. There is surely artistic license, but they look rather East Asian to me, as opposed to the hybrid Eurasian appearance modal among modern Uygurs. I won’t touch upon the rather fraught and complex ethnology and ethnogenesis of modern Uygurs, and their relationship to Russian and Chinese ethnographers, but suffice it to say that one needs to be careful about excessive reliance on the literality of historical documents in this area, because of semantic confusions.

So let’s move to the main course: what’s going on in Europe? Before putting the spotlight on the macro picture, let’s highlight one secondary aspect: the authors detect evidence of massive gene flow into Spain from Northern Europe ~4,000 years before the present. I’ll let them speak here:

We hypothesize that we are seeing here a genetic signal of the ‘Bell-Beaker culture’ (HARRISON, 1980). Initial cultural flow of the Bell-Beakers appears to have been from South to North, but the full story may be complex. Indeed one hypothesis is that after an initial expansion from Iberia there was a reverse flow back to Iberia (CZEBRESZUK, 2003); this ‘reflux’ model is broadly concordant with our genetic results, and if this is the correct explanation it suggests that this reverse flow may have been accompanied by substantial population movement.

Two things to hammer home here. First, pots move with people. That’s the inference being drawn from the results. It’s not pots-not-people, it’s people-and-pots. Second, the idea of reversals in the direction of gene flow are intriguing, and, I think need to be taken more seriously. It seems the most plausible candidate here are the people who later became the Celtiberians. Celts have been associated with the Bell Beakers before.

But the bigger shock is that Europeans, and especially Northern Europeans, seem to have a substantial Northeast Asian component. From the nature of the prose I feel that the authors were definitely taken aback. They basically say so in so many words. In the process of resolving their confusion they skinned the cat every which way. And it does look to me that Northern Europeans are truly descended in part from a population which has affinities to the “First Americans.” I say this specifically because the Siberian samples they tested actually gave a weaker result than the South American Amerindians on the 3-population test.

So what’s the proportion of ancestry? Using the Siberian population they came up with an interval of 5-18 percent in Northern Europeans. The authors used the Sardinians as their “pure” European reference, and admit that it is likely that their admixture estimate is lower than real value due to this fact. Inference is inference, do you trust this result? As it happens the authors also checked Ötzi the Iceman, and found that like the modern Sardinians he had very little Northeast Asian ancestry. Ötzi is dated to ~5,000 years before he present. Using rolloff the authors estimate an admixture date of ~4,000 years before the present, with an error of nearly 1,000. Additionally, using a different data set they came with an admixture date of ~2,000 years before the present. The latter is obviously wrong (they explain why this could happen in the text). But Ötzi seems to put a boundary on how early it could have been, at least in Southern Europe.

As of publication the authors did not have time to include a reference to this interesting nugget from the abstracts of ASHG 2012:

The complete genome of the 5,300 year old mummy of the Tyrolean Iceman, found in 1991 on a glacier near the border of Italy and Austria, has recently been published and yielded new insights into his origin and relationship to modern European populations. A key finding of this study has been an apparent recent common ancestry with individuals from Southern Europe, in particular Sardinians…We used unpublished data from whole genome sequencing of 452 Sardinian individuals, together with publicly available data from Complete Genomics and the 1000 Genomes project, to confirm that the Iceman is most closely related to contemporary Sardinians. An analysis of these data together with ancient DNA data from a recently published study on Neolithic farmers and hunter-gatherers from Sweden shows the Iceman most closely related to the farmer individual, but not the hunter-gatherers, with the Sardinians again being the contemporary Europeans with the highest affinity. Strikingly, an analysis including novel ancient DNA data from an early Iron Age individual from Bulgaria also shows the strongest affinity of this individual with modern-day Sardinians. Our results show that the Tyrolean Iceman was not a recent migrant from Sardinia, but rather that among contemporary Europeans, Sardinians represent the population most closely related to populations present in the Southern Alpine region around 5000 years ago. The genetic affinity of ancient DNA samples from distant parts of Europe with Sardinians also suggests that this genetic signature was much more widespread across Europe during the Bronze Age.

I’m betting that this Bulgarian sample won’t exhibit Northeast Asian ancestry, though who knows?

There is a definite geographic pattern within Europe to the strength of the signature of admixture. Northern European populations have the greatest, Southern European populations less, and islanders like Cypriots hardly any. Recall that Sardinians seem to be the best reference, so the ~0 floor may just be a statistical artifact of the measuring stick we have. All that being said, what went on <5,000 years before the present to reorder the European landscape?

The answer may sound crazy, but I think the most probable explanation (even if it is unlikely) is something to do with the Indo-Europeans. We know that Indo-European languages were spoken in Greece by ~1500 BC at the latest. One thing that is clear from less advanced clustering algorithms is that Basques and Finns are somewhat distinctive in relation to their neighbors. Though they are not genetically that different, they still lack some “interesting”elements. The results to the left are from Dienekes, though I’ve replicated it. You can see a similar difference between French, and French Basques. The Basques seem to lack something which has affinities with West Asia. These results, and hints elsewhere, imply that the Basque may not be descended from hunter-gatherers, but the first European farmers. So who came after them?

Though it strikes me as a bizarre conjecture, but I can’t help but imagine the rapid expansion of Indo-European populations into Europe, pushing into the peninsulas of the south. These people may have been a newly formed cosmopolitan mix of West Asians, Northern European Mesolithics, and Northeast Asians. I am at a loss to hazard a guess as to who the First American-like Northeast Asians were, though perhaps they were a western offshoot of the Kets? These people were then absorbed into a melange of tribes who themselves emerged from a synthesis between immigrant West Asian farmers and Northern Europeans. In shorthand: perhaps the Indo-Europeans were mongrels! This is not an entirely crazy proposition if you look at the historical record. Conquest populations often synthesized and absorbed those who they conquered. Sometimes they even became the conquered in deep cultural ways (e.g., the Bulgars).


To ward off accusations of glib and facile speculations, I well understand that much of what I suggest above is likely wrong. But bizarre results are going to elicit unhinged hypotheses. And I shouldn’t overplay how strange these results are, I think they are going to stand the test of time. The authors are top notch, and Dr. Joseph Pickrell found the same pattern (a connection between Europeans and Native Americans) with TreeMix! If we sit back and reflect on phenotype it shouldn’t be entirely surprising. Some Scandinavians have always struck me as having a generalized Eurasian cast to their features. Obviously this tendency is stronger among the Sami and Finns, but you can see it in Swedes and others. This is far less evident to me among Southern European peoples. I doubt one would ever confuse a Sardinian for a Eurasian, and I never had that feeling when I spent some time in Italy a few years back (in contrast, some Finns did look Asiatic to me).

Finally, this paper highlights the reality that population genetics has little to do with Plato. A population within a species is simply not clear and distinct in a sense which would satisfy an Idealist. The authors of the above paper nod to this, illustrating how their tests for admixture are confounded and confused by constant gene flow via isolation-by-distance dynamics. These results indicate that Northern Europeans are on the order of 10% Northeast Asian. Does this mean that Northern Europeans are 10% non-white? Well, it turns out that white people were always 10% non-white! We just didn’t know. Is my daughter (who is 50% Northern European) now majority non-white? Oh wait, I’m South Asian. That means I’m ~50% white! Is my friend who is 25% Japanese now more than 25% Northeast Asian? Words and concepts fail us on the boundary of unfamiliarity, in time and space. Populations and genealogies don’t brook our categorizations. On a deep level we are all admixtures, and partitioning of ancestry along phylogenetic trees are useful and comprehensible fictions. These techniques put flesh upon the bones of archaeology and smoke out the outlines of history. But we always need to be aware that that history is not made by humans, rather, we excavating it, and then giving it appropriate glosses in our museums. And yet it is.

Related: Dienekes has much to say (obviously).

Image credit: Wikipedia, Wikipedia, and Wikipedia.

Cite: 10.1534/genetics.112.145037

(Republished from Discover/GNXP by permission of author or representative)
 
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There’s a new ancient DNA paper out which examines the maternal lineage and the autosomal background of two individuals extracted from a Spanish site dated to 7,000 years before the present. That is, during the European Mesolithic. In other words, these are the last wave of Iberian hunter-gatherers before agriculture. I have placed the PCA, with some informative labels, to illustrate the peculiarity of these samples. Here’s the abstract:

The genetic background of the European Mesolithic and the extent of population replacement during the Neolithic…is poorly understood, both due to the scarcity of human remains from that period…The mitochondria of both individuals are assigned to U5b2c1, a haplotype common among the small number of other previously studied Mesolithic individuals from Northern and Central Europe. This suggests a remarkable genetic uniformity and little phylogeographic structure over a large geographic area of the pre-Neolithic populations. Using Approximate Bayesian Computation, a model of genetic continuity from Mesolithic to Neolithic populations is poorly supported. Furthermore, analyses of 1.34% and 0.53% of their nuclear genomes, containing about 50,000 and 20,000 ancestry informative SNPs, respectively, show that these two Mesolithic individuals are not related to current populations from either the Iberian Peninsula or Southern Europe.

