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 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!