It’s normal to be lactose intolerant. Mammals evolved to be able to drink their mother’s milk until weaning, when the gene that breaks down lactose into simpler sugars that can be readily absorbed is switched off.
Around 70 per cent of the world’s adults are lactose intolerant and the remainder who aren’t owe their fortunes to two key events.
The first is the emergence of dairy farming some 9,000 years ago, which gave ready access to nutritional dairy products.
The second is the appearance of a handful of mutations, which kept the relevant gene switched on beyond infancy, making milk drinking a lifelong possibility.
This article is an answer to the question (asked by Faye Martinez, Manchester) 'Why are some people and animals lactose intolerant, despite drinking their mother’s milk when young?'
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