Here’s another PCA showing one individual on a more fine-grained representation of European populations:


Perhaps the most interesting aspect here is something Dienekes pointed out from the supplements: these two individuals are not only outside of the range of extant European populations in their positioning on a global PCA plot, but they are shifted toward East Asians.

The fact that these two individuals, who really come close to being one data point because they are likely rather closely related, are outside of the modern European population distribution isn’t too surprising. 7,000 years is a long time, and we can’t assume that ancient populations can be recomposed as combinations of the variation of modern populations. But, the shift toward East Asians is surprising to me, because they are Iberian individuals. In PCA and model-based (e.g., ADMIXTURE) clustering frameworks modern Iberians, and populations from Southwest Europe in general, are the most distant from East Eurasians of all West Eurasians. Dienekes opines:

It now appears clear that the Mesolithic substratum in Europe was:

1. Well outside the modern range, contributing a little to extant populations
2. Its contribution in northern populations was higher than in southern ones
3. It may be responsible for the pattern of Asian-shift observed for non-Mediterranean European populations

…It seems that this was the composition of the pre-Neolithic population of Europe that was later supplanted first by the “Mediterranean”/”Southern” components during the early Neolithic, and later by the “West_Asian”/”Caucasus”/”Gedrosia” components, perhaps during the Copper Age. We’ll see whether my prediction pans out soon enough.

On the broadest level I think Dienekes model is entirely possible. I’d give it the highest probability of the range of options, though I have a high uncertainty. The question is the weight of the contributions. Let’s rename the various groups A, B, and C, for the three waves in chronology (hunter-gatherers, first wave farmers, and second wave farmers). Then any European population is:

xA + yB + zC, where x + y + z = 1

About 10 years ago B + C would be one class, so B, and you had a pan-European estimate of:

0.75A + 0.25B

Now some scholars are trying to revise that, and reduce the weight for A. I think we need to be very careful, because we’ve already been burned by overly elegant and simplistic models of the settlement of Europe. For example, here are my made-up estimates quantitatively from everything I’d read so far:

Finns = 0.90A + 0.05B + 0.05C

Lithuanians = 0.85 A + 0.05B + 0.10C

Irish = 0.60 A + 0.30B + 0.10C

Germans = 0.60 A + 0.20B + 0.20C

Basques = 0.40 A + 0.60B

North Italians = 0.40 A + 0.30B + 0.30C

The exercise above was not to give you accurate numbers that I’m sure of, but to give you numbers instead of verbal labels which are imprecise. I’m quite willing to “update” my estimate in the future, and expect to. In the paper the authors highlight that the mtDNA lineages that the two individuals carry is modal in the Sami of northern Finland. Genetically the Sami seem more Finnish that the Finns. But, they also clearly have an “eastern” affinity (diminished, but still present in Finns as well). This dovetails with linguistic connections to North-Central Eurasia, and the margins of Western Siberia. What’s going on here? One hypothesis has been that the Finnic languages (and the Sami) are culturally intrusive, and the Siberian genetic affinity is a signal of this ancient core admixed with the local substrate, which resembles that of Scandinavians.

These results make me update my assessment, and increase my own probability that the Finnic people have deep cultural roots in Northeast Europe. This was already my hunch based on model-based clustering which seemed to show that there were “southern” modal elements present in Scandinavians lacking in Finns or Sami. If Finns or Sami were relatively late arrivals, I would have expected to be more diverse in the complement of ancestral elements, not less. Now the set of results form Mesolithic European genomes indicate to me that what we may be seeing in the Siberian affinity of the Sami (and to a lesser extent the Finns) are the echos of a post-Ice Age expansion of Palearctic peoples from the center of Eurasia which ranged west toward the Atlantic and east toward the Sea of Japan. I suspect that this Palearctic population did not move into a totally empty landscape, so the Mesolithic peoples which the West Asian and East Asia farmers displaced or assimilated were not entirely similar. Rather, they were themselves syntheses between hunter-gatherer groups.

Obviously this is an interesting time to moot these questions. The major issue we need to keep in mind is not to move from one enthusiasm to another. A model of preponderant biological continuity between European hunter-gatherers and early agriculturalists is now in serious doubt, especially for southern Europe. But that does not mean that we should move to a model where the hunter-gatherers were replaced in totality by agriculturalists. This does not seem to have happened in northeast Europe, and the model of replacement itself is probably more complex than a single-step transition.

Citation: Current Biology, 28 June 2012 doi: 10.1016/j.cub.2012.06.00

(Republished from Discover/GNXP by permission of author or representative)
 
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The figure to the left is from a new paper in Science, When the World’s Population Took Off: The Springboard of the Neolithic Demographic Transition. It reports the findings from 133 cemeteries in the northern hemisphere in regards to the proportion of 5-19 year old individuals. When calibrated to period when agriculture was introduced into a specific region there seems to be a clear alignment in terms of a demographic transition toward a “youth bulge.” Why? A standard model of land surplus explains part of it surely. When farmers settle “virgin land” there is often a rapid “catch up” phase toward the Malthusian limit, the carrying capacity. Another possibility though is that sedentary populations did not need to space their offspring nearly as much as mobile hunter-gatherers. Whatever the details, the facts remain that the data do point to a shift in the age pyramid during this period. The author wonders as to the possible cultural implications of this. There is an a priori assumption that a young vs. old age profile in a society constrains its choices and channels its energies (e.g., think the “baby boom” generation in the USA). A final interesting point is that the authors note that today we are seeing the last gasp of this transition toward large numbers of children, as fertility drops toward replacement all across the world. That too may have some cultural consequences.

Here’s a podcast with the author. Link via Dienekes.

(Republished from Discover/GNXP by permission of author or representative)
 
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As many of you know when you have two adjacent demes, breeding populations, they often rapidly equilibrate in gene frequencies if they were originally distinct. There are plenty of good concrete examples of this. The Hui of China are Muslims who speak local Chinese dialects. The most probable root of this community goes back to the enormous population of Central Asia Muslims brought by the Mongol Yuan dynasty that ruled ruled China for over a century from the late 1200s to 1300s. Genetic studies of this group that I’ve seen indicate that a high bound estimate for West Eurasian ancestry is ~10%. The other ~90% is interchangeable with the Han Chinese. So let’s assume that the Hui are ~10% West Asian. If you assume that in the year 1400 the Hui were “pure,” you have 24 generations (25 years per generation). The original population of “Central Asian Muslims” were heterogeneous, including Iranians and Turks. But let’s take it granted that they were 50% East Eurasian and 50% West Eurasian in ancestry at the time of their arrival. What would the intermarriage rate per generation have to be so that the Hui are ~10% West Eurasian at t = 24 (24 generations after the beginning of intermarriage assuming 50/50 West vs. East Eurasian splits)? Turns out all you need is a constant 7% intermarriage rate per generation (the Han Chinese population is so large in relation to the Hui that you can model it as infinite in size).

The situation gets even simpler when you have one population which divides into two. For example, imagine that the Serbs and Croats fissioned from a set of unstructured South Slavic tribes which filtered into ancient Illyria ~600 A.D. Soon enough there was a cultural division between the two in terms of religion (Western vs. Eastern Christian) which threw up a population genetic barrier. If you assume that genetically the two groups were totally similar at t = 0, and you separated them perfectly, over time they would diverge due to drift in their allele frequencies. But the reality is that barriers between geographically close groups do not prevent all intermarriage. Even extremely insular groups in a cultural sense such as the Roma of Eastern Europe are clearly heavily admixed with their surrounding populations, as they seem to be no more than ~50% South Asian in total genome content. Going back to the South Slavs, who start out very similar in our putative scenario, how much intermarriage will be necessary for them to not diverge? The issue is not the rate of intermarriage, rather, one migrant per generation across the two demes will be sufficient to equilibrate allele frequencies. On the face of it this seems implausible, but recall that divergence is driven mostly by drifting of genes as well as new variation (whether through other exogenous migratory sources or mutation). Very small populations are subject to a lot of drift, and so diverge rapidly, but only very few migrants are needed to bring it back into alignment, because they are proportionally significant. In contrast, the frequencies of large populations are less buffeted by generation-to-generation sample variance (e.g., 10 tosses of a coin will deviate more from 50/50 proportionally than 100 tosses), requiring less gene flow proportionally to maintain parity.


