I probably spend about an hour of every day feeding. I can do this because of cooked food. This morning I had a burrito utilizing cooked grains, eggs, cheese, and spinach. It took about 15 minutes to house, and I’m a food-picker. An ape, however, might spend as much as eight hours a day feeding. This is partially due to the ape’s raw diet — a raw diet means the foods you eat are drastically reduced in caloric density and getting a survivable number of calories into your hairy ape body means spending a whole lot of time munching on leaves.
Every other primate eats a raw diet as well, and shows a very different relationship between brain size and body size compared to us. Simply, humans have the largest among primates, yet we aren’t the largest primates. Our brains and bodies have evolved at different rates than those of our primate ancestors.
This last part is the result at least in part of our diets, according to a paper just out in PNAS. Humans are able to consume nutrient-dense foods in a short amount of time, which means we have nutrition available to support larger brains. Why brains, specifically? Because brains require more energy than other body parts. The brain uses “20 percent of the total body resting metabolic rate, even though it represents only 2 percent of total body mass,” explains the paper, So it uses about 20 percent of the calories our body consumes, even though it’s tiny. It’s the third-most energy-expensive organ in the body. (Our brains average about three times the size of primate brains.)
“For a gorilla to have a brain corresponding to 2 percent of its body mass would cost the animal an extra 733 kCal, which we estimate would require another 2 h 12 min of feeding—when a gorilla already spends as much as 80 percent of 12 h of day in feeding,” the paper reads. And it’s highly unlikely that the gorilla has an extra couple of hours to spend eating, so it’s thus limited to those calories it can consume in about eight hours. Unless it learns to cook, in which case the gorilla might evolve into something like human. In science-speak:
By using a simple model of scaling of energetic intake and expenditure with increasing MBD and number of brain neurons, here we demonstrate that, for primates feeding exclusively on a raw diet, metabolism is indeed a hysiologically relevant limiting factor in evolution such that a tradeoff between MBD and number of brain neurons is imposed. This tradeoff is particularly clear in animals the size of great apes, as sustaining an MBD of ∼50 kg already requires feeding approximately 8 h/d, beyond which increasing both MBD and the number of brain neurons becomes rapidly dangerous. Such a tradeoff provides a simple explanation for why great apes have the smallest relative brain sizes among primates: on their diet based on raw foods, and given that, per gram of tissue, larger brains came at a higher metabolic cost than larger bodies, the largest great apes cannot afford both a large body and a larger number of neurons.
Of course, caloric density is also a curse and we should all be eating a lot more greens, like our dim friends the apes, and less hyper-dense things like sugar. And, I think this makes its own point about fashionable “ancestral” diets, so I’ll just let it go there.