[AlotOfReading]

The vast majority of the human population is lactose intolerant, both historically and today. Genetically intolerant populations in South and Central Asia have microbiotic help with their dairy-heavy diets, but for people who didn't spend thousands of years developing a culture around it, dairy is just a quick road to an upset stomach and/or food poisoning.

[mmooss]

That makes some sense. Given the historic sometime scarcity of food and pressure of starvation, and the widespread availability of milk, I would think people would adapt to it.

I guess that lactose-intolerent people today would drink milk rather than starve - do they get zero nutrients from it? - and that evolution would select for those who could survive that way.

[AlotOfReading]

Not going to get into the social darwinism stuff. We can empirically measure an apparent selective pressure for lactase persistence, but it's an open question without clear answers what the factors driving that are.

I think you're missing why milk is useful though. Dairy allows you to take resources that aren't calorically useful like grasslands and turn them into food. You can consume it either immediately or later via preservation techniques like cheese. Even if you consume it immediately, milk is a seasonal product.

Dairy also isn't the only way of turning unusable resources into food though. You can eat the animal, for example. That's less efficient if you're limited to a single species, but cattle and other large livestock suitable for the scale of milk production you're talking about are so phenomenally inefficient that you're likely better off if you consume more efficient animals instead.

[mmooss]

> social darwinism

There is none of that in my comment.

> I think you're missing why milk is useful though.

? I was saying it is useful, and therefore I expect Homo sapiens would adapt to it.

After writing the GP I was told that humans, and some or all mammals, have a gene that disables lactose tolerance when they reach the stage of life where they no longer need milk. A miniority of humans have a mutation that stops that process, making them lactose-tolerent.

Why haven't we evolved to consume milk lifelong, given its obvious advantages (or why have we evolved to become lactose-intolerent past early childhood)?

A guess: Obviously milk consumption is inherited from mammal ancestors. That provides plenty of time (66 million years +) and population to evolve lifelong lactose digestion.

But other mammals don't have much need for that adaptation - for the most part, they can't figure out obtaining milk from another species as a regular food source. Human ancestors didn't figure out tool use until 2.6-3.3 million years ago; would we have figured it out then?

My guess is that it required domestication of animals ~12 thousand years ago before non-childhood milk consumption was commonplace. 12,000 years isn't much time to evolve much.