These models of how genes flow between populations and how they diverge are analytically very useful. They allow us to conceptualize how gene flow and population substructure could work to generate allele frequency clines on a large geographical scale. Aggregating and averaging allele frequencies in deme-by-deme bins you can perceive the changes as a function of space as smooth continuities. This is certainly the take away from the synthetic maps derived from PCA results in L. L. Cavalli-Sforza’s History and Geography of Human Genes. This gradualistic space of variation can be well accounted for by a few rapid expansion events and bottlenecks as well as geographic isolation building up variation, which eventually “smooths” over through gene flow (with a few exceptions such as Oceania and the New World). Imagine for example the New World. From what I have read the shortest time frame for the push of modern humans from north to the very far south is on the order of ~1,000 years. That’s 40 generations. Depending on how you look at it, that’s very short, or very long. If our understanding of pre-agricultural populations is correct, this would have occurred via a demic fission where tribes would rapidly expand demographically to fill the “empty space,” and move the frontier forward due to classic Malthusian pressures which would manifest in tribal fissioning.

To a great extent this model of small-scale demic diffusion can be projected into the future, and down to the recent past. From what I can gather L. L. Cavalli-Sforza and Albert Ammerman seem to be positing a shift in terms of quantity and not quality when it came to agriculture and demographics. Because agriculturalists could extract more per unit calories out of the same amount of land their population would surpass that of hunter-gatherers, and they would expand into a relatively “empty” land of hunter-gatherers through the bottom-up force of numbers, admixing with the indigenes as the wave of advance pushed on. As an American this aligns with some of the narrative of the recent history of our own nation, with the myth of pioneer families moving to the frontier drawn by the fruit which could be won by mixing of their labor with their land. But the reality is that such a narrative is mythic in that it focuses too exclusively on individuals. The settling of the frontier was not simply a matter of mass action and demographic pressure, as individuals or families expanded into new territory. It was rather a complex of individual, subcultural, cultural, and governmental dynamics which operated in concert to expand the frontier of settlement! The national, and to a lesser extent in the earlier years state, government set the institutional parameters by which Amerindian populations were cleared off the land and white settlers were allowed to start up their frontier homesteads with relative security. Additionally, there were pulses of settlement which followed broader cyclical dynamics in American and European society. Not only is reducing the expansion to purely individual level natural increase writ large probably not accurate, but it has little inferential power to explain the true arc of the phenomenon of how the West was won.

I believe this institutional parameter in the prehistoric past is far too often ignored. We we can not see, we can not imagine. In particular, I suspect that complex institutions between the level of the tribe and the state came to the fore with the rise of agriculture. The map at the beginning of the post shows the migrations of several historically attested people. You see where they start, and where they end, and you see how long they took. And importantly, these were folk wanderings, not just small bands of mobile males. Because all of these were nomadic populations heavily dependent on the horse it would be easy to fall into technological determinism. That is, we attribute the mobility to the horse (or camel) which these groups had access to. But the difference between the time taken by the Banu Hilal and the Alans was not the speed of their mount, but historical contingency. Rapid population movements of whole people require technologically necessary preconditions, but these are not sufficient. The 200 years that it took the Alans to sweep from the plains of Eurasian heartland to the North African Maghreb occurred in starts and stops. In Spain the Alans were decimated by Roman armies in the 5th century, and were absorbed into the confederacy of the Vandals, who eventually conquered North Africa. All this was possible due to institutional collapse on the part of the Romans. Similarly, the rapidity of the Banu Hilal transfer from Arabia to North Africa had to do with the facilitation on the part of the Fatimids of Egypt. The historical details of this folk wandering need not concern us, suffice it to say that without knowing the specifics one would be at a loss to understand this mass translocation of a whole society.

And it is the issue of translocation which I think is critical. A mass-action and bottom-up model usually entails some synthesis across the source population expanding, and the target population being absorbed. But the Banu Hilal were simply Bedouin who had relocated, pure and simple. They had not had a long sojourn in Egypt, or been influenced by the cities of Cyrenaica. It is as if they had taken a worm-hole from point A to point E without encountering B, C, and D. They were a literal “culture shock” to the Maghreb, as well as a genetic shock. The Banu Hilal, and nomadic peoples in general, are perfectly suited to “leapfrog” in nearly an instant from position A to position D.

But not just the Banu Hilal. The expansion of Europeans during the Age of Discovery to all suitable points across the globe is viewed to a great extent as sui generis. I do not think it we should see it as so exceptional in quality. Rather, it is an extension of an ancient pattern. Water transport is cheap, and one can shift matériel in bulk. In antiquity Egypt fed both Rome and Constantinople in turn because its surplus was easily accessible via river and sea. It seems entirely possible that the expansion of farming across the Mediterranean also occurred to a large extent through jumps from fertile locale to locale, facilitated by the ease of water transport. Instead of a demic diffusion one can model this as a series demic of pulses, which eventually filled in. Our perception of diffusion has to do with the coarseness of our measurements.

This sort of translocation process en masse could not have occurred simply through the ingenuity of groups of a few families. Ancient hunter-gatherers were resident on far more fertile territory than modern ones, and so they were likely more well organized and numerous than we might think. Just as the Russian Empire had to pacify the black lands of Ukraine before peasants could farm without being molested by Cossack or Turk, perhaps military expeditions of some sort cleared out the way for the ancient farmers?

But the power of institutions is not just military. One of the peculiar aspects of agriculture is that the regions which we perceive as rich and fertile today were often settled later than more marginal territories in terms of peak production. To give an example, farming began in the marginal uplands of the Fertile Crescent, only later to expand to the lowland territories of Mesopotamia. A more recent case is Thailand, where the uplands were the center of gravity before the shift began to the modern rice-basket of the lower Chao Phraya. Why? Because small-scale farming is far easier in drylands with easily tillable soil. The more potentially productive territory is often more intensive in capital and requires greater coordination of resources and population. In other words, without institutional scaffolding the frontiers of the production possibility are not exploited. One family or set of families can only do so much. One requires more elaborated structure to leverage the technology to its full range of possibilities.

What does all this have to do with genetics? I believe that a relatively simple isolation-by-distance model re-equilibrating after a few major human population genetic fissions (e.g., “Out-of-Africa”) is a very good first approximation to the patterns of variation we see around us. There’s well over a generation of research in this area. But there are details and deviations on the margins which I think need a more complex model to explain. Some of these are in deep prehistory, such as the possibility of admixture between very divergent human lineages (Neo-Africans and Denisovans). But many of them are very recent. We are only ~10 years into the post-genomic era. I suspect that in a few years we’ll feel that the coverage geographically given by data sets such as the HGDP are coarse indeed. As we drill-down to a finer-grain I suspect we’ll get a better sense of the deviation of human genetic variation from the null, the tortoise of constant and continuous genetic exchange and banal fission of tribe upon tribe, buffeted by animal Malthusian pressures. Into the landscape of the tortoise ~10,000 years ago arose the dynamic of the frog, protean, leaping to and fro, exploring the ecological niches on the margins, and creating them anew. Whereas behavioral modernity ~40,000 years B.P. is termed the “Great Leap Forward,” the past ~10,000 years have been an ever more rapid succession of leaps and lunges. Most of this is clearly cultural, due to the flexibility and plasticity of memes. But some of this has almost certainly has been genetic, as the gentle accumulation of thousands of years of genetic variation upon a demographic palimpsest is torn to shreds by cultural revolutions which have genetic import.

In short, the distance between 8,000 B.C. and 12,000 B.C. in terms of the range of potential possibilities may have been far greater than that between today and 8,000 B.C. Agriculture may have heralded the era of morbid misery, but it also unlocked the keys to startling possibilities.

Image credit: Francois Marchal

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Back when this sort of thing was cutting edge mtDNA haplogroup J was a pretty big deal. This was the haplogroup often associated with the demic diffusion of Middle Eastern farmers into Europe. This was the “Jasmine” clade in Seven Daughters of Eve. A new paper in PLoS ONE makes an audacious claim: that J is not a lineage which underwent recent demographic expansion, but rather one which has been subject to a specific set of evolutionary dynamics which have skewed the interpretations due to a false “molecular clock” assumption. By this assumption, I mean that mtDNA, which is passed down in an unbroken chain from mother to daughter, is by and large neutral to forces like natural selection and subject to a constant mutational rate which can serve as a calibration clock to the last common ancestor between two different lineages. Additionally, mtDNA has a high mutational rate, so it accumulates lots of variation to sample, and, it is copious, so easy to extract. What’s not to like?

First, the paper, Mutation Rate Switch inside Eurasian Mitochondrial Haplogroups: Impact of Selection and Consequences for Dating Settlement in Europe:

R-lineage mitochondrial DNA represents over 90% of the European population and is significantly present all around the planet (North Africa, Asia, Oceania, and America). This lineage played a major role in migration “out of Africa” and colonization in Europe. In order to determine an accurate dating of the R lineage and its sublineages, we analyzed 1173 individuals and complete mtDNA sequences from Mitomap. This analysis revealed a new coalescence age for R at 54.500 years, as well as several limitations of standard dating methods, likely to lead to false interpretations. These findings highlight the association of a striking under-accumulation of synonymous mutations, an over-accumulation of non-synonymous mutations, and the phenotypic effect on haplogroup J. Consequently, haplogroup J is apparently not a Neolithic group but an older haplogroup (Paleolithic) that was subjected to an underestimated selective force. These findings also indicated an under-accumulation of synonymous and non-synonymous mutations localized on coding and non-coding (HVS1) sequences for haplogroup R0, which contains the major haplogroups H and V. These new dates are likely to impact the present colonization model for Europe and confirm the late glacial resettlement scenario.

