Brains are calorically demanding organs. Our distant ancestors had small ones. Australopithecus afarensis, for example, who lived some three million years ago, had a cranial capacity of about four hundred cubic centimetres, which is roughly the same as a chimpanzee’s. Modern humans have a cranial capacity of about thirteen hundred cubic centimetres. How, as their brains got bigger, did our forebears keep them running? According to what’s known as the Expensive Tissue Hypothesis, early humans compensated for the energy used in their heads by cutting back on the energy used in their guts; as man’s cranium grew, his digestive tract shrank. This forced him to obtain more energy-dense foods than his fellow-primates were subsisting on, which put a premium on adding further brain power. The result of this self-reinforcing process was a strong taste for foods that are high in calories and easy to digest; just as it is natural for gorillas to love leaves, it is natural for people to love funnel cakes.
Although no one really knows what life was like in the Pleistocene, it seems reasonable to assume that early humans lived, as it were, hand to mouth. In good times, they needed to stockpile food for use in hard times, but the only place they had to store it was on themselves. Body fat is energy-rich and at the same time lightweight: when the water is taken out, a gram of fat contains 9.4 kilocalories, compared with 4.3 kilocalories for a gram of protein, and when the water is left in, as it is on the human belly, a gram of fat still contains 9.1 kilocalories, while a gram of protein has just 1.2. As a consequence, a person with a genetic knack for storing fat would have had a competitive advantage. Power and Schulkin are both researchers at the American College of Obstetricians and Gynecologists, and they argue that this advantage would have been especially strong for women. Human infants are unusually portly; among mammals, only hooded seals have a higher percentage of body fat at birth. (Presumably, babies need the extra reserves to fuel their oversized brains.) Tellingly, humans, unlike most other animals, have no set season of fertility. Instead, ovulation is tied to a woman’s fat stores: those who are very thin simply fail to menstruate.
Of course, for early humans putting on too many pounds would have been a significant disadvantage; it’s hard to chase down a mastodon or track through a forest if you’re tubby. Thus, there would appear to be a Darwinian argument against obesity as well. Power and Schulkin get around this problem by noting that, as a practical matter, opportunities for eating too much were limited. Austerity was the rule for hunter-gatherer societies, and that didn’t change when people started to form farming communities, some ten thousand years ago. In fact, human remains from many parts of the world show that early agriculturalists were less well fed than their Paleolithic forebears; their skeletons are several inches shorter and often show signs of nutrition-related diseases, like anemia. Genes that controlled weight gain wouldn’t have been selected for because they simply weren’t needed.
In America today, by contrast, obtaining calories is very nearly effortless …