Why are most adults lactose intolerant?
In: Biology
*All* adult mammals are lactose intolerant except for some humans.
Lactose sugar is only found naturally in one place – mammalian milk. Milk is produced exclusively by mothers for young. It’s very beneficial for mammalian young, because it’s full of calories and nutrients. But that also makes it hugely problematic for mammalian mothers. Those calories and nutrients have to come from somewhere, and that somewhere is extra food that the mother has to eat.
She does not benefit from that extra food, because she has to use it all for milk. For that reason, mammals only produce milk for young, and only when they are young. Once the offspring are old enough and strong enough to fend for themselves, mothers stop producing milk. The young don’t need it and continuing to make has a cost. Mom *could* keep trying to take care of the young, but that becomes increasingly dangerous for her. At some point she has to worry about possible future generations and let the current generation figure out its own survival, whether it wants to or not.
Lactose requires a special enzyme to break it down – lactase. All (normal) mammal babies make lactase. That enzyme is *only* useful for breaking down lactose. And building lactase also has a cost. If an animal isn’t ingesting any lactose, there’s no reason to waste energy making lactase. So once mom stops giving it lactose, it stops making lactase. No other animal gets milk as an adult, so there’s no reason to keep making it.
That has been the pattern for *hundreds of millions of years*, since milk and lactose evolved. Humans began domestication of animals only 11,000 years ago. And it was only about 5,000 years ago that a mutation entered the human genome that delays when we stop producing lactase. That’s only 5,000 years for that mutation to pass down and get spread around the human population. That mutation appeared in Europe, so it’s only people with European ancestry in their history that have it. Granted, Europeans have gotten around to most of the world so people all over the world have European ancestry, even if it’s only a fraction of it. Despite that, most of Asia and Africa do not have the gene and become lactose intolerant as adults. And, it’s not *everyone* with European ancestry. If you have the gene you are guaranteed to have European ancestry, but having European ancestry does not guarantee that you will have the gene.
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