Our world is killing us. We feed on sugars and carbs that give us cancer and diabetes, and in everything else, from our love-lives to our exercise, we yearn for a better, more wholesome way to live. How now shall the caveman live?

Paleo dieters, exercisers, lovers, and health nuts assert we were made for something better than our space age culture. Their rationale: human evolution has distanced us from our roots. There was a time when we were more in tune with our environment. A golden age of human health and fitness. Alas, like Adam and Eve banished from the Garden, we have been cut adrift to become the prey of modernity. Or have we?

Marlene Zuk, a professor of ecology, evolution, and behavior at the University of Minnesota, calls this caveman craze a fantasy. In Paleofantasy: What Evolution Really Tells Us About Sex, Diet, and How We Live, she attempts a takedown of the fad.

Zuk's main argument is that evolution actually happens pretty quickly. We are not stuck in Stone Age bodies. Our recently evolved ability to digest milk is a perfect example. Paleo diet advocates shun dairy products, asserting that humans never consumed dairy before animals were domesticated. It is true that most humans (about 65 percent), like all mammals, cannot digest milk into adulthood. A specific gene has been linked to shutting off the production of lactase, the enzyme that allows us to digest milk. But some humans, specifically those with a shepherding heritage, have evolved a mutation in that gene, allowing them to drink milk and eat yogurt and cheese (or ice cream!) for a lifetime. Because this change seems to have happened in the last 7,000 years, scientists have been able to study the way this milk-friendly gene spread, analyzing the DNA of the skeletons of our recent ancestors. Yes, our bodies continue to evolve. Far from a degeneration of the purity of caveman digestion, our ability to drink milkshakes and eat Gruyere is a recent development that, by chance, has helped us survive.

Zuk applies this line of reasoning to barefoot running, co-sleeping with infants, polygyny, and a number of other arguments made by paleo advocates about how we are supposed to live. She marshals plenty of evidence that modern behaviors are adaptive—there's no need to buy a pair of foot gloves if your shoes work just fine. But in many cases, it doesn't really matter. Evolution has no point, she insists: "[I]t is not goal-oriented. It is not as if we were on a predestined path toward enlightenment when agriculture suddenly threw a plow into the works."

Time and again, Zuk shows, evolution can provide insight into our behavior, but evolution doesn't make value judgments. It never tells us how we should live. Which makes me wonder why paleo dieters and their antagonists insist on there being a proper way to live. Given the purportedly aimless tinkering of evolution, why would anyone feel so profoundly at odds with their world that they insist on carrying large rocks as a form of exercise and eating several pounds of beef daily? If there's no should in evolution, why do we feel there should be?

Rob Moll is editor at large for Christianity Today magazine and author of the forthcoming book What Your Body Knows about God. His book The Art of Dying was published by InterVarsity Press in 2010.

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