And they took data collected by Callum Ross and others on the number of hours that these species spend eating.įrom these figures, they calculated the maximum number of neurons that each species can afford, given different amounts of feeding time. They worked out the number of neurons in various primate brains, by painstakingly inspecting their own samples. They collected figures on the average brain and body sizes of many primates from past papers. That, in turn, fuelled more neurons and larger brains.įonseca-Azevedo and Herculano-Houzel developed their argument using data on 17 species of primates, from the tiny common marmoset to the huge gorilla. These early chefs could gain more energy from the same amount of eating time. Cooked food offers more calories than raw food, and is easier to chew and digest. Our ancestors overcame this constraint when they learned how to cook. Their energy budget simply wouldn’t balance.
An ape can’t evolve a brain as big as a human’s, while still eating like an ape. To do so, they would need to spend an implausible amount of time on foraging and feeding. On their typical diets of raw foods, great apes can’t afford to fuel more neurons than they already have. And bigger brains, which have more neurons, consume more fuel. Each gram of brain uses up more energy than each gram of body. Their argument is simple: brains demand exceptional amounts of energy. (Yes, humans are apes for this piece, I am using “apes” to mean “apes other than us”). But Karina Fonseca-Azevedo and Suzana Herculano-Houzel from the Federal University of Rio de Janeiro have turned that question on its head-they want to know why other apes haven’t evolved bigger brains. Many scientists ask why our brains have become so big. And the gorilla, which can grow to be three times bigger than us, has a smaller brain than we do. Our close relative, the chimpanzee, has a brain that’s just twice as big as expected for its size. But the human brain is seven times bigger than that of other similarly sized animals. As animals get bigger, so do their brains.
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