John Hawks has written at length of the possible distortions that selection might produce in our understanding of the history of mtDNA lineages, and therefore our understanding of the history of the population groups which these genealogies are used as proxies for. So I won’t review that much. I find the dynamics that they’re detecting possible, even plausible. But I don’t see why the authors having introduced skepticism start to conjure up positive visions of what is the true nature of the demographics which underpin these mtDNA phylogenies, now that they’ve “corrected” for variation in the power of the molecular clock to let use look through the glass clearly.

Readers with more fluency in the mtDNA literature can probably pick it apart. At the end of the day I’m always wondering what do the subfossils tell us? In other words, ancient DNA. Inferences from contemporary populations have been a total hash at a finer grain than that of continents, so you probably shouldn’t rest on that leg alone.

Finally, I thought this paper was of interest because it’s an inversion of R1b1b2. That’s a Y chromosomal haplogroup which was once presumed to be Paleolithic but now seems likely to be Neolithic. These authors are claiming that a mtDNA haplogroup which was once presumed to be Neolithic is actually Paleolithic. All this I think indicates that we should be modulating outward our error bars whenever we make assertions based on uniparental data with any time depth and below a very coarse level of spatial granularity.

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The image above is adapted from the 2010 paper A Predominantly Neolithic Origin for European Paternal Lineages, and it shows the frequencies of Y chromosomal haplogroup R1b1b2 across Europe. As you can see as you approach the Atlantic the frequency converges upon ~100%. Interestingly the fraction of R1b1b2 is highest among populations such as the Basque and the Welsh. This was taken by some researchers in the late 1990s and early 2000s as evidence that the Welsh adopted a Celtic language, prior to which they spoke a dialect distantly related to Basque. Additionally, the assumption was that the Basques were the ur-Europeans. Descendants of the Paleolithic populations of the continent both biologically and culturally, so that the peculiar aspects of the Basque language were attributed by some to its ancient Stone Age origins.

As indicated by the title the above paper overturned such assumptions, and rather implied that the origin of R1b1b2 haplogroup was in the Near East, and associated with the expansion of Middle Eastern farmers from the eastern Mediterranean toward western Europe ~10,000 years ago. Instead of the high frequency of R1b1b2 being a confident peg for the dominance of Paleolithic rootedness of contemporary Europeans, as well as the spread of farming mostly though cultural diffusion, now it had become a lynch pin for the case that Europe had seen one, and perhaps more than one, demographic revolutions over the past 10,000 years.

This is made very evident in the results from ancient DNA, which are hard to superimpose upon a simplistic model of a two way admixture between a Paleolithic substrate and a Neolithic overlay. Rather, it may be that there were multiple pulses into a European cul-de-sac since the rise of agriculture from different starting points. We need to be careful of overly broad pronouncements at this point, because as they say this is a “developing” area. But, I want to go back to the western European fringe for a moment.


As I stated above the Basques were long used as a Paleolithic “reference” by historical geneticists. That is, the deviation of a population from the Basques would be a good measure of how much admixture there had been from post-Paleolithic sources. Connections between Iberian populations and those of western and northern Europe were used to trace expansions out of the ecological refuges of modern humans during the Last Glacial Maximum ~20,000 years ago. Just goes to show how reliant we are on axioms which are squishier than we’d like to think.

Last fall I posted a result from Dodecad on the difference between French and French Basques (both from the HGDP). I’ve replicated this myself a few times now too:

The striking aspect is that the Basque are less cosmopolitan than the other French. This is evident in most of the runs of the HGDP Basque; they just have a “simpler” genetic heritage than other Western Europeans. Today Dienekes posted some results from the IBS Spanish data set in the 1000 Genomes. He suggests there are clearly a few Spanish Basques in there (I’ve highlighted them):

Recall that the Basques were exempt from inspection for “cleanliness of blood”, because they were presumed to lack Jewish or Moorish ancestry by virtue of being Basque. It seems that the Spanish IBS sample, like the Behar et al. Spaniards and Portuguese, do have some Moorish genetic imprint. This is not too surprising. The Moriscos might have been expelled in the early 17th century, but not before the majority had converted to Christianity over the centuries (in fact, some of the most virulent anti-Morisco partisans had Moorish ancestry themselves, and were particularly tainted by association with the remaining culturally unassimilated crypto-Muslims). All that being said, I suspect that the “West Asian” ancestry amongst the majority of the Spaniards is not due mostly to the Arab period (when of the majority of the settlers probably were Berbers or Arabicized Berbers), but to population impacts prior to that. By the time of the Roman conquest much of Spain was Celtiberian. I have low confidence in this assertion, but I am coming to believe that the Indo-Europeans brought a mix of East European and West Asian ancestry (or at least those two distinct strands which tend to shake out of ADMIXTURE in a broad array of European samples) to western Europe.

On a related note, Wave-of-Advance Models of the Diffusion of the Y Chromosome Haplogroup R1b1b2 in Europe:

Whether or not the spread of agriculture in Europe was accompanied by movements of people is a long-standing question in archeology and anthropology, which has been frequently addressed with the help of population genetic data. Estimates on dates of expansion and geographic origins obtained from genetic data are however sensitive to the calibration of mutation rates and to the mathematical models used to perform inference. For instance, recent data on the Y chromosome haplogroup R1b1b2 (M269) have either suggested a Neolithic origin for European paternal lineages or a more ancient Paleolithic origin depending on the calibration of Y-STR mutation rates. Here we examine the date of expansion and the geographic origin of hgR1b1b2 considering two current estimates of mutation rates in a total of fourteen realistic wave-of-advance models. We report that a range expansion dating to the Paleolithic is unlikely to explain the observed geographical distribution of microsatellite diversity, and that whether the data is informative with respect to the spread of agriculture in Europe depends on the mutation rate assumption in a critical way.

Really I’m waiting for more ancient DNA. These sorts of studies are starting to feel like rewarming cold pizza. Edible, but suboptimal. Next, Phylogeography of a Land Snail Suggests Trans-Mediterranean Neolithic Transport:

Background
Fragmented distribution ranges of species with little active dispersal capacity raise the question about their place of origin and the processes and timing of either range fragmentation or dispersal. The peculiar distribution of the land snail Tudorella sulcata s. str. in Southern France, Sardinia and Algeria is such a challenging case.

Methodology
Statistical phylogeographic analyses with mitochondrial COI and nuclear hsp70 haplotypes were used to answer the questions of the species’ origin, sequence and timing of dispersal. The origin of the species was on Sardinia. Starting from there, a first expansion to Algeria and then to France took place. Abiotic and zoochorous dispersal could be excluded by considering the species’ life style, leaving only anthropogenic translocation as parsimonious explanation. The geographic expansion could be dated to approximately 8,000 years before present with a 95% confidence interval of 10,000 to 3,000 years before present.

Conclusions
This period coincides with the Neolithic expansion in the Western Mediterranean, suggesting a role of these settlers as vectors. Our findings thus propose that non-domesticated animals and plants may give hints on the direction and timing of early human expansion routes.

So basically the snail hitched a ride from Sardinia to Algeria to France. I don’t think this is that surprising. First, it seems pretty obvious that a lot of the cultural expansion in the prehistoric period did not consist of the fission of villages along a continuous wave of advance, but involved leap-frogging to suitable nuclei from which the populations expanded. Imagine a rising flood where the lowest zones are inundated first, and then the higher peaks. Additionally, we shouldn’t presume that these expansion events were without conflict and institutional support. Consider that the expansion of farming across much of southern European Russia and Ukraine could only occur after the state had pacified, expelled, or assimilated, the mobile Turkic populations which were wont to extract unsustainable rents out of isolated and vulnerable peasant populations.

Finally, what’s up with the strong north-south differentiation across the Mediterranean basin, peaking in the west? It’s as if there were two waves of demographic and cultural advance which laid the ground work, and later perturbations haven’t disrupted that bedrock. It suggests to me the critical importance of lateral coastal transport in connecting cultural colonies, as opposed to more long distance jumps across the open sea. The latter were probably important for the transport of luxury goods and the exchange of memes, but not so much for the exchange of genes.

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ResearchBlogging.org Seriously, sometimes history matches fiction a lot more than we’d have expected, or wished. In the early 2000s the Oxford geneticist Bryan Sykes observed a pattern of discordance between the spatial distribution of male mediated ancestry on the nonrecombinant Y chromosome (NRY) and female mediated ancestry in the mitochondrial DNA (mtDNA). To explains this he offered a somewhat sensationalist narrative to the press about possible repeated instances of male genocide against lineage groups who lost in conflicts.

