Thursday, January 24, 2008

The Neolithic transitions   posted by Razib @ 1/24/2008 10:40:00 PM

I've been posting a fair amount on the transition from hunter-gatherer to farmer in northern Europe lately. Though I'm obviously interested in historical scholarship in and of itself, my focus on this period has been triggered by the spate of recent papers on selection within the last 10,000 years or so. It seems that the overwhelming shift of humans from hunter-gathering toward agricultural lifestyles within the period between 10,000 and 2,000 years ago had to have had a major impact on evolutionary pressures; just as fire might have hundreds of thousands (or millions) of years in the past. The amylase and lactase persistence stories are pretty straightforward derivations of the change in lifestyle; different food inputs will result in different optimal digestive propensities. Then there are pretty obvious second order concerns; farming societies are usually characterized by more individuals per unit area because less land is needed to support one person.1 The implications of this for disease are clear from the dependence of endemic diseases on particular density thresholds. Additionally, the domestication of herbivores also likely cranked up the rate of production of new diseases as pathogens crossed the species barrier. Finally, there are more nebulous possibilities such as various in alleles which are known to have behavioral correlates, such as DRD4, and their possible relationship to a local human ecology.

That being said, attention to details is important. The farming lifestyle in Denmark is very different from the farming lifestyle on the North China plain. In Farewell to Alms Greg Clark made a few general claims derived from assumptions which I think are pretty easy to refute. For example:
...Chinese adults, despite their very long history of settled agriculture and the variety of climate zones within China, generally lack the ability to absorb lactase, suggesting that milk was never a large part of the Chinese diet, and that by implication Chinese living standards were generally low in the preindustrial era.

Clark assumes that cattle culture is a sign of local wealth, and that the gene-culture co-evolutionary process was driven by economic parameters. The reality is of course that there are ecological considerations; the distribution of cattle culture in Africa is the clearest example, but it seems likely that it was an issue elsewhere. I'm reading A Concise Economic History of the World, and the author notes before the Romans cattle raising and slash & burn farming were the norm in Gaul. With the spread of the Roman empire two-course rotation was introduced, but the thick clay rich soil of northern Europe did not yield easily to the Mediterranean plow. A more powerful heavy wheeled eventually did open up northwest Europe to intense three-course rotation, and resulted in very high population densities and the flourishing of the manorial system by the peak of the Medieval Climate Optimum. That being said, the manorial system did not spread to the Celtic Fringe or most of Scandinavia because the cereal based system was less optimal in extremely moist or cool climates; there a cattle based form of agriculture remained dominant not because of the wealth of the Irish or Norwegians, rather the local ecology placed constraints on the options they could follow.

The point of all this is that the spread of agriculture to northern Europe 8,000 years ago did not mean that the hunter-gatherer with his bow immediately became the medieval peasant on his plow, so some of my presentation comes rather close to implying this. No doubt the shift was across a continuous range, and the local configuration was subject to historical contingency and ecological constraint. I do hold that it is likely that endemic disease became much more significant with the rise of agricultural communities, but we should be cautious about projecting from the extremely productive period before the modern era, when better technology (plow, horse collar, etc.) and diversified crops combined to drive the Malthusian limit very high indeed.

With that in mind, I'd like to point to a dissertation that Paul found where there are some interest results being reported:

Paper V: Different allele frequencies in the lactase gene in Scandinavian Neolithic populations and the development of dairy product consumption

Even though genetics and culture may interact...finding direct evidence of this is difficult. The genetics behind lactase persistence...the ability to consume unrefined milk during adulthood, may be an exception. The frequency of a mutation...linked to this trait is strongly associated with present-day dairy cultures and the geographical areas where they have developed. For example, cattle genes related to milk shows a geographical affinity for these areas...and fat residues in archaeological pottery indicating actual usage of milk have been found here...It has been suggested that present day geographical distribution of lactase persistence and the -13910 substitution is a result of culturally induced selection over a short period of time, especially in Europe...and that this selection was due to the introduction of farming....

Here we investigate if the ability to consume unrefined milk was present in populations with different nutritional specialisation in Scandinavia during the middle Neolithic (5,300 to 4,500 years ago).

Materials and methods

The material consisted of duplicate extractions from 36 middle Neolithic humans that had previously yielded high-quality mitochondrial DNA data...The samples derived from four hunter-gatherer sites (n=14) and one farming site (n=4) in Sweden were extracted together with negative controls...The -13910 C/T substitution in the nuclear lactase gene was amplified and alleles were identified using pyrosequencing. The ancient lactase data was statistically compared to earlier published data from modern Swedes. The previously yielded HVSI data...was compared to modern Swedish, Norwegian and Saami populations using pair wise Fst.

Results and discussion

Even after thorough sample quality pre-selection only 50% (18 out of 36) of our samples yielded reproducible results and among these, as also noted in other ancient DNA studies...allelic dropout was high. A lower number of positive results were detected in negative controls (in 4/43 seal amplicons, 4/41 extraction blanks and 6/39 PCR blanks). The allele associated with lactase persistence was found in 50% of the farmer samples and in 10% of the hunter-gatherer samples. The farming samples did not differ from modern Swedes whereas the hunter-gatherers did. Further, the Neolithic samples were significantly different from modern Norwegian and Saami samples when mitochondrial HVSI was compared. These results may, however, be influenced by five of the hunter-gatherer samples sharing a haplotype not found among any published modern populations. We base the authenticity of our results on the fact that the samples had been pre-screened for contamination in a previous study...that the success rate for retrieving alleles differ between the human samples and the negative controls, and that the allele frequencies differ between these two sample types.

Our data suggests that the frequency of the allele linked to lactase persistence in the investigated farmer population was, already 5,500 years ago, closer to modern Swedish frequencies than to those seen in the contemporary hunter-gatherers. This may be caused by cultural induced selection in the farmers. An alternative explanation that would not rely on an extreme cultural induced selection pressure, but with limited archaeological support...would be that only those with the genetic base for consuming unrefined milk became farmers while remaining people stayed hunter-gatherers.

I'll you digest this, but, do keep in mind that histories of populations and particular genes do not always align. A major problem in modeling the past seems to be a disregard for this distinction. Demic diffusion may not be necessarily the substantial replacement of ancestral genome content. Rather, long distance colonies from southern Europe might have brought both a new lifestyle and new genes. Most of their cultural and genetic distinctiveness might have been swamped out, but a few extremely salient elements may have remained and become dominant.

1 - There are exceptions, such as the coastal Pacific Northwest where a dense and affluent society grew up around the abundance of salmon.