Abstract
Milk is a major food of global economic importance and its consumption is regarded as a classic example of gene-culture evolution. Humans have exploited animal milk as a food resource for at least 8500 years, but the origins, spread and scale of dairying remain poorly understood. Indirect lines of evidence, such as lipid isotopic ratios of pottery residues, faunal mortality profiles and lactase persistence allele frequencies, provide a partial picture of this process; however, in order to understand how, where and when humans consumed milk products, it is necessary to link evidence of consumption directly to individuals and their dairy livestock. Here we report the first direct evidence of milk consumption, the whey protein β-lactoglobulin (BLG), preserved in human dental calculus from the Bronze Age (ca. 3000 BCE) to the present day. Using protein tandem mass spectrometry, we demonstrate that BLG is a species-specific biomarker of dairy consumption and we identify individuals consuming cattle, sheep and goat milk products in the archaeological record. We then apply this method to human dental calculus from Greenland’s medieval Norse colonies and report a decline of this biomarker leading up to the abandonment of the Norse Greenland colonies in the 15th century CE.
Milk is an interesting foodstuff. The sugar in it is called lactose and lactose requires a chemical or enzyme called lactase to allow it to pass across the walls of the gut into the blood stream. When we are babies, we all produce plenty of the lactase enzyme which allows us to absorb our mother’s milk. In populations where milk consumption has been historically low, such as Japan and China, most children will have stopped producing lactase soon after weaning and – producing almost entire populations that may be unable to absorb the lactose in milk – this we call “lactose intolerance”.
In populations where milk consumption has always been high, such as in Europe, most adults continue to produce lactase for their whole lives and can digest milk quite happily with only around 5% of the population being lactose intolerant.
Continuing to produce lactase into adulthood is actually an inherited genetic variation which has become so common because being able to tolerate milk has a selective advantage. Milk is a useful source of protein, energy, calcium, phosphate, B vitamins and iodine, meaning that those with the mutation were generally healthier and produced more children than those who couldn’t tolerate milk, and so the presence of the mutation increased.