Here is a portion of the book of Numbers in the Bible:

15 – And Moses said unto them, Have ye saved all the women alive?

16 – Behold, these caused the children of Israel, through the counsel of Balaam, to commit trespass against the LORD in the matter of Peor, and there was a plague among the congregation of the LORD.

17 – Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him.

18 – But all the women children, that have not known a man by lying with him, keep alive for yourselves.

Then there is the rape of the Sabine women. The ethnogenesis of the mestizo and mulatto populations of the New World in large part was the union between non-European women and European men. These are hard brutal myths and hard brutal facts. But do they reflect an essential aspect of the dynamics which have shaped our species’ past?

I’m not willing quite yet to add a confident weight upon this possibility, but this seems to be part at least part of the picture. You see a major disjunction on male and female lineages among South Asians for example. A new paper in PNAS adds weight to this possibility, albeit only incrementally. Ancient DNA reveals male diffusion through the Neolithic Mediterranean route:

The Neolithic is a key period in the history of the European settlement. Although archaeological and present-day genetic data suggest several hypotheses regarding the human migration patterns at this period, validation of these hypotheses with the use of ancient genetic data has been limited. In this context, we studied DNA extracted from 53 individuals buried in a necropolis used by a French local community 5,000 y ago. The relatively good DNA preservation of the samples allowed us to obtain autosomal, Y-chromosomal, and/or mtDNA data for 29 of the 53 samples studied. From these datasets, we established close parental relationships within the necropolis and determined maternal and paternal lineages as well as the absence of an allele associated with lactase persistence, probably carried by Neolithic cultures of central Europe. Our study provides an integrative view of the genetic past in southern France at the end of the Neolithic period. Furthermore, the Y-haplotype lineages characterized and the study of their current repartition in European populations confirm a greater influence of the Mediterranean than the Central European route in the peopling of southern Europe during the Neolithic transition.

First, the easy stuff. This is another datum which should make us skeptical of the idea of Neolithicization as an overwhelmingly indigenous process, spreading via cultural emulation. The Y chromosomal lineages sequenced here are very homogeneous, and seem to belong to a patrilocal kinship group. In contrast, the mtDNA lineages, which tell us about female ancestry, are much more diverse. They cover a much better sweep of contemporary European genetic diversity. The authors note that a minority of mtDNA lineages are of Middle Eastern origin, but the majority are of lineages which are presumed to have a deeper Paleolithic root, as supported by their greater variance. I think we should still be cautious of even this interpretation, but there does seem to be a notable difference in this one community between males and females which may be indicative of a particular social and cultural system.

The maps to the left show the relationship of mtDNA and Y lineages to modern patterns of European genetic variation. The darker the shading the higher proportion of lineages shared. The top figure illustrates female mtDNA, and you can see the broad correspondences between the ancient southwest French sample and modern groups. But observe the big difference in the second figure, which shows the male distributions. This is much more localized to particular regions of Iberia and Turkey. The overwhelming haplogroup in the cemetery was G2a-P15, which is rather rare in Europe today, and the region. What happened to these men? Genetic drift or population replacement perhaps. If one posits a model of long term smaller male effective populations then Y chromosomal lineages will be subject to more stochastic extinction and fixation events than mtDNA. I’m not sure if I believe this, but that is one model which doesn’t necessarily involve a conventional replacement of the male lineages a la Conan.

But the dispersal of G2a-P15 at low frequencies around the Mediterranean is also consistent with the possibility of repeated replacement of male lineages across the arc of history. This has historical precedent, the Greek colonies alonge fringes of the Mediterranean were founded by men, sometimes explicitly exiled from their home polis. They had often had to “obtain” local women to perpetuate themselves. This isn’t supposition or conjecture, but outlined in some of the texts which record how colonies were founded in the Archaic pre-Classical period. Of course we do know that these sorts of transplantations could also involve women, they seem to have in the case of the Etruscans.

Additionally, this may also be a case of male “leap-frog” migration patterns, which break apart the null model of genetic variation which is modeled by isolation-by-distance. The argument is that the expansion of farming from the eastern Mediterranean did not occur via demic diffusion by land, but rather through a process of maritime transplantation, and then subsequent expansion from the nascent nuclei. Again, we can look at the expansion of the maritime Greeks. There was no contiguous region of Greek settlement between Greece proper and “Magna Graecia” in southern Italy and Sicily. Connections were by sea, which makes sense insofar as long distance sea transport was far cheaper energetically than land migration. I see no reason why these ancient farming Diasporas couldn’t have maintained a sort of cultural continuity for centuries through ritual or regular contacts via maritime transit.

A second point in this paper is that this population seems to have lacked in totality the allele which is diagnostic of lactase persistence across much of Europe today. The authors observe that that allele has a frequency of ~43% in the modern French population (it’s dominant, so that means that ~35% of the French are lactose intolerance). I would be curious about the frequency in the south of France, as traditionally the north of France was the domain of butter, while that to the south more of olive oil (historically the south of France witnessed the preservation of the Gallo-Roman aristocracy, which explicitly adopted Frankish modes of dress in 5th century as a way to assimilate into the non-clerical administrative apparatus of the Merovingian monarchy, but maintained their cultural distinction). The authors conclude from the lack of the LP allele that the individuals buried in the cemetery were from a different migration stream than those which founded the LBK society in central Europe, and who may have invented the dairy culture.

I think the “big picture” that is shaping up is that there were multiple intrusions, eruptions, replacements, and assimilations, across prehistoric Europe, just as there were across many regions of the world. On the whole I suspect males played more of a role in this process than females, though I’m not confident that we will see a consistent pattern of female lineages in a given area being markers for the Paleolithic populations. There may have been so much shifting and layering that the original people, the oldest of old, may only be accessible via ancient DNA. Speaking of which, thank god we’re finally entering the golden age of ancient DNA! Many questions will no doubt finally be resolved.

Dienekes Pontikos also has a lot to say about this. I’m sure some of you who are more versed in mtDNA and NRY haplogroups will also offer you 2 cents!

Citation: Marie Lacan, Christine Keyser, François-Xavier Ricaut, Nicolas Brucato, Francis Duranthon, Jean Guilaine, Eric Crubézy, & Bertrand Ludes (2011). Ancient DNA reveals male diffusion through the Neolithic Mediterranean route PNAS : 10.1073/pnas.1100723108

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Thanks to the fact that northern Europe is cool and archaeological research is rather well developed in the region due to quirks of history, there are lots of findings from ancient DNA which are answering long-standing questions. In particular Scandinavia is of special interest in regards to the transition of Europeans from a hunter-gatherer lifestyle to an agricultural one. We know that hunting and gathering as dominant modes of economic production persisted relatively late in European history in this region, up to ~5,000 years before the present. From my cursory reading of the material on the spread of agriculture in northern Europe one dynamic which seems clear is that the rate of expansion was not always constant, and that at the northern fringes in particular social or ecological frontiers served to demarcate the limits to the expansion of farming groups, which often originated from the south and east. Additionally, on the maritime fringes of the North Sea and Baltic there seem to have been relatively dense agglomerations of hunter-gatherers which resisted or coexisted with farming populations for long periods of time (perhaps they were more accurately termed fisher-gatherers!).

This is where Anna Linderholm’s research comes into the picture. I’ve blogged some of her work before. Linderholm’s goal seems to be to synthesize a range of results from disparate fields in understanding how two partially contemporaneous prehistoric Scandinavian cultures related to each other: the Pitted Ware Culture (PWC) and the Funnelbeaker Culture (TRB, which is an acronym for the German name for the culture). The former were hunter-gatherers who tended to rely upon marine resources, while the latter were agriculturalists who engaged in a great deal of animal husbandry.

You can find her contribution to the book Human Bioarchaeology of the Transition to Agriculture online. It’s pretty accessible for an ignorant lay person, and in the chapter she outlines some really interesting detail about the relationship between the PCW, TRB, modern northern European populations, and the functional genetic characteristics of these ancient groups.


The basic chronological outline seems to be that around 3000 BCE there was a period of hundreds of years of contemporaneous habitation of southern Sweden by the TRB and PCW cultures, though they were spatially segregated. TRB finds seem to be concentrated in inland regions, while PCW were found on the maritime fringe. Additionally, the island of Oland in the Baltic exhibited nearly 1,000 years of coexistence of the two cultures. After 2000 BCE these cultures eventually disappeared and gave way to a homogeneous agricultural Bronze Age society.

Were the two cultures of southern Sweden during the Neolithic simply two modes of production of the same people? And were these the ancestors of modern day Swedes? And what can biological anthropology tell us about what they ate and how they ate?

