ARGH stream ended and no one explained.
OKAY so here are three questions that came up:
- Why do we milk cows?
- Why does lactose intolerance exist?
- Does human milk not have lactose?
In semi-random order:
Why does lactose intolerance exist?
It kinda dosen't! The more correct term is lactase persistence: Basically, most mammals have an enzyme called lactase that lets them digest lactose which goes away after weaning. This is because your body has to produce lactase to be able to digest lactose, the main sugar in milk. As most mammals don't drink milk after weening, they stop making lactase (or just greatly decrease how much they make) as they don't need it, and it wastes energy to make lactase when it's not useful.
Humans, because we have been drinking animal milk for a long while, have two gene variants that mean we keep making lactase after weaning. Interestingly, the majority of humans do not have these gene variants! Only like 35% of humans have lactase persistence.
The fact it seems so common to be able to drink milk and the fact we named the most common state for humans to be as "lactose intolerance" is because lactase persistence varies across ethnicities. Europeans have the highest percentage of lactase persistence, so lactase persistence is just "normal" and the majority of humans supposedly suffer from "lactose intolerance".
This is also a new mutation: Sometime in the last 10,000 years, with the variant of the gene must europeans have evolving something like 7,500 years ago. This is so recent that there are buildings still standing that predate this mutation.
There's a whole other rant here about theories on how lactase persistence evolved, but I can't go on forever.
Does human milk not have lactose?
The assumption from this is that humans have always been able to drink human milk, so it must not have lactose. Nah. Human milk is pretty similar to the milk of other mammals. The reason we (as in, babies) can drink human milk while being lactose intolerant (like how everyone was for most of human history) is not that there's anything special about human milk: it's that it's drank by babies. Humans (and most mammals) start out with plenty of lactase so they can digest milk, then they lose it later (unless they have lactase persistence as explained above)
BUT WHY COWS? WHY DO WE DRINK MILK FROM COWS?
Grass. It's grass.
Humans can't eat grass. Grass is hard to digest, and it has a bunch of silica in it that'll destroy your teeth. So we can't eat most grasses (we do eat a lot of grasses, though: corn, wheat, rice, and barley are all grasses).
But cows can eat clover and grasses we can't. And if we can milk a cow that's fed on clover & grass, we can then drink the milk. We're basically using a cow to turn inedible (to us) food into something we can digest.
Cows have very complex stomachs that are nothing like our own, because they need it to eat grasses. Instead of evolving our own stomachs like that, we domesticated the cow to have them eat the grass and turn it into milk, which we can consume. They're a biological conversion step in the factory that lets use eat grass.
So yeah. The ultimate answer to the starting question of "why do we milk cows?" is THEY EAT GRASS.
