Before cows and sheep and goats were domesticated, it made sense for humans to be lactose-intolerant as adults. Otherwise, during a time of scarcity and near-starvation, the strongest members of the group could forcefully take all the available breast milk, or the older siblings would demand it, and the babies and infants would die.
Nature's solution was to make sure that anyone old enough to eat real food would suffer greatly if they refused to wean themselves.
But once people had animal milk available, it was a gigantic natural advantage to be able to drink it as adults -- not just for nutrients and calories, but much more significantly, to avoid dehydration, which was a major killer back then.
You may not know much about lactation, but nutrition is a big part of it. If there is a famine/drought, there is no milk to be had, doesn't matter how strong you may be.
"There is a common misconception that malnutrition greatly reduces the amount of milk a mother produces. Although malnutrition may affect the quality of milk, studies show that the amount of breastmilk produced depends mainly on how often and how effectively the baby sucks on the breast. If a mother temporarily produces less milk than the infant needs, the infant responds by suckling more vigorously, more frequently, or longer at each feeding. This stimulates greater milk production."
No I don't. Perhaps I should do more homework. I was appealing to common sense. If there is a famine or drought, people are starving and thirsty. Making milk is a lot of work and requires at least marginally decent nutrition. If someone is only eating one "meal" a day and drinking 2-4 cups of water a day, there's just not much with which to make milk.
N=1 anecdote, my daughters mother was very easily able to tell how her diet affected breastfeeding. A lot of it had to do with water intake.
Why do you think lactose intolerance came about, if not as evolution's way to ensure that the available milk went to those who needed it because they were not yet old enough to eat normal food?
Producing lactase is costly, those proteins could be doing other things. What's the evolutionary selection pressure to continue digesting a food source that's only present in the infant diet? None.
That's circular reasoning. The only reason it was "only present in the infant diet" is because of the evolution of lactose intolerance. Without that development, children would keep drinking milk for as long as they could get away with it.
But don't take my word for it; here's what Slate has to say: "With Cain weaned, Abel could claim more of his mother's attention and all of her milk. This kept a lid on sibling rivalry [...] while allowing women to bear more young."
> … it made sense for humans to be lactose-intolerant as adults
> Nature's solution was …
What made sense for an individual human is for everyone to be lactose-intolerant except them. This is what gives the best results for the individual in the near-starvation scenario.
Your suggestion sounds very much like “group selection”, an idea with many high-profile detractors (http://en.wikipedia.org/wiki/Group_selection ). The only argument I read against it was in “The Selfish Gene”, but according to the Wikipedia page, I could have picked from other high-profile scientists. Something looking like group selection may work in some cases (“The Selfish Gene” itself describes John Maynard Smith's counter-proposal of Evolutionary Stable Strategies), but offering such an explanation as obvious amounts to ignoring a lot of debunking by many bright people.
Nature's solution was to make sure that anyone old enough to eat real food would suffer greatly if they refused to wean themselves.
But once people had animal milk available, it was a gigantic natural advantage to be able to drink it as adults -- not just for nutrients and calories, but much more significantly, to avoid dehydration, which was a major killer back then.