A new ancient DNA study found that 800 years ago in Dalheim (Western Germany) lactase persistence was already similar to modern day frequencies (h/t to Chad):
Annina Krüttli et al., Ancient DNA Analysis Reveals High Frequency of European Lactase Persistence Allele (T-13910) in Medieval Central Europe. PLoS ONE 2014. Open access → LINK
Ruminant milk and dairy products are important food resources in many European, African, and Middle Eastern societies. These regions are also associated with derived genetic variants for lactase persistence. In mammals, lactase, the enzyme that hydrolyzes the milk sugar lactose, is normally down-regulated after weaning, but at least five human populations around the world have independently evolved mutations regulating the expression of the lactase-phlorizin-hydrolase gene. These mutations result in a dominant lactase persistence phenotype and continued lactase tolerance in adulthood. A single nucleotide polymorphism (SNP) at C/T-13910 is responsible for most lactase persistence in European populations, but when and where the T-13910 polymorphism originated and the evolutionary processes by which it rose to high frequency in Europe have been the subject of strong debate. A history of dairying is presumed to be a prerequisite, but archaeological evidence is lacking. In this study, DNA was extracted from the dentine of 36 individuals excavated at a medieval cemetery in Dalheim, Germany. Eighteen individuals were successfully genotyped for the C/T-13910 SNP by molecular cloning and sequencing, of which 13 (72%) exhibited a European lactase persistence genotype: 44% CT, 28% TT. Previous ancient DNA-based studies found that lactase persistence genotypes fall below detection levels in most regions of Neolithic Europe. Our research shows that by AD 1200, lactase persistence frequency had risen to over 70% in this community in western Central Europe. Given that lactase persistence genotype frequency in present-day Germany and Austria is estimated at 71–80%, our results suggest that genetic lactase persistence likely reached modern levels before the historic population declines associated with the Black Death, thus excluding plague-associated evolutionary forces in the rise of lactase persistence in this region. This new evidence sheds light on the dynamic evolutionary history of the European lactase persistence trait and its global cultural implications.
|Table 2. Results of genetic sex and LP allele genotyping.|
So lactase persistence was already highly prevalent in West-Central Germany 800 years ago, much as it is today.
Very interesting also is their mention of a previous study in Medieval Hungarians (Nagy 2011, PPV):
A study of medieval Hungary found moderate levels of LP in local commoners (33%) ca. AD 900–1100, but extrapolating from these results is complicated by the region's history of conquest by lactase non-persistent Asian invaders.
While these frequencies are clearly much higher than Neolithic ones (zero), they were still much lower than present day (c. 60%).
They also mention the oldest know lactase persistence alleles in Europe, which correspond to Chalcolithic findings in Götland and the Basque Country, albeit still at low frequencies and, in the Basque case, showing strong linkage disequilibrium pointing to an initial admixture episode between two different populations: one lactose-tolerant and the other intolerant. See this previous entry for more details.
As I see it, these two data points help us to better understand the still very wide window when lactose tolerance spread among Europeans, which begins in the Chalcolithic and, at least in the case of Germany, seems closed by the Middle Ages. Although in the Hungarian case remained still half-way in that period.
It is quite possible that instead of a single selective swap affecting this trait, the process took place in several bouts, each one with their own geography and timeline.
Still, the reasons behind this apparent positive selection for milk-digesting genes, remain ill-explained at academic level. Recently I tried to articulate a consistent theory on it, based on the fact that the Metal Ages, when this sweep happened almost certainly, were characterized by the accumulation of agricultural resources, wealth and power in few hands, producing a class-structured society in which the vast majority were poor and lived precarious lives, in which the general availability of, particularly, goat milk may have been an important nutritional relief (calories and proteins). See: Is the ability to digest milk in Europeans caused by ancient social inequality?