Let’s start with mtDNA relationships:

The mtDNA results point to the fact that the TRB and PWC are two genetically distinct populations (p G 0.001) (Linderholm, 2008; Linderholm, unpublished). Furthermore, the PWC population appears to be more closely related to the modern Latvian population than the contemporaneous TRB population or any of the other modern population examined. This could imply that the PWC population may have had an eastern/central European origin, whereas the TRB population may have had a continental European origin (Linderholm, 2008; Malmstr€om et al., 2009; Linderholm unpublished). These results could also imply that in Sweden the PWC were part of a large hunter-gatherer complex that spanned vast areas of the central and eastern parts of Europe…

Neither of the two populations (PWC and TRB) shows any genetic affinities to the Sami population when compared on the basis of mtDNA sequence. It therefore appears that the ancient mtDNA analyses provide genetic evidence that the TRB and the PWC cultures were Neither of the two populations (PWC and TRB) shows any genetic affinities to the Sami population when compared on the basis of mtDNA sequence….

There’s a little ambiguity here about the TRB’s relationship to modern Swedes, so let me jump to the discussion where this is clarified:

Over 50 individuals were successfully typed and based on these results it has been shown that the PWC and TRB cultures were genetically distinct populations. The genetic legacy of the TRB continued into the Bronze Age and further in time, suggesting that they made a genetic contribution to modern Swedish populations. In contrast, the PWC seem to have left no genetic legacy to the modern Swedish population, but their genetic signals can be detected in modern north-eastern European populations…..

Modern Sweden was settled ~10,000 years ago after the retreat of the Ice Age. The TRB culture first arrived in Sweden ~6,000 years ago. From these results one might infer then that modern Swedes derive predominantly from a migration of farmers from the north European plain ~6,000 years before the present. In these analyses the identity of the hunter-gatherers resident in region (e.g., Skane) for the 4,000 years after the Ice Age and before the arrival of farming is left unclear. The PWC itself may have been intrusive, and closely related to a broader group of northeast European hunter-gatherer societies. Do recall that this study focused on southern Sweden. We know that the Sami were indigenous to northern Sweden relatively late into the historic period. Whether they too were exogenous on this sort of time scale, we don’t know yet.

Next Linderholm reviews previous results on the prevalence of lactose tolerance among these two societies. The results are pretty straightforward: the PWC generally lack the ability to digest lactose. This makes sense, why would they need this mutation if they weren’t engaged in animal husbandry? There’s an allele in northern Europeans which is strongly correlated with lactose tolerance. It operates in a dominant fashion, so the fact that the frequency of the allele in modern Swedes is 75% means that lactose tolerance is nearly at 95%. But the question left unanswered is the state of the situation in the TRB. It seems that this is work that is proceeding now (from what I can tell there was less success amplifying the TRB remains for whatever reason), but the author politely hinted strongly: “Analysis has also been performed on individuals representing the TRB population; the outcome of these analyses has not yet been totally verified. In the dataset we can detect a large rise in T-allele frequency in Sweden amongst the farming community (unpublished data).” This is the reasonable expectation, as the TRB kept cattle. It seems plausible that the rise of farming on the northern European frontier was in large part a function of the innovation of dairying, as in these regions the primary productivity of Middle Eastern cereal crops was far less. Rather, less palatable crops needed to be cultivated, and it may be that it was more efficient to use this as feed for cattle which then produced milk (e.g., oats).

Finally, they assayed the CCR5-D32 variant in their PWC and TRB samples. This is the “HIV resistance” allele found in many Europeans, whose origin has been hypothesized to be 1400 CE or 3000 BCE, depending on which coalescence model you trust. Interestingly Linderholm found that both of these ancient groups carried the resistant allele, likely pushing back the origin of the variant.

So what does this tell us in the end? We need to keep in mind other facts, and not lose the big picture. The populations of Norden, whether Finnic or Scandinavian, are genetically similar. But clearly there is a distinction as one moves from Sweden to Finland. The relationship between the Baltic populations and the Nordic ones is somewhat confused. What I can say from ADMIXTURE runs performed by genome bloggers is that it seems that Scandinavian, as opposed to Finnic, populations seem to have an ancestral component at low, but consistent, levels which is modal in the Middle East. The lack of this distant Middle Eastern affinity among the Finns, and to a lesser extent the Lithuanians, is to me telling. I suspect what you’re seeing is an ancient admixture event, greatly diluted by the time one approaches the far north, which was carried from the southwest by the ancestors of modern Scandinavian populations. The Finns were not impacted by this migration, and so do not have any imprint of that distant Middle Eastern ancestral component.

Second, the relationship of the PWC, hunter-gatherers, and TRB, farmers, needs to be teased apart. It does not seem this was a classic replacement of hunter-gatherers by farmers. Both these groups were intrusive to southern Sweden in relation to the Mesolithic hunter-gatherers who originally repopulated the peninsula. It could be that there was a symbiosis between the PCW and TRB, but it is notable that the genetic distinction between these groups is very strong. Additionally, the authors note that there is no connection between the PCW and modern Swedes, though there is one between the TRB and modern Swedes (in the late Neolithic and early Bronze Age the TRB were replaced by the “Battle-Axe” culture, which may or may not have been an organic outgrowth of the TRB). It doesn’t take rocket science to make an inference of “who won.” All that said, I wonder if one could make the case for the PWC being highly efficient superior hunter-gatherers, the “last stand” so to speak of a non-marginal manifestation of this lifestyle in Europe. Their concentration in coastal fringes suggests that they were focusing on regions where their per unit labor had the greatest returns. It may be that for a period they were at demographic parity with the TRB because the farmers had simply not perfect their cultural toolkit. An analysis of isotope ratios in the bones of the remains points to the possibility that the TRB took a very long time to acclimate to the north: “The results show a gradual transition from a hunter-gatherer diet to a diet based on agriculture took place by the TRB people (which is thought to be associated with the introduction of farming on the island) only by the end of the Neolithic and hence much later than had been previously thought.” If the TRB were facultatively engaging in behavior more typical of foragers then it seems understandable that they didn’t push aside the PWC as easily as they might have, because they lacked the traditional farmer demographic advantage.

There’s a lot more to come I assume. Though there was some analysis of nuclear ancient DNA the focus here was on mtDNA. That’s because mtDNA is copious. But as the techniques get better I assume museums might be more willing to part with parts of their specimens. Overall these results make me more cautious of maximalist clean models of simple replacements. Here’s a speculative scenario which I think might have been:

- TRB emerges out of a hybridization of native north European plain marine foragers and farmers from central Europe, who are themselves a hybridization of southern migrants and indigenes.

- Once TRB crystallizes with an agricultural toolkit more appropriate to the far north it quickly sweeps back the older farming frontier. The old-stock hunter-gatherers of the north melt away. But the TRB then are challenged by the “climax” culture of the northern hunter-gatherers, the very efficient lifestyle of which the PWC were partaking of, focusing on marine resources which were easy to obtain in large quantities. The rise of these hunter-gatherers may partly have been competition, but their lifestyle may also have been dependent upon trade with the dense agricultural societies to their south and west.

(Republished from Discover/GNXP by permission of author or representative)
 
• Category: Science • Tags: Anthropology, Genomics, Neolithic, Scandinavia, Sweden 
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A new paper in Proceedings of the Royal Society dovetails with some posts I’ve put up on the peopling of Japan of late. The paper is Bayesian phylogenetic analysis supports an agricultural origin of Japonic languages:

Languages, like genes, evolve by a process of descent with modification. This striking similarity between biological and linguistic evolution allows us to apply phylogenetic methods to explore how languages, as well as the people who speak them, are related to one another through evolutionary history. Language phylogenies constructed with lexical data have so far revealed population expansions of Austronesian, Indo-European and Bantu speakers. However, how robustly a phylogenetic approach can chart the history of language evolution and what language phylogenies reveal about human prehistory must be investigated more thoroughly on a global scale. Here we report a phylogeny of 59 Japonic languages and dialects. We used this phylogeny to estimate time depth of its root and compared it with the time suggested by an agricultural expansion scenario for Japanese origin. In agreement with the scenario, our results indicate that Japonic languages descended from a common ancestor approximately 2182 years ago. Together with archaeological and biological evidence, our results suggest that the first farmers of Japan had a profound impact on the origins of both people and languages. On a broader level, our results are consistent with a theory that agricultural expansion is the principal factor for shaping global linguistic diversity.

I don’t know the technical details of linguistics to comment, but the alignment between the linguistic model and archeology is pretty impressive to me. There’s a 95% confidence interval which can push the time back to 4,000 years, so there’s some fudge factor too. The basic technique is borrowed from phylogenetics. This is pretty clear when you notice that one of the algorithms seems to be the same one used in the rice genomics paper. Nick Wade covers the paper in The New York Times, so no need for me to give a blow-by-blow in a domain where I don’t have much insight anyway.


Dienekes Pontikos really likes these results and the method which they use. He, rightly in my opinion, believes that they lend more credence to the thesis promoted in the early 2000s using the same technique that the last common ancestor of Indo-European languages is very far back in time. I’m skeptical of this model, at least in its simple general form, but these results do push me into thinking that that model is more plausible. But to really understand this stuff I probably need to teach myself some rudimentary linguistics, so I guess we’ll see.

