GENETICS × HUMAN EVOLUTION

Drinking Milk as an Adult

~7,500 years ago

Phanerozoic · Cenozoic · Quaternary · Holocene · Northgrippian

Most mammals stop making the enzyme that breaks down milk sugar once they are weaned. In Europe, a change to a single letter of DNA leaves it switched on for life, and that letter spread faster than almost any other human variant; several other populations, in Africa and beyond, reached the same tolerance through mutations of their own. The odd part is the timing: people had been drinking milk for thousands of years before the gene became common.

Why it matters

This is the example people reach for when they want to show that a habit can change a genome. But the reason the variant spread is not the obvious one. Milk was already in the diet, so the advantage cannot simply have been nutrition; the best statistical fit involves famine and disease, when drinking milk you cannot digest stops being a discomfort and turns dangerous.

Dating & uncertainty

The date is that of the variant coming under selection, among dairying farmers in a region between the central Balkans and central Europe; it is estimated from ancient DNA and from the length of the chromosome segment carried along with the mutation. Why it spread so fast is contested: milk had been drunk for thousands of years before the gene became common, and the models that fit the data best point to famine and disease rather than to dairying alone.

Sources

1. Evershed et al. (2022), Nature · Dairying, diseases and the evolution of lactase persistence in Europe
2. Itan et al. (2009), PLoS Computational Biology · The origins of lactase persistence in Europe
3. Krüttli et al. (2014), PLoS ONE · Ancient DNA analysis reveals high frequency of European lactase persistence allele in medieval central Europe

Earth history as one day

23:59:59.86143 ms before midnight

if Earth’s 4.54 billion years were compressed into a single day