Why Milk Matters in Mammals
Female mammals produce milk containing lactose, a sugar that nourishes infants.
Infants have an enzyme called lactase that breaks down lactose into simpler sugars for digestion.
As babies grow and are weaned, they lose the ability to produce lactase.
Adults who drink milk without enough lactase can suffer from lactose intolerance (bloating, gas, diarrhoea).
Lactase Persistence: An Evolutionary Adaptation
Some adults, especially in Europe and Africa, can still digest milk due to a genetic mutation called lactase persistence.
These mutations likely appeared around 11,000 years ago, during the time humans started domesticating animals.
Scientists believed this was a case of convergent evolution — similar traits evolving in unrelated groups due to shared lifestyles (like drinking milk).
A New Twist: Neanderthal Origins in East Asia
New research shows East Asians (like Chinese and Japanese) developed lactase persistence differently — their version came from Neanderthals.
This mutation appeared over 30,000 years ago, before domestication began.
It may have been selected for immune benefits, not milk digestion originally.
Unlike African and European cases, cultural milk-drinking may not have caused the mutation in East Asians.
What This Tells Us About Human Evolution
Humans and Neanderthals interbred 80,000–120,000 years ago, contributing up to 4% of DNA in modern Eurasian people.
The lactase gene from Neanderthals spread slowly and is now found in nearly 29% of East Asians.
New data shows the gene’s rise long before animal domestication, challenging the old story that milk-drinking caused these mutations.
This finding complicates the textbook example of gene–culture coevolution and makes the story of human adaptation richer and more complex.
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