Computing the spread of lactase persistence

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As most readers of this weblog know most humans as adults cannot digest lactose. The ability to digest lactose via the persistence of the enzyme lactase is differentially distributed. Both inferential methods and a small number of ancient genetic extractions suggest that this ability arose within the last 10,000 years. A new paper, The Origins of Lactase Persistence in Europe:

Most adults worldwide do not produce the enzyme lactase and so are unable to digest the milk sugar lactose. However, most people in Europe and many from other populations continue to produce lactase throughout their life (lactase persistence). In Europe, a single genetic variant, −13,910*T, is strongly associated with lactase persistence and appears to have been favoured by natural selection in the last 10,000 years. Since adult consumption of fresh milk was only possible after the domestication of animals, it is likely that lactase persistence coevolved with the cultural practice of dairying, although it is not known when lactase persistence first arose in Europe or what factors drove its rapid spread. To address these questions, we have developed a simulation model of the spread of lactase persistence, dairying, and farmers in Europe, and have integrated genetic and archaeological data using newly developed statistical approaches. We infer that lactase persistence/dairying coevolution began around 7,500 years ago between the central Balkans and central Europe, probably among people of the Linearbandkeramik culture. We also find that lactase persistence was not more favoured in northern latitudes through an increased requirement for dietary vitamin D. Our results illustrate the possibility of integrating genetic and archaeological data to address important questions on human evolution.

Here’s a graphical illustration of their conclusion:

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12 Comments

  1. This would seem to undermine Cochran’s theory that lactase persistence was the driving force behind the Indo-European expansion, since most scholars place the IE homeland in the Pontic-Caspian steppe (or Anatolia).

  2. The hills are alive 
    With the sound of milking 
    A sound they have heard  
    For ten thousand years…..

  3. I know a lactose-intolerant Norwegian American. It seems odd, since that demographic is gig into dairying. But look at the map.

  4. Very interesting. Some babes just never wane in germany.

  5. The characteristic European lactase-tolerance allele is known to be fairly common in Pakistan: since lactase tolerance is also common in North India, just next door, I’d guess it is the same allele. A long and winding road from Bavaria.

  6. John Hawks expresses some skepticism.

  7. >>This would seem to undermine Cochran’s theory that lactase persistence was the driving force behind the Indo-European expansion, since most scholars place the IE homeland in the Pontic-Caspian steppe (or Anatolia).

  8. didn’t dairy economies spread into the pontic caspian steppes from central europe via the Bug-Deinster (Deinster valley) and Cucuteni Tripolye (eastern Carpathians) cultures? this was much before the Indo European expansion so that theory would till hold. 
     
    David Anthony’s Horses Wheel and Language makes a case for that.

  9. I do wonder how much fresh milk most early agriculturalists would drink. Wouldn’t they usually turn it into cheese or butter, which keep longer?

  10. I mentioned the cheese issue last winter, and the issue of its tolerability by lactase-negative people and whether that might negate the advantage of lactase persistence.  
     
    But on the other hand, I later mused, if the allele was strongly selected, that in itself shows that there was a big advantage in being able to take your milk unfermented. Probably. It’s conceivable that the allele was selected for something else, but not likely. 
     
    Are there any butters or cheeses that will keep well in the summer? This might be the crux of the matter. They would keep if immersed in a snowmelt stream, but one might be a semi-nomad and such amenities wouldn’t be available.

  11. What does the map purport to show? Is it a probability distribution for the single origin point of lactose tolerance, or is it a distribution map for frequency of lactose tolerance at some date thereafter? Either way, the remarkably smooth nature of the eliptical geometric shape projected onto the map of Europe seems a little odd. What am I missing?

  12. John – from a quick scan of the paper, it’s a posterior density of region of origin, not for the allele, but for co-evolution of the gene and a dairy culture! Each point represents the outcome of a simulation; the heat map was generated using a kernel density estimator (presumably Gaussian), and looks smooth because kernel density estimators don’t recover high frequency information. They’ve really created a very detailed simulation – it even contains topographic data to model the effect on mobility of terrain.

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