For mammals to lose the ability to digest lactose after they stop nursing is the normal default, established by millions of years of evolution.
It’s the persistence of the ability to digest lactose after weaning that is the new mutation, which appears to have occurred just a few thousand years ago.
Also of interest:
and more interesting info!
Yes.
A friend had to put his baby on non-dairy formula because she was severely lactose intolerant. It’s unusual for newborns not to make lactase, but it can happen.
For that matter, my daughter had “colic” and cried all the time as a baby. She was diagnosed as lactose intolerant as a small child. She digested my milk well enough that she grew and all that, but my guess is she was partially lactose intolerant even as a nursing infant.
Lactose intolerance is an issue digesting a specific protein for which you need a specific enzyme, and that enzyme works equally well on milk from you mother or from a cow or from any other source.
Very interesting. Thank you. So maybe it was “reverse” evolution. People who could tolerate milk products didn’t starve to death.
Digesting a specific carbohydrate, actually. Lactose, or ‘milk sugar’.
It’s a disaccharide, formed by joining one molecule of glucose and one molecule of galactose.
Lactase cleaves the lactose into its component simple sugars, and then those can be readily absorbed. Lack of lactase means lactose stays in the gut where bacteria feast on it and make a ton of gas and other volatile molecules in the process.
Right, my bad. Thanks for the correction.
I shouldn’t post “science” before I have my morning caffeine.
I would just call it “evolution”, personally, but yes. A genetic mutation arose among people who kept meat animals that made abundant milk, allowing them to digest milk as adults. And that mutation was selected for, as they had more food sources than those without the mutation.
Happened more than once, too, with different groups developing lactose tolerance in different ways.
I just read a thread on travel stack exchange, on the same topic, where there were two answers: one was something about center of gravity, which seems like it would be less of a concern on an A330, but what do I know and the other was
In long flights, in airplanes that has no crew rest area (bunks) such as A330, airlines reserve the seats at the rear of the airplane for the crew to take rests (shifts).
So that might be it, though I don’t know if a little over 7 hours counts as a long flight.
It would be a long flight if you’re having severe lactose intolerance. Or a raw milk induced illness. Especially if it’s a bumpy flight with limited access to lavatories.
Pretty long for one’s fellow passengers as well.
Aw great. We just herded this thread back from the “rectal exam” drift, and now we got airplanes?
On topic, somewhat, this thread inspired such a dairy craving in me that there’s now a (partially killed) carton of heavy cream in my fridge, for no reason beyond the fact that I wanted to drink straight up cream.
Back to the “raw milk” discussion …
lol. Somehow I screwed up the respond to email feature of discourse.
I had home made yogurt for lunch. 
BUT!! Did you make the yogurt from raw milk, and if so, are you 1a) healthier, or 2b) dead?
Not only did we start with pasteurized milk, the first step in our recipe is to briefly heat the milk to boiling (using the “steam” setting on the instant pot.) So I think we are re-pasteurizing it.
Nope, it’s that nearly all mammals stop producing lactase after weaning because why would they ever need it after that?
Lactose “intolerance” is the norm in nature. Those of us who can still digest it in adulthood are the freaky mutants. It has nothing to do with “this is not mom”, it’s “this is baby food and I’m not a baby anymore” except some of us retain the ability to digest it far longer than we “should”.
No, because nearly all mammals produce lactase as infants.
There is a condition called galactosemia in which individuals can’t properly digest milk that, prior to the invention of non-dairy baby formula, was usually fatal in infancy. But even they have lactase, it’s further down the processing chain that they have a problem.
As others have mentioned, it’s possible to be lactose intolerant from birth but very rare.
And if it’s ice cream 
we’re talking about I’m damn glad to be defective.
Not “defective” - different. It’s not a bad thing.
Remember to consider unforseen circumstances.
I would suspect that the raw milk is protected from the spoiling bacteria by the benign bacteria. The bacteria that turn milk into yogurt (mostly lactobacillus) will form an inhospitable environment for the bacteria that make the more objectionable smells and tastes. Pasteurization gets rid of all the bacteria, then when it is colonized by whatever is in the air in your home you get bad bacteria as well. When you make yogurt at home you’ll sterilize the milk first then add a culture to introduce the lactobacillus. Or just use the unwashed mason jars from the last batch of yogurt, that’s what I do.