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. A change to a single letter of DNA leaves it switched on for life, and that letter spread through European and some African populations faster than almost any other human variant. The odd part is the timing: people had been drinking milk for thousands of years before the gene became common.

Why it matters

It is the clearest case of a culture changing its own genome. But the reason it 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 an undrinkable food becoming drinkable is the difference between surviving a bad year and not.

Dating & uncertainty

The mutation is dated by ancient DNA and by the length of the chromosome segment carried with it. Why it spread so fast is contested: milk was already being drunk for thousands of years before the gene became common, and the best statistical fit involves famine and disease rather than 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