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  • Have you ever found yourself, I don't know, thinking about the X-Men, secretly wishing

  • you were a mutant who could control the weather, or read minds, or teleport, or do something cool like that?

  • Let me ask you this, then: Do you drink and enjoy milk? How about ice cream? Can you do

  • that without getting ill?

  • Well then, congratulations, you are a mutant with a special superpower after all!

  • [INTRO]

  • Milk is produced by mammalian mammary glands, mainly to feed babies until they can digest

  • other foods.

  • Although milk is very nutritious -- full of protein, calcium, potassium, vitamin-D -- it's

  • also full of lactose, or milk sugar, which can be hard to digest.

  • Luckily, baby mammals, including human babies, produce tons of the lactose-digesting enzyme

  • lactase. If they didn't, they wouldn't be able to process their sole food source, and

  • they'd die.

  • Back in the day, when a child was four or five years old, their bodies started easing

  • off the production of lactase.

  • And by the time the kid was seven or eight years old, nursing or drinking another animal's

  • milk would have made him really sick.

  • If you or someone you know is lactose intolerant, you know what kind of miserable stomach-cramping

  • bathroom blowout awaits you if you dare succumb to a double-scoop of chocolate chip ice cream.

  • And those sufferers are actually in the majority -- about 70 percent of the world's population

  • cannot produce lactase after childhood.

  • And without lactase around to digest lactose, milk basically becomes toxic.

  • So what changed? How did milk go from being a weird food that only babies could appreciate

  • to a supermarket staple?

  • Two words: Mutant. Farmers.

  • Humans started to domesticate animals around 11,000 years ago in the Middle East. And traces

  • of milk fat have been discovered on artifacts in the Fertile Crescent going back about 8,500

  • years ago, and in central Europe, around 7,000 years ago.

  • The chemistry of these traces suggested that Neolithic herders had discovered a neat new

  • way to reduce the concentrations of lactose in milk -- by fermenting it. -- turning it

  • into cheese and yogurt.

  • But that only got them so far. They still could not drink the actual milk, be it goat,

  • or cow, or whatever.

  • And then, everything changed, when a unique genetic mutation popped up.

  • It's known as the lactase persistence trait, carried by what's called the LP allele, and

  • scientists think it first appeared about 7,500 years ago in central Europe.

  • That one little gene variant allowed its bearers to continue producing lactase into adulthood.

  • It probably spread as those Neolithic groups trekked north and west through Europe.

  • The allele did particularly well in the north, probably for several reasons. For one thing,

  • dairy products store well in colder climates, and they're extremely handy in places where

  • food may have been harder to come by, or grow.

  • And it may even be that milk's high concentrations of vitamin D provided a health advantage in

  • areas with little winter sun, since our bodies typically need sunlight to make vitamin D.

  • So the lactase persistence trait may have helped make this wave of human migration possible,

  • but still, it wasn't necessary everywhere. Today, in Britain and Scandinavia, nearly

  • 90 percent of adults can chug all the milk they want, whereas down toward the Mediterranean,

  • probably less than 40 percent of people have lactase persistence. And in some populations

  • in Africa and Asia, it shows up in less than 10 percent.

  • So if you're one of the 30 percent of the world's mutants who can eat ice cream with

  • impunity, enjoy that evolutionary perk for everybody else!

  • Thanks for watching this SciShow Dose -- especially to our Subbable subscribers. To learn how

  • YOU can help us keep sharing delicious science like this, just go to subbable.com. And don't

  • forget to go to YouTube.com/scishow and subscribe!

Have you ever found yourself, I don't know, thinking about the X-Men, secretly wishing

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