Tuesday, February 12, 2013

Balkan Neolithic Has Large Demic Component

Strontium isotopes document greater human mobility at the start of the Balkan Neolithic
Dušan Borić and T. Douglas Price
Questions about how farming and the Neolithic way of life spread across Europe have been hotly debated topics in archaeology for decades. For a very long time, two models have dominated the discussion: migrations of farming groups from southwestern Asia versus diffusion of domesticates and new ideas through the existing networks of local forager populations. New strontium isotope data from the Danube Gorges in the north-central Balkans, an area characterized by a rich burial record spanning the Mesolithic–Neolithic transition, show a significant increase in nonlocal individuals from ∼6200 calibrated B.C., with several waves of migrants into this region. These results are further enhanced by dietary evidence based on carbon and nitrogen isotopes and an increasingly high chronological resolution obtained on a large sample of directly dated individuals. This dataset provides robust evidence for a brief period of coexistence between indigenous groups and early farmers before farming communities absorbed the foragers completely in the first half of the sixth millennium B.C.

From here.

The strontium isotype analysis of human skeletal remains provides an independent confirmation of the genetic evidence (including ancient DNA evidence) tending to show that farming came to Europe with mass migrations of farming populations, rather than primarily through technology transfers to existing hunter-gather populations. 

Within seven hundred years of their arrival (the press release suggests more like a couple of hundred years), the indigeneous hunter-gather populations of the region were exiled, had died off, or were assimilated in the farming communities. 

Neither the press release, nor the abstract set forth above, quantify the extent to which indigeneous hunter-gathering populations of the Balkans were assimilated into farming communities as opposed to being exiled or dying off (a variety of scenarios from genocide to declining game habits that reduce carrying capacity are possible within that category).

We can infer from lines of evidence not included in this study that women were probably more likely to be assimilated into the farming communities than men - leading to greater conservation of mtDNA lineages than Y-DNA lineages across the Mesolithic-Neolithic transition.  This study shows a gender differential that is somewhat different although the phrasing could be more clear:
An interesting finding of the study is that 8,000 years ago, when Neolithic farmers were beginning to migrate into the Danube Gorges and overlap with Mesolithic hunter-gatherers, more women than men were identified as foreigners. A possible explanation for the variance, according to the study, is that women came to these sites from Neolithic farming communities as part of an ongoing social exchange.
As I read this, the researchers seem to intepret their findings as showing that young male farmers born on the Neolithic frontier (and hence having "local" strontium isotypes) procured wives from more settled Neolithic lands who appeared to be "foreigners" in their strontium isotypes, rather than seeing this as evidence of indigneous men transitioning to farming and then taking Neolithic culture women as wives.


Va_Highlander said...

Not sure why we must dismiss what the abstract explicitly states, that "farming communities absorbed the foragers completely".

As for your reading of the reported gender differential, I concur and that does seem to be what they're claiming. I also tend to agree that a greater preponderance of "foreign" women seems counterintuitive, at least at first glance.

On reflection, though, I'm less convinced that my intuition is correct. The question seems to be, how well do we understand the spread of the Neolithic? I think the answer remains, "not very". I doubt we have anything more than vague and ahistoric ideas of what gender roles and relations might have been like in Europe 8,000 years ago. Consequently, it seems difficult to imagine how a phrase like "social exchange" can have meaning in such a poorly-defined context.

andrew said...

When the abstract states that "farming communities absorbed the foragers completely", in context, it implies that "farming communities absorbed the [remaining] foragers completely[.]", in a manner that is agnostic on the question of whether the foreigners who were absorbed were the sole descendants of the foragers who were there when the farming communities were established and is likewise agnostic on the question of what proportion of the foragers who were there when the farming communities were established left descendants who could be absorbed into the farming communities a couple of centuries later.

The abstract's statement makes clear that some descendants of the foragers who were there when the farming community was established were absorbed into the farming community, and that after a couple of centuries that there were no longer any foragers existing independently of the farming community. But, it says no more than that.

Va_Highlander said...

Which, with all due respect, seems a fancy way of saying that you don't believe that the foraging population was absorbed.

I did some digging and it turns out that Dr Borić probably knows his subject: "Mesolithic-Neolithic Interactions in the Danube Gorges", 2007 -- PDF. Seems the Mesolithic peoples of the Iron Gate were in contact with the Neolithic world from cal 6300/6200 BCE until 5900 BCE, yet the adoption of Neolithic culture was a slow process. It's interesting to now learn that "foreigners" were already present in significant numbers at such an early point in time. Borić had suspected some immigrants were present, despite the fact that these individuals were buried in a Mesolithic fashion. Now his suspicions are confirmed and apparently emphasized.

andrew said...

