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.
For the time being, the mythical version of the story isn't so bad. In the Garden, Adam and Eve were gatherers, collecting fruits as they fell from the tree. Cain the farmer and Abel the pastoralist represented two paths into the future: agriculture and civilization versus animal husbandry and nomadism. Cain offered God his cultivated fruits and vegetables, Abel an animal sacrifice that Flavius Josephus tells us was milk. Agriculture, in its earliest form, brought disease, deformation, and death, so God rejected it for the milk from Abel's flocks. Cain grew enraged and, being your prototypically amoral city-dweller, did his brother in. God cursed Cain with exile, commanding him to wander the earth like the pastoralist brother he'd killed. Cain and agriculture ultimately won the day—humans settled into cities sustained by farms—but only by becoming a little like Abel. And civilization moved forward.