The ability for adults to digest the milk sugar, lactose, is often referred to as lactase persistence (LP), describing the continued (persistent) production of lactase into adulthood. LP is an autosomal dominant trait that is most often associated with a T allele situated 13,910 base pairs (–13,910*T allele) upstream of the lactase gene, LCT. Archaeogenetic data indicates that pre- and early Neolithic populations were largely LP-negative, and that the frequency of the LP phenotype rose dramatically in Europe around 8,000 years ago, coinciding with the Neolithic transition from a hunter-gather to an agricultural-based lifestyle (1) and the appearance of domesticated dairying animals. Today roughly 35% of adults are lactose persistent. The frequency varies dramatically by geographic region, from a high prevalence in Europe (89–90%) and to a relatively low prevalence in the eastern Mediterranean (15%)(1). The spread of lactase persistence is an often cited example of gene-culture co-evolution. You can’t separate the history of domestic dairying and the evolution of lactase persistence, but scientists are still trying to understand how these two worked together.
Early Neolithic People Lacked the –13,910*T Allele But Kept Dairy Animals
The age of the –13,910*T allele has been estimated using both long-range haplotype conservation (2,188–20,650 years) and variation in closely linked microsatellites (7,459–12,300 years). These age estimates are consistent with other palaeogenetic data. For example, palaeogenetic studies on one Mesolithic and eight early Neolithic skeletons showed that they all lacked the LP allele, suggesting that it is unlikely that the early Neolithic populations would have been able to digest milk (2). The rapid increase in LP frequencies in these regions (northern and central Europe) from the Neolithic era to present day is indicative of strong natural selection and cannot be explained by genetic drift alone.
Part of the Neolithic transition was the introduction of domestic animals such as sheep, goats and cattle. These animals were a reliable source of food, not just through their meat, but also through their milk. Despite the absence of lactase persistence, archaeozoological evidence of slaughtering age profiles (3,4) of the domestic animals are consistent with populations that are actively involved in dairying. Lacking the LP allele, milk would have been almost toxic to these people, begging the question: What did they do with the milk if they couldn’t drink it?
7,000-Year-Old Pottery Shards Hint at Milk Use in the Kuyavia Region of Poland
Oddly perforated pottery fragments excavated from a site in central Poland in the 1970s offer an answer to how these early farmers used milk. The fragmented vessels resembled crude sieves like those used by later peoples for cheese making. In 2012 a team of geochemists and archeologists analyzed the absorbed organic residues found in the perforated fragments (5). The lipid residues extracted from the perforated pottery were compared to those extracted from “cooking pots” excavated from the same region in Poland. Because metabolism of adipose and milk fat result in significantly different δ13C ratios, the researchers were able to distinguish the lipids in the proposed sieves as dairy fats and those in cooking pots as adipose fats. These results support the idea that early Neolithic populations were producing fermented milk products (cheese and yogurt).
Cheese making would have offered several advantages; it would make it easier to store milk products for use at times when milk was not readily available, as well as improve the ease of transport of dairy products. Perhaps most importantly for the early Neolithic populations, the process would dramatically reduce or eliminate the amount of lactose present in the final dairy product, and thus making them more digestible to a population that was most likely largely LP-negative.
The Cows Came First. Why Did LP Follow?
The presence of milk use in early Neolithic populations is an important piece of the co-evolution puzzle because it offers strong evidence that the use of milk and milk products was present from very early on and did not develop after the population had become lactase persistent. Phylogenetic models have shown that the keeping of domestic animals (pastoralism) best explains LP frequencies seen world wide (6). The results suggest not only that the evolution of LP is strongly associated with the presence of domesticated livestock, but also that keeping livestock always precedes lactase tolerance in a population.
Positive Selection is widely accepted as the explanation for the increase in LP frequencies, but the type of selective forces is still unclear. The evidence of early adoption of milk fermentation suggests that simple selection for fitness doesn’t make sense, because LP-negative people would have had access to most of the same nutritional benefits of milk as LP-positive individuals. Some have speculated that there may have been socio-economic selection pressures. For example, if only the more elite social classes kept livestock, milk drinking could have been limited to a smaller segment of the population. If the LP allele was also present in this population, natural selection could be amplified, assuming a higher successful reproductive rate in the more elite classes (7). Unfortunately, it is virtually impossible to identify archeological and palaeogentic samples from the Neolithic era as being from distinct social classes.
Although we don’t have all the answers, this body of work demonstrates the value of integrating different disciplines to gain a more complete picture. By working together archeologists, archaeozoologists, paleogeneticists, geochemists and population geneticists were able to build a much more complete picture of the co-evolution of lactase persistence and dairying than any one field could have produced alone. And although we don’t quite understand how it happened, at least now we know the cow came first.
- Leonardi, M. et al. (2012) The evolution of lactase persistence in Europe. A synthesis of archaeological and genetic evidence. Intl. Dariy J. 22, 88–97.
- Burger, J. et al. (2007) Absence of the lactase-persistence-associated allele in early Neolithic Europeans. Proc. Natl. Acad. Sci. USA 104, 3736–41.
- Vigne, J.D. (2008) In J.P. Bocuet-Appel and O. Bar-Yosef (Eds) The Neolithic demographic transition and its consequences (pp. 179–205) New York, NY: Springer.
- Vigne, J.D. and Helmer, D. (2007) Was milk a ‘second product’ in the old world Neolithisation process? Its role in the domestication of cattle, sheep and goats. Anthropozoologica 42, 9–40.
- Salgue, M. et al. (2013) Earliest evidence for cheese making in the sixth millennium BC in northern Europe. Nature 493, 522–5.
- Holden, C and Mace, R. (1997) Phylogenetic analysis of the evolution of lactose digestion in adults. Human Biol. 69, 605–28.
- Heyer, E. et al. (2005) Cultural transmission of fitness: genes took take the fast lane. Trends in Genetics 21, 234–9.