A new study has calculated the energetic cost of growing a bigger brain. If humans had been eating a raw food diet exclusively, they would have had to spend more than 9 hours a day eating in order to get enough energy from unprocessed raw food alone to support their large brains.
The scientists published their findings in the journal Proceedings of the National Academy of Sciences. Human ancestors managed to get enough energy to grow brains that have three times as many neurons as gorillas, which was only possible with the invention of cooking.
There aren’t enough hours in the day to build up enough energy to sustain the building of such a large brain, states Suzana Herculano-Houzel, neuroscientist at the Federal University of Rio de Janeiro, Brazil, who co-wrote the report. Humans have 86 billion neurons on average, and gorillas have 33 billion neurons. These extra neurons come at a price in energy consumption. The brain consumes 20% of the human body’s energy when resting, compared to 9% in other primates.
The brain began to expand rapidly 1.6 to 1.8 million years ago in Homo erectus because they learned how to roast meat and vegetables over fires. Cooking effectively predigested food, making it easier and more efficient for the gut to absorb calories more rapidly. Lab studies in rodents and pythons have shown that these animals grow up bigger and faster when eating cooked food, since it takes less energy to digest it.
Herculano-Houzel decided to see if a diet of raw food would put limits on how a large a primate’s brain or body could grow. They counted the number of neurons in the brains of 13 species of primates and 30 species of mammals, finding that the size of the brain is directly linked to the number of neurons in the brain, and this correlates to the amount of energy needed to feed the brain.
They calculated how many hours per day it would take for various primates to eat enough calories to fuel their brains. It would take 8.8 hours for gorillas, 7.8 hours for orangutans, 7.3 hours for chimps and 9.3 hours for humans.
The data shows that there is an upper limit on how much energy primates can get from an unprocessed raw diet. Naturally, an ape’s diet in the wild differs significantly from a human raw food diet, in which humans get sufficient calories from processing raw food in blenders and adding protein and other nutrients to get the energy they