More broadly this gets to the question: did farming spread through demographic expansion or cultural diffusion? Obviously it’s not an either/or. There’s a small residual of Amerindian ancestry in American whites, so there was some diffusion through genetic assimilation. The Xhosa tribe of South Africa seem to have ~20% Khoisan ancestry. They’re the group on the Bantu farming frontier, the last before the Bantu toolkit ceased to be effective and the Khoisan managed to maintain their hold before the whites arrived. Some of the admixture is from pastoralist Khoi, but some of it may also be from hunter-gatherer Bushmen. But here’s my issue at this point: what are the examples where we know that hunter-gatherers picked up agriculture? The instances of Japan and the Bantu expansion are two where we’re now rather sure that it was demographic expansion and replacement. Was it so different in the past? I think it may have been insofar as farming was less advanced a cultural toolkit in terms of its ability to overpower hunter-gatherers. And yet still I am becoming more convinced of the thesis of that farming spread through procreation, not propagation. My hesitation is mostly due to the reality that our understanding of the past is so clouded as a fundamental matter.

(Republished from Discover/GNXP by permission of author or representative)
 
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pc1
Synthetic map

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.

ResearchBlogging.org 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:

In a series of highly influential publications, Cavalli-Sforza and colleagues used principal component (PC) analysis to produce maps depicting how human genetic diversity varies across geographic space. Within Europe, the first axis of variation (PC1) was interpreted as evidence for the demic diffusion model of agriculture, in which farmers expanded from the Near East ∼10,000 years ago and replaced the resident hunter-gatherer populations with little or no interbreeding. These interpretations of the PC maps have been recently questioned as the original results can be reproduced under models of spatially covarying allele frequencies without any expansion. Here, we study PC maps for data simulated under models of range expansion and admixture. Our simulations include a spatially realistic model of Neolithic farmer expansion and assume various levels of interbreeding between farmer and resident hunter-gatherer populations. An important result is that under a broad range of conditions, the gradients in PC1 maps are oriented along a direction perpendicular to the axis of the expansion, rather than along the same axis as the expansion. We propose that this surprising pattern is an outcome of the “allele surfing” phenomenon, which creates sectors of high allele-frequency differentiation that align perpendicular to the direction of the expansion.

The first figure shows the general framework with which they performed the simulations:

pcab1

You have a lattice which consists of demes, population units, all across Europe. They modulated parameters such as population growth (r), carrying capacity (C), and migration (m). Additionally, they had various scenarios of expansion from the southwest or southeast, as well as two expansions one after another to mimic the re-population of Europe after the Ice Age by Paleolithic groups, and their later replacement by Neolithic groups. They modulated admixture and introgression of genes from the Paleolithic group to the Neolithics so that you had the full range where the final European were mostly Neolithic or mostly Paleolithic.

Below are some of the figures which show the results:

[nggallery id=25]

allesurAs you can see the strange thing is that in some models the synthetic map gradient is rotated 90 degrees from the axis of demographic expansion! In this telling the famous synthetic map showing Neolithic expansion might be showing expansion from Iberia. Perhaps a radiation from a post-Ice Age southern refuge?

One explanation might be “allele surfing” on the demographic “wave of advance.” Basically as a population expands very rapidly stochastic forces such as random genetic drift and bottlenecks could produce diversification along the edge of the population wave front. The reason for this is that these rapidly expanding populations explode out of serial bottlenecks and demographic expansions, which will produce genetic distinctiveness among the many differentiated demes bubbling along the edge of expansion. Alleles which may have been at low frequency in the ancestral population can “fix” in descendant populations on the edge of the demographic wave of advance. This is the explanation, more or less, that one group gave last year for the very high frequencies of R1b1b2 in Western Europe. With this, they overturned the classic assumption that R1b1b2 was a Paleolithic marker, and suggested it was a Neolithic one.

Here’s their conclusion from the paper:

A previous study showed that the original patterns observed in PCA might not reflect any expansion events (Novembre and Stephens 2008). Here, we find that under very general conditions, the pattern of molecular diversity produced by an expansion may be different than what was expected in the literature. In particular, we find conditions where an expansion of Neolithic farmers from the southeast produces a greatest axis of differentiation running from the southwest to the northeast. This surprising result is seemingly due to allele surfing leading to sectors that create differentiation perpendicular to the expansion axis. Although a lot of our results can be explained by the surfing phenomenon, some interesting questions remain open. For example, the phase transition observed for relatively small admixture rates between Paleolithic resident and Neolithic migrant populations occurs at a value that is dependent on our simulation settings, and further investigations would be needed to better characterize this critical value as a function of all the model parameters. Another unsolved question is to know why the patterns generally observed in PC2 maps for our simulation settings sometimes arise in PC1 maps instead. These unexplained examples remind us that PCA is summarizing patterns of variation in the sample due to multiple factors (ancestral expansions and admixture, ongoing limited migration, habitat boundary effects, and the spatial distribution of samples). In complex models such as our expansion models with admixture in Europe, it may be difficult to tease apart what processes give rise to any particular PCA pattern. Our study emphasizes that PC (and AM) should be viewed as tools for exploring the data but that the reverse process of interpreting PC and AM maps in terms of past routes of migration remains a complicated exercise. Additional analyses—with more explicit demographic models—are more than ever essential to discriminate between multiple explanations available for the patterns observed in PC and AM maps. We speculate that methods exploiting the signature of alleles that have undergone surfing may be a powerful approach to study range expansions.

What’s the big picture here? In the textbook Human Evolutionary Genetics it is asserted that synthetic maps never became very popular compared to PCA itself. I think this is correct. But, the original synthetic maps have become prominent for many outside of genetics. They figure in Peter Bellwood’s First Farmers, and are taken as a given by many pre-historians, such as Colin Renfrew. And yet a reliance on these sorts of tools must not be blind to the reality that the more layers of abstraction you put between your perception and comprehension of concrete reality, the more likely you are to be led astray by quirks and biases of method.

In this case I do think first-order intuition would tell us that synthetic maps which display PCs would be showing gradients as a function of demographic pulses. And yet the intuition may not be right, and with the overturning of old orthodoxies in the past generation of inferences from the variation patterns in modern populations, we should be very cautious.

Citation: Olivier François, Mathias Currat, Nicolas Ray, Eunjung Han, Laurent Excoffier, & John Novembre (2010). Principal Component Analysis under Population Genetic
Models of Range Expansion and Admixture Mol Biol Evol

(Republished from Discover/GNXP by permission of author or representative)
 
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After linking to Marnie Dunsmore’s blog on the Neolithic expansion, and reading Peter Bellwood’s First Farmers, I’ve been thinking a bit on how we might integrate some models of the rise and spread of agriculture with the new genomic findings. Bellwood’s thesis basically seems to be that the contemporary world pattern of expansive macro-language families (e.g., Indo-European, Sino-Tibetan, Afro-Asiatic, etc.) are shadows of the rapid demographic expansions in prehistory of farmers. In particular, hoe-farmers rapidly pushing into virgin lands. First Farmers was published in 2005, and so it had access mostly to mtDNA and Y chromosomal studies. Today we have a richer data set, from hundreds of thousands of markers per person, to mtDNA and Y chromosomal results from ancient DNA. I would argue that the new findings tend to reinforce the plausibility of Bellwood’s thesis somewhat.

The primary datum I want to enter into the record in this post, which was news to me, is this: the island of Cyprus seems to have been first settled (at least in anything but trivial numbers) by Neolithic populations from mainland Southwest Asia.* In fact, the first farmers in Cyprus perfectly replicated the physical culture of the nearby mainland in toto. This implies that the genetic heritage of modern Cypriots is probably attributable in the whole to expansions of farmers from Southwest Asia. With this in mind let’s look at Dienekes’ Dodecad results at K = 10 for Eurasian populations (I’ve reedited a bit):


neolith

Modern Cypriots exhibit genetic signatures which shake out into three putative ancestral groups. West Asian, which is modal in the Caucasus region. South European, modal in Sardinia. And Southwest Asian, which is modal in the Arabian peninsula. Cypriots basically look like Syrians, but with less Southwest Asian, more balance between West Asian and South European, and far less of the minor components of ancestry.

Just because an island was settled by one group of farmers, it does not mean that subsequent invasions or migrations could not have an impact. The indigenous tribes of Taiwan seem to be the original agriculturalists of that island, and after their settlement there were thousands of years of gradual and continuous cultural change in situ. But within the last 300 years settlers from Fujian on the Chinese mainland have demographically overwhelmed the native Taiwanese peoples.

During the Bronze Age it seems Cyprus was part of the Near East political and cultural system. The notional kings of Cyprus had close diplomatic relations with the pharaohs of Egypt. But between the end of the Bronze Age and the Classical Age Cyprus became part of the Greek cultural zone. Despite centuries of Latin and Ottoman rule, it has remained so, albeit with a prominent Turkish minority.

