six impossible things before breakfast 137gassy havoc. The effect is somewhat like eating any other indigestible
carbohydrate, such as cardboard or grass.
This is not an allergy or even, technically, a disorder. Physical anthro-
pologists tell us that age four, when lactose intolerance typically starts, is
about when nature intended for our kind to be wholly weaned onto solid
food; in other words, a gradual cessation of milk digestion is normal. In all
other mammals the milk- digesting enzyme shuts down soon after wean-
ing. So when people refer to this as an illness, I’m inclined to point out we
L.I.’s can very well digest the sugars in grown- up human foods like fruits
and vegetables, thank you, we just can’t nurse. From a cow. Okay?
But there is no animal weirder than Homo sapiens. Over thousands of
years of history, a few isolated populations developed intimate relation-
ships with their domestic animals and a genetic mutation gave them a
peculiar new adaptation: they kept their lactose- digesting enzymes past
childhood. Geneticists have confirmed that milk- drinking adults are the
exception to the norm, identifying a deviant gene on the second chromo-
some that causes lactase persistence. (The gene is SNP C/T13910, if you
care.) This relatively recent mutation occurred about ten thousand years
ago, soon after humans began to domesticate milk- producing animals.
The gene rapidly increased in these herding populations because of the
unique advantage it conferred, allowing them to breast- feed for life from
another species.
The gene for lifelong lactose digestion has an 86 percent frequency
among northern Europeans. By contrast, it shows up in only about one-
third of southern Europeans, who historically were not big herders. In the
Far East, where dairy cattle were unknown, the gene is absent. Even now,
Southeast Asians have virtually zero tolerance for lactose. Only about 10
percent of Asian Americans can digest milk as adults, along with fewer
than half of American Jews and about a quarter of rural Mexicans. Among
Native Americans it’s sketchily documented—estimates range from 20 to
40 percent. Among African Americans, adult milk- drinking tolerance is
high, nearly 50 percent, owing to another interesting piece of human his-
tory. The mutation for lactase persistence emerged several times inde-
pendently, alongside the behavior of adult milk- drinking. It shows up in