Ah, the traditions of Thanksgiving. The satisfaction of appreciating little things with family and friends. The aroma of roasting turkey. And the inability to take just one helping of anything! We asked Bill Leonard, Abraham Harris Professor of Anthropology at Northwestern and a leading authority on the role nutrition plays in evolution, about this last custom.
You study biological anthropology. What is that?
It's the study of the origins and maintenance of human biological diversity, which is how our species varies over time and geographical distance. In other words, we study the evolution of the human species, as well as how human diversity is promoted and maintained today.
What’s really interesting about human biological diversity is that even though human beings are highly variable when it comes to things like height, weight, and skin color, we are extremely similar genetically. If you take any two human beings from opposite sides of the world, they will, on average, have less genetic variation than two chimpanzees from the forests of Africa. How do you reconcile this apparent contradiction of high physical variation, yet low genetic diversity? The answers partly reflect the fact that humans, more so than other species, use social and cultural strategies to create and adapt to their environments. That’s what we study—evolution as a “bio-cultural” process.
You’re particularly interested in nutrition in this context. Why?
Because diet showcases the interface of biology and culture.
Take something as seemingly simple as drinking milk. As infants, all mammals can digest milk, but this ability is lost after weaning. Among humans, some groups retain this ability as they grow up. As it turns out, the ability is more common in humans from populations with a history of dairy production. In cultures that raise milk-producing animals, we find more people with the genes that allow them to digest dairy. For example, we even see these genes begin to appear in central Europe at about the same time as they began domesticating cows and using vessels to carry milk. That is, cultural innovations in food production favored genes that allowed for milk digestion in adulthood.
Basically, the story of human nutrition is the story of the interplay of environment, human culture, and genetics.
Sounds a little like the story of Thanksgiving, where we celebrate that food and friends meant survival. Care to comment?
Yes, human subsistence societies, or cultures that live off the land, experience times of feast as well as famine. The ability to store resources during times of plenty allowed for survival during times of famine.
Individuals who had this ability were more likely to thrive in conditions of feast or famine, and so would be more likely to reproduce and pass along their genes. We call that being favored by natural selection, a prime force that promotes evolutionary change.
What traits are well suited to survival on the feast or famine roller coaster?
Natural selection favored those individuals who had a taste for high-fat, high-calorie foods, and those who were good at storing excess calories as fat reserves to use in the lean times. Yet in our modern world, the “lean times” occur more rarely, but our cravings for high-calorie foods and our ability to efficiently store those excess calories remain. This is why we tend to put on weight, especially in climates like ours with cold winters.
I’m getting discouraged. Based on your understanding, is it possible to get through winter without gaining a few pounds?
Absolutely! The most encouraging data come from my research comparing diet and exercise levels in traditional societies with those in America. In the last 40 years, the obesity boom in America has often been blamed largely on availability of junk food. While this is certainly a contributing factor, the available data show the average increase in calories is not enough to account for the increase in weight. This suggests that we also need to look at the other half of the nutrition equation—exercise and energy expenditure.
How much activity patterns and energy expenditure levels have changed over the last 40 years is hard to gauge, since there is limited comparative data. We compared current activity levels of individuals in traditional societies with their American counterparts. Adjusted for differences in body mass, women in traditional societies only burned about 200-300 more calories per day than American women. The men burned about 400-500 more calories than American men. They achieve this through a slow and steady pace of doing small things like walking instead of driving. They’re not running around at breakneck speeds. Adopting small changes, like taking the stairs or riding a bike to work, make a big difference.