One thing notable about Cyprus, and which distinguishes it from mainland Greece, is the near total absence of a Northern European ancestral component. Therefore we can make the banal inference that Northern Europeans were not initially associated with the demographic expansions of farmers from the Middle East. Rather, I want to focus on the West Asian and Southern European ancestral components. One model for the re-population of Europe after the last Ice Age is that hunter-gatherers expanded from the peninsular “refugia” of Iberia and Italy, later being overlain by expansions of farmers from the Middle East, and perhaps Indo-Europeans from the Pontic steppe. I have a sneaking suspicion though that what we’re seeing among Mediterranean populations are several waves of expansion out of the Near East. I now would offer the tentative hypothesis that the South European ancestral element at K = 10 is a signature of the first wave of farmers which issued out of the Near East. The West Asians were a subsequent wave. I assume that the two groups must correlate to some sort of cultural or technological shift, though I have no hypothesis as to that.

From the above assertions, it is clear that I believe modern Sardinians are descendants of that first wave of farmers, unaffected by later demographic perturbations. I believe that Basques then are a people who emerge from an amalgamation of the same wave of seafaring agriculturalists with the indigenous populations preceding them (the indigenes were likely the descendants of a broad group of northern Eurasians who expanded after the end of the last Ice Age from the aforementioned refugia). They leap-frogged across fertile regions of the Mediterranean and pushed up valleys of southern France, and out of the Straits of Gibraltar. Interestingly, the Basque lack the West Asian minority element evident in Dienekes’ Spaniards, Portuguese, as well as the HGDP French (even up to K = 15 they don’t shake out as anything but a two way admixture, while the Sardinians show a minor West Asian component). Also, the West Asian and Southern European elements are several times more well represented proportionally among Scandinavians than Finns. The Southern European element is not found among the Uyghur, though the Northern European and West Asian one is. I infer from all these patterns that the Southern European element derived from pre-Indo-European farmers who pushed west from the Near East. It is the second largest component across much of the Northwestern Europe, the largest across much of Southern European, including Greece.

A second issue which First Farmers clarified are differences between the spread of agriculture from the Near East to Europe and South Asia. It seems that the spread of agriculture across South Asia was more gradual, or least had a longer pause, than in Europe. A clear West Asian transplanted culture arrived in what is today Pakistan ~9,000 years ago. But it does not seem that the Neolithic arrived to the far south of India until ~4,000 years ago. I think that a period of “incubation” in the northwest part of the subcontinent explains the putative hybridization between “Ancient North Indians” and “Ancient South Indians” described in Reconstructing Indian population history. The high proportion of “Ancestral North Indian,” on the order of ~40%, as well as Y chromosomal markers such as R1a1a, among South Indian tribal populations, is a function of the fact that these groups are themselves secondary amalgamations between shifting cultivators expanding from the Northwest along with local resident hunter-gatherer groups which were related to the ASI which the original West Asian agriculturalists encountered and assimilated in ancient Pakistan (Pathans are ~25% ASI). I believe that the Dravidian languages arrived from the Northwest to the south of India only within the last 4-5,000 with the farmers (some of whom may have reverted to facultative hunter-gathering, as is common among tribals). This relatively late arrival of Dravidian speaking groups explains why Sri Lanka has an Indo-European presence to my mind; the island was probably only lightly settled by farming Dravidian speakers, if at all, allowing Indo-European speakers from Gujarat and Sindh to leap-frog and quickly replace the native Veddas, who were hunter-gatherers.

Note: Here is K = 15.

* Wikipedia says there were hunter-gatherers, but even here the numbers were likely very small.

(Republished from Discover/GNXP by permission of author or representative)
 
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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:
fareq

Just click through above. Rather, I am curious more about their nice visualization of the archaeological data:


euroneolithic

Note how much variance there is in terms of the rate of change of the clines. As I’ve observed before there was a “break out” of the LBK farmers into Central Europe nearly 7,000 years ago, but it took much longer to close the gap between the farms on the frontier and the sea. This is well known from the archaeology, as there seems to have been a pause of ~1,000 years across much of the north European plain. On the scale of 10,000 years that’s not much time, but that’s about 40 generations. In Frisia it looks like the spreading of farming stopped for nearly ~2000 years!

Why the abatement of the spread of farming? I think the authors of the above paper are correct in their acceptance of the conventional wisdom of greater Mesolithic densities in Northern Europe. But I think perhaps a better description might be maritime Northern Europe. We often imagine early farmers displacing hunters and gatherers of game and herb, but what if in much of the world the main clash numerically was between dense populations oriented toward the sea, and those who were depended on the land? About seven years ago a study came out which argued for a rapid transition from seafood to meat in the diets of early Britons, Why Did Ancient Britons Stop Eating Fish?:

When cattle, sheep, pigs, and wheat arrived on the shores of Great Britain about 5,000 years ago, fish quickly fell off the Neolithic menu, according to an analysis of human bones scattered throughout the island.

“Farming really took off in Britain during the Neolithic. The main questions concerning the speed of change relates to how quickly Mesolithic peoples adapted—or otherwise—to the new farming methods and/or the spread of farming into Britain by new farming communities,” he said.

The research by Richards and colleagues Rick Schulting at Queen’s University Belfast and Robert Hedges at the University of Oxford tracks the shift in diet by examining the dietary signature stored in the bones.

They find that the shift was rapid and complete at the onset of the Neolithic. “Marine foods, for whatever reason, seem to have been comprehensively abandoned,” the researchers conclude in the September 25 issue of the journal Nature.

“We determined that after the introduction of domesticates, as well as the other artifacts associated with the Neolithic, the isotope values showed that marine foods were not used anymore,” he said. “We then infer that this is a switch from wild foods such as fish and shellfish to the new domesticates that arrive at this time.”

Richards said there are three plausible reasons why the British abandoned seafood from the beginning of the Neolithic: the domesticated plants and animals presented a steady source of food; the shift was forced by a climate change; or cultural pressure.

In the early 2000s the idea of wholesale rapid demographic replacement was not in the air. I think we need to put that back on the table. Here is the chart on isotope ratios from the 2003 paper:
culwar

Notice the sharp discontinuity. Richards et al. in 2003 interpreted this as a rapid cultural acquisition of the Neolithic lifestyle ~2500-3000 BC. They note in the media reports that later Britons, for example at the time of the Roman conquest, seem to have utilized fish a bit more in their diet than these early Neolithics. This stands to reason, much of Britain is not too far from the sea. To me the very sharp drop in marine consumption is indicative more of a food taboo, than a practical shift. Obviously farmers would primarily be subsistent on grain, but there’s no necessary reason to avoid meat or fish, but as it happens in many parts of the world societies preserve and perpetuate exactly such norms. These norms may have spread through cultural diffusion, for example through an adoption of a new religion. Or, the norms may have been brought by a new group which arrived in large numbers and replaced the indigenous population.

Here is an equivalent chart from Denmark from an earlier paper by the same group:

denmark

800px-Saami_Family_1900pacnortWhen we think of peoples who aren’t farmers, we often think of marginalized nomadic or semi-nomadic groups. Many of the remaining hunter-gatherers such as Bushmen, as well societies which supplement their conventional lifestyle with a lot of hunting & gathering, such as the indigenous peoples of Siberia or the Sami of northern Scandinavia, occupy territory which is simply not viable for conventional agriculture. But this was not so in the past. Before the farmers arrived the rich bottom-lands were occupied by hunters & gatherers, of fish, game, grain, and nuts. In certain ecologies, such as around productive estuaries one could imagine enormous aggregations of these peoples. Additionally, it seems likely that a sedentary lifestyle predates farming. A good contemporary analog for what ancient Northern Europe may have been like was the Pacific Northwest before the European settlement. These native tribes were relatively affluent because of the abundance of salmon runs, and engaged in lavish signalling, such as with their famous potlatches. Seeing as how there are Atlantic salmon runs in places like Norway and Scotland one can make even closer correspondences perhaps!

Stonehenge-GreenAs I have stated before just because we have no written records of this period, we can not assume that these were necessarily the fragmented and scattered “small-scale societies” which we’re familiar with today. There may have been ideologically motivated political coalitions and alliances which broke down along ethnic and cultural lines. In the paper above the authors argue that there is evidence that a climatic constraint, crops which do not have a good yield in cooler or warmer temperatures, is a weak hypothesis. If so I wonder if it is a bit too pat to simply model the dynamics as a diffusive “bottom up” process. Seems plausible enough for much of Europe where Mesolithic populations were thin on the ground because of local carrying capacity, but I suspect that the encounter between dense agglomerations of farmers and fishermen resulted in an inevitable ramp up of political integration and consolidation, as villages and tribes had to coordinate together because of a positive feedback loop of conflict.

Image Credit: Lordkinbote, Mactographer

(Republished from Discover/GNXP by permission of author or representative)
 
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