Over the last decade, milk has become a bit controversial.

Some people say it's a necessary and nutritious food, vital for healthy bones,

but others say it can cause cancer and lead to an early death.

So, who's right?

And why are we drinking it anyway?

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Milk is the basis of every mammal's diet after birth,

when our digestive systems are immature and small.

Basically, it's power food to kick-start our bodies and help us grow.

Milk is rich in fat, vitamins, minerals, and milk-sugar: lactose.

On top of that, for a while after birth, it also contains antibodies and proteins

that protect us from infections and regulate our immune system.

But it's a lot of effort for mothers to produce.

Eventually, humans stop drinking mother's milk

and transition to the diet of their parents.

This is how it's been for thousands of years.

Until about eleven thousand years ago,

when our ancestors settled down in the first agricultural communities.

Soon, they domesticated the first dairy animals:

goats, sheep, and cattle.

They found that dairy animals are able to eat useless and abundant stuff

and turn it into nutritious and tasty food.

This made a huge difference in terms of survival,

especially in hard times.

So groups that had milk available had an evolutionary advantage.

And through natural selection,

it changed the genes of communities who consumed a lot of it.

This adaptation has to do with a specialized enzyme: lactase.

Babies have a lot of it in their system,

so they can break down the milk-sugar lactose and digest milk easily.

But the older we grow, the fewer lactase enzymes our body produces.

Worldwide, about 65% of the population do not have the enzyme after infancy,

which means they are not able to digest more than about 150 milliliters each day.

This lactose intolerance is not spread evenly around the world, though.

In some East Asian communities, for example, it's up to 90%.

In Northern Europe and North America, the rates are the lowest overall.

There are probably a few reasons for this uneven distribution.

The trait was first introduced by random mutation,

which happened independently of each other in a few populations.