You aren’t what you eat, exactly. But over many generations, what we eat does shape our evolutionary path. “Diet,” says anthropologist John Hawks, of the University of Wisconsin-Madison, “has been a fundamental story throughout our evolutionary history. Over the last million years there have been changes in human anatomy, teeth and the skull, that we think are probably related to changes in diet.”
As our evolution continues, the crucial role of diet hasn’t gone away. Genetic studies show that humans are still evolving, with evidence of natural selection pressures on genes impacting everything from Alzheimer’s disease to skin color to menstruation age. And what we eat today will influence the direction we will take tomorrow.
When mammals are young, they produce an enzyme called lactase to help digest the sugary lactose found in their mothers’ milk. But once most mammals come of age, milk disappears from the menu. That means enzymes to digest it are no longer needed, so adult mammals typically stop producing them.
Thanks to recent evolution, however, some humans defy this trend.
Around two-thirds of adult humans are lactose intolerant or have reduced lactose tolerance after infancy. But tolerance varies dramatically depending on geography. Among some East Asian communities, intolerance can reach 90 percent; people of West African, Arab, Greek, Jewish and Italian descent are also especially prone to lactose intolerance.
Northern Europeans, on the other hand, seem to love their lactose—95 percent of them are tolerant, meaning they continue to produce lactase as adults. And those numbers are increasing. “In at least different five cases, populations have tweaked the gene responsible for digesting that sugar so that it remains active in adults,” Hawks says, noting it is most common among peoples in Europe, the Middle East and East Africa.
Ancient DNA shows how recent this adult lactose tolerance is, in evolutionary terms. Twenty-thousand years ago, it was non-existent. Today, about one-third of all adults have tolerance.
That lightning-fast evolutionary change suggests that direct milk consumption must have provided a serious survival advantage over peoples who had to ferment dairy into yogurt or cheese. During fermentation, bacteria break down milk sugars including lactase, turning them into acids and easing digestion for those with lactose intolerance. Gone with those sugars, however, is a good chunk of the food’s caloric content.
Hawks explains why being able to digest milk would have been such a boon in the past: “You’re in a nutrition limited environment, except you have cattle, or sheep, or goats, or camels, and that gives you access to a high energy food that infants can digest but adults can’t,” he says. “What it does is allow people to get 30 percent more calories out of milk, and you don’t have the digestive issues that come from milk consumption.”
A recent genetic study found that adult lactose tolerance was less common in Roman Britain than today, meaning its evolution has continued throughout Europe’s recorded history.
These days, many humans have access to plentiful alternative foods as well as lactose-free milk or lactase pills that help them digest regular dairy. In other words, we can circumvent some impacts of natural selection. That means traits like lactose tolerance might not have the same direct impacts on survival or reproduction that they once did—at least in some parts of the world.
“As far as we know, it makes no difference to your survival and reproduction in Sweden if you can digest milk or not. If you’re eating out of a supermarket (your dairy tolerance doesn’t affect your survival). But it still makes a difference in East Africa,” Hawks says.
Wheat, Starch and Alcohol
These days, it isn’t uncommon to find an entire grocery store aisle devoted to gluten-free cookies, bread and crackers. Yet trouble digesting gluten—the main protein found in wheat—is another relatively recent snag in human evolution. Humans didn’t start storing and eating grains regularly until around 20,000 years ago, and wheat domestication didn’t begin in earnest until about 10,000 years ago.