"Which, with all due respect, seems a fancy way of saying that you don't believe that the foraging population was absorbed."

Not necessarily. Mostly, I am not comfortable that I know (or that anyone knows) enough to really be sure, although I'm sure that some point, like Dr Borić have a more solid grasp of what is within the realm of plausibility than I do.

I think that eventually a mix of ancient DNA and population genetic methods supplemented by evidence from archaeology and methods like this isotype study will be able to tell us fairly accurately what percentage of the population ca. 5900 BCE (i.e. at the end of the process) was derived from Mesolithic peoples.

I think that we will be able to infer some minimum exile/extinction component based on the observed gender differentials in assimiliation rates.

I am almost certain that the exile/extinction component is greater than zero and is non-negligible.

For example, I would be stunned if there were not at least some instances of farmer community diseases disproportionately causing mortality in Mesolithic populations at some point close to the time of first contact ca. 6200 BCE, and I would be stunned if there was not some degree of warfare between the two groups in the 6200 BCE to 5900 BCE time period, with the foragers ultimately fairing less well than the farmers.

But, you really need, at least, both a fairly precise estimate of the extent to which the farmers assimilated Mesolithic peoples (my best guess would be something on the order of 20% of women and somewhat fewer men), and a fairly precise understanding of population densities for both farmers and foragers, to back out of the analysis a fairly solid estimate of the exile or extinction component. The farmer population density is the easier number of estimate accurately (although certainly still a daunting task) from direct evidence. Population densities for foragers in fringe habitats aren't good proxies for population densities in prime habitats that farmers appropriated early on. Probably the best way to estimate forage population density would be based on diversity in ancient Mesolithic DNA which can be used to approximate an effective population size in a fairly robust way with fairly small sample sizes and some reasonable supportable assumptions.

The scope of the population genetic, ancient DNA and archaelogical data necessary to discriminate between exile and extinction components once you were able to estimate the overall size of this component would be much larger. You would need genetic and archaelogical culture evidence of Mesolithic migration to establish an exile component effectively. You'd look for difficult to consistently preserve forensic physical anthropology evidence and fortification trends and trends towards declining ancient DNA diversity to prove extinction theories. But, much of the data necessary to resolve those questions may never been avaialable in sufficient quantities, so we have to resort to analogies to better documented cases that we can be comfortable are parallel to understand the process in the Neolithic transition in Europe.

andrew said...

A few more order of magnitude numbers suggest the hard parts of the analysis. In Egypt and North Africa, the increase in population density in the early Neolithic was about 100-1. This is a pretty harsh environment where foraging may have been more difficult than the Danube basin, but even in the Danube basin, a 10-1 population density ratio would probably be too low. I would guess ratios in the range of 20-1 to 80-1; not very exact but at least down to a factor of four rather than a full order of magnitude of uncertainty.

If estimates of 10% to 20% assimilation for both genders combined based on Y-DNA and mtDNA haplogroup frequencies in modern populations is right (and given a later demic copper age or early bronze age migration wave it should be low) are correct, there have to be founder effects from admixture very early on by pioneer Neolithic people with local foragers before the farmers have reached peak Neolithic population density, followed by probably lower rates of assimilation of foragers per generation relative to overall farmer population size in the area later on (because farmer wives are available at that point) to get percentages that high.

We have basically no Mesolithic ancient Y-DNA and almost no Mesolithic autosomal DNA, so we can only infer Mesolithic components in later populations based on contexts and distributions and phylogenies and mutation rates. The mtDNA pool in the Mesolithic, which we have a meaningful amount of ancient DNA to support, was not very diverse at all compared to any modern population almost anywhere. Mesolithic Europeans were hardly more diverse in their mtDNA than the modern Andamanese islanders are.

On the other hand, there is some reason to believe that Mesolithic modern humans may have had a somewhat higher population density than Neanderthals who had a narrower diet (and thus a smaller spot near the top of the food pyramid). We have pretty decent automsomal DNA based estimates of the effective population size of the Neanderthals, so that gives us a minimum expected modern human Mesolithic population size.

Rather abstract (but empircally validated) bottom up estimates of how many pounds of top predators could be supported by estimated prey species in these areas ecologically could provide another means by which to make an order of magnitude estimate but is limited by issues related to the mix of species in the top level predator spots and the details of the Mesolithic diet - meat heavy diets means smaller populations; nut and vegetable heavy diets and small game and fish heavy diets tend to favor larger populations.

