Lactose is the main carbohydrate in milk and is a major energy source for most young mammals. The enzyme responsible for hydrolysis of lactose into glucose and galactose is lactase. Without this enzyme, mammals are unable to Pascalbreak down and thus utilize lactose. After the weaning period is over, lactase production usually declines. However, some humans continue to produce lactase throughout adult life, and are thus able to digest the lactose found in fresh milk; a trait which is called lactase persistence (LP). Genetic studies on modernpopulations have shown that a common variant of the lactase gene (LCT) known as *13 9100T, is strongly associated with, and probably causative of LP in Europe and appears to be one of the most strongly selected alleles in the human genome in Europeans in the last 30,000 years. In Europe, clines are seen with LP found at frequencies of 10 to 30% in the south-eastern part of the continent, 50 to 60% in Central Europe, rising to 70 to 95% in north-western continental Europe and the British Isles. A similar trend is seen for the *13,9100 T allele. Palaeogenetic typing of this variant site in various Meso- and Neolithic skeletons revealed an absence of the *13,9100T allele in all cases. Statistical analysis has demonstrated significantly lower frequencies of LP when compared to modern Europeans from the same region. This is consistent with previous studies, inferring high selection coefficients acting on this trait. It is unlikely that natural selection would have driven the *13,9100T allele to high frequencies without a supply of fresh milk and this ties the biological evolution of LP to the culture of dairying through a gene-culture co-evolution process. Clear evidence of milk production can be seen in South-east Europe as early as 6,200 calBC, using lipid analysis on pot sherds.
In order to better understand the co-evolution of LP and dairying in Europe, Itan and colleagues developed a demic computer simulation model in order to examine how demographic and evolutionary parameters could have shaped both the modern distribution of LP in Europe and the timing of the arrival of farming at different locations throughout Europe. The study modelled the spread of dairying and non-dairying farmers into a Europe that was previously occupied by hunter-gatherers, under the plausible assumption that an LP-associated allele would only be selected in dairying farmers. Values for a number of different parameters must have shaped this process, including: 1. the extent of sporadic unidirectional migration; 2. the extent of gene flow between different cultural groups; 3. the extent of gene flow between neighbouring demes; 4. the extent to which people take up the culture of their neighbours; 5. the strength of selection favouring LP; and 6. the origin time and location of LP-dairying co-evolution.
By choosing random values for these parameters (within reasonable ranges), performing the simulation, and then comparing outcomes to observed data using Approximate Bayesian Computation, the authors were able to identify parameter values that best explained the modern distribution of LP in Europe and the timing of the arrival of farming at different locations throughout Europe. Although the LP allele is most frequent in Northern Europe today, the simulations that best explained the observed data (on the distribution of LP and the arrival time of farming at different locations) required LP- dairying co-evolution to start in an area between the Carpathian Basin and Central Europe between 6,260 and 8,680 years ago. The LP selection coefficient had inferred values between 0.0518 and 0.159 (in dairying farmers only). The inferred location and dates for the co-evolution of LP and dairying correspond well with the origins of the LBK culture in the Lake Balaton region. This again is in accordance with Bramanti et al. (2009) who inferred a massive immigration of the LBK farmers from this region.

(after Burger & Thomas 2011)

Fig. 1. Earliest known LBK sites (5,700–5,500 calBC; white squares) north of Lake Balaton after Pavuk (2005) and the inferred geographical origin of selection of lactase persistence, after Itan et al., 2009 (about 4,310=966,730 calBC, concentric ellipses).