Globally, lactose intolerance is the norm; around two-thirds of humans cannot drink milk in adulthood
Photograph by Valentyn Volkov/iStockphoto/Thinkstock.
To repurpose a handy metaphor, let’s call two of the first Homo sapiens Adam and Eve. By the time they welcomed their firstborn, that rascal Cain, into the world, 2 million centuries of evolution had established how his infancy would play out. For the first few years of his life, he would take his nourishment from Eve’s breast. Once he reached about 4 or 5 years old, his body would begin to slow its production of lactase, the enzyme that allows mammals to digest the lactose in milk. Thereafter, nursing or drinking another animal’s milk would have given the little hell-raiser stomach cramps and potentially life-threatening diarrhea; in the absence of lactase, lactose simply rots in the guts. With Cain weaned, Abel could claim more of his mother’s attention and all of her milk. This kept a lid on sibling rivalry—though it didn’t quell the animus between these particular sibs—while allowing women to bear more young. The pattern was the same for all mammals: At the end of infancy, we became lactose-intolerant for life.
Two hundred thousand years later, around 10,000 B.C., this began to change. A genetic mutation appeared, somewhere near modern-day Turkey, that jammed the lactase-production gene permanently in the “on” position. The original mutant was probably a male who passed the gene on to his children. People carrying the mutation could drink milk their entire lives. Genomic analyses have shown that within a few thousand years, at a rate that evolutionary biologists had thought impossibly rapid, this mutation spread throughout Eurasia, to Great Britain, Scandinavia, the Mediterranean, India and all points in between, stopping only at the Himalayas. Independently, other mutations for lactose tolerance arose in Africa and the Middle East, though not in the Americas, Australia, or the Far East.
In an evolutionary eye-blink, 80 percent of Europeans became milk-drinkers; in somepopulations, the proportion is close to 100 percent. (Though globally, lactose intolerance is the norm; around two-thirds of humans cannot drink milk in adulthood.) The speed of this transformation is one of the weirder mysteries in the story of human evolution, more so because it’s not clear why anybody needed the mutation to begin with. Through their cleverness, our lactose-intolerant forebears had already found a way to consume dairy without getting sick, irrespective of genetics.
Mark Thomas, an evolutionary geneticist at University College London, points out that in modern-day Turkey, where the mutation seems to have arisen, the warm climate causes fresh milk to rapidly change its composition. “If you milk a cow in the morning,” he says, “by lunchtime it’s yogurt.”
Yogurt has plenty of benefits to confer, among them large testicles, swagger, and glossy fur—at least if you’re a mouse—but most salient to our ancestors was that the fermentation process that transforms milk into yogurt consumes lactose, which is a sugar. This is why many lactose-intolerant people can eat yogurt without difficulty. As milk ascends what Thomas calls the “fermentation ladder,” which begins with yogurt and culminates with virtually lactose-free hard cheeses, ever more lactose is fermented out. “If you’re at a party and someone says, ‘Oh, I can’t eat that—I’m lactose intolerant,’ ” he says, “you can tell them to shut up and eat the Parmigiano.”
Analysis of potsherds from Eurasia and parts of Africa have shown that humans were fermenting the lactose out of dairy for thousands of years before lactose tolerance was widespread. Here is the heart of the mystery: If we could consume dairy by simply letting it sit around for a few hours or days, it doesn’t appear to make much sense for evolution to have propagated the lactose-tolerance mutation at all, much less as vigorously as it did. Culture had already found a way around our biology. Various ideas are being kicked around to explain why natural selection promoted milk-drinking, but evolutionary biologists are still puzzled.
“I’ve probably worked more on the evolution of lactose tolerance than anyone in the world,” says Thomas. “I can give you a bunch of informed and sensible suggestions about why it’s such an advantage, but we just don’t know. It’s a ridiculously high selection differential, just insane, for the last several thousand years.”
A “high selection differential” is something of a Darwinian euphemism. It means that those who couldn’t drink milk were apt to die before they could reproduce. At best they were having fewer, sicklier children. That kind of life-or-death selection differential seems necessary to explain the speed with which the mutation swept across Eurasia and spread even faster in Africa. The unfit must have been taking their lactose-intolerant genomes to the grave.
Milk, by itself, somehow saved lives. This is odd, because milk is just food, just one source of nutrients and calories among many others. It’s not medicine. But there was a time in human history when our diet and environment conspired to create conditions that mimicked those of a disease epidemic. Milk, in such circumstances, may well have performed the function of a life-saving drug.
There are no written records from the period when humans invented agriculture, but if there were, they would tell a tale of woe. Agriculture, in Jared Diamond’s phrase, was the “worst mistake in human history.” The previous system of nourishment—hunting and gathering—had all but guaranteed a healthy diet, as it was defined by variety. But it made us a rootless species of nomads. Agriculture offered stability. It also transformed nature into a machine for cranking out human beings, though there was a cost. Once humans began to rely on the few crops that we knew how to grow reliably, our collective health collapsed. The remains of the first Neolithic farmers show clear signs of dramatic tooth decay, anemia, and low bone-density. Average height dropped by about 5 inches, while infant mortality rose. Diseases of deficiency like scurvy, rickets, beriberi, and pellagra were serious problems that would have been totally perplexing. We are still reeling from the change: Heart disease, diabetes, alcoholism, celiac disease, and perhaps even acne are direct results of the switch to agriculture.
Meanwhile, agriculture’s alter ego, civilization, was forcing people for the first time to live in cities, which were perfect environments for the rapid spread of infectious disease. No one living through these tribulations would have had any idea that things had ever been, or could be, different. Pestilence was the water we swam in for millennia.
It was in these horrendous conditions that the lactose tolerance mutation took hold. Reconstructed migration patterns make it clear that the wave of lactose tolerance that washed over Eurasia was carried by later generations of farmers who were healthier than their milk-abstaining neighbors. Everywhere that agriculture and civilization went, lactose tolerance came along. Agriculture-plus-dairying became the backbone of Western civilization.
But it’s hard to know with any kind of certainty why milk was so beneficial. It may have been the case that milk provided nutrients that weren’t present in the first wave of domesticated crops. An early, probably incorrect, hypothesis sought to link lactose tolerance to vitamin D and calcium deficiencies. The lactose-intolerant MIT geneticist Pardis Sabeti believes that milk boosted women’s fat stores and thus their fertility, contributing directly to Darwinian fitness, though she and others allow that milk’s highest value to subsisting Homo sapiens may have been that it provided fresh drinking water: A stream or pond might look clean yet harbor dangerous pathogens, while the milk coming out of a healthy-looking goat is likely to be healthy, too.
Each of these hypotheses makes rough-and-ready sense, but not even their creators find them totally convincing. “The drinking-water argument works in Africa, but not so much in Europe,” says Thomas. He favors the idea that milk supplemented food supplies. “If your crops failed and you couldn’t drink milk, you were dead,” he says. “But none of the explanations that are out there are sufficient.”
The plot is still fuzzy, but we know a few things: The rise of civilization coincided with a strange twist in our evolutionary history. We became, in the coinage of one paleoanthropologist, “mampires” who feed on the fluids of other animals. Western civilization, which is twinned with agriculture, seems to have required milk to begin functioning. No one can say why. We know much less than we think about why we eat what we do. The puzzle is not merely academic. If we knew more, we might learn something about why our relationship to food can be so strange.