New World ethnography and archaeology provides a third line of evidence for forager population densities in ecologies like those of Daubian Europe in the Mesolithic, although one has to be wary of first contact effects and you have to be careful to distinguish New World Neolithic peoples from New World forager peoples.

Direct archaeological evidence form the Mesolithic is just too thin to make meaningful estimates of population density that do anything but corroborate the other estimates as possible.

andrew said...

The end percentages of forager source DNA in farmer populations ca. 5900 BCE is very sensitive to timing and to migration rates relative to what were probably quite high levels of natural increase (since the areas farmed were the most prime virgin lands and land was in excess to farmers at first) during the transition period. The later in time that a particular instance of admixture occurs in this population with lots of natural increase, the less impact it would have on the fully admixed population genetics of that population.

One mixed couple ca. 6200 BCE has as ten generation edge on a mixed couple ca. 5900 BCE, has 2.59 times more demographic impact on the end state gene pool than a last generation mixed copule at 10% natural growth per generation (0.33% per year), for example, which is probably far too low.

I suspect you are looking at natural increase rates of closer to 50% to 100% per generation (i.e. 3-4 children per couple who survive and have descedants of their own on average during the virigin territory settling expansion phase). At 100% per generation, a first generation mixed couple has about 4000 times as much demographic impact as a last generation mixed couple. At 50% per generation, a first generation mixed couple has about 57.6 times as much demographic impact as a last generation mixed couple.

So, estimating the number of admixing forages relative to total number of foragers, even to an order of magnitude, is tricky business from population genetic evidence informed by the kind of ancient DNA evidence that we have so far, alone. But, basically, with reasonable estimates of natural increase rates almost all of the admixture in the end state is going to be derived from the first two or three of the ten generations of the assimilation period.

So, if the end state has 20% Mesolithic source mtDNA, assimilation is probably happening at a time when the admixture rate is perhaps 10%-15% per generation (and falling) perhaps 15% in the first generation. This suggests that one in six pioneer farmers are marrying forager women. And, since it is reasonable to assume that both foragers and farmers had some endogamy preference, the ratio of farmers to foragers at the time of first (and most demographically influential) admixture, was probably less than that 5-1 you would infer if there was no endogamy prefernce in either group. If it was a 3-1 farmer to forager ratio at that time (meaning farmers are getting almost 50% of forager women - a level seen in modern second generation Koreans and Japanese and Hispanic women in the U.S., i.e. too high), and that the end ratio of farmer to forager population density is 60-1, you are talking about admixture mostly happening at a time when farmers were at just 5% of their end state population density after ten generations of farming in the Danubian basin (which is about right for demographically influential admixture mostly happening in the first three generations during a period of high natural increase for farmer communities).

andrew said...

The number, indeed, suggest that using farmer population density within 30-60 years of the first Neolithic evidence of any kind based on archaeological evidence might be another decent way to estimate total forager populations to an order of magnitude level.

You might also be able to bin the Neolithic evidence during the 6200 BCE to 5900 BCE time period to make better estimates of natural increase + additional migration rates combined, and then use the foreigner isotype analysis to ballpark the relative contributions from natural increase and new migration over this time period, which would provide a more accurately multiplier to put into demographic models of admixture at each of ten generations.

You'd also ideally fit the farmer population growth curve to a logistic curve that has a mature Neolithic pre-copper age population density carrying capacity, rather that an exponential curve as I did, and would want to experiment with various functions of admixture assuming some endogamy based on relative forager and farmer population sizes to see which models make the most sense.

I suspect that the effects of larger farmer populations relative to forager populations in later generations is going to dwarf any possible weakening endogamy assumption (it has to be zero at full assimiluation after ten generations and I suggest quite a bit more than 50% endogamy in the first generation to fit realistic ethnographic examples that are compable probably at least 75% endogamy - a linear model of declining endogamy rate would probably be a decent place to start even though the reality is probably non-linear), so a decline relative rates of introgression into farm communities under reasonable assumptions at each new generation of farmers amplifies the founder effect multipliers for earlier generation admixture demographic impacts already discussed. Mostly, better estimates of endogamy rates and natural increase rates provide better estimates of the size of the forager population in the first three generations or so of contact.

Of course, you'd like to compare all of the different methodologies for estimating forager population densities pre-Neolithic and see if they more or less confirm each other (and if not, what levels of empirical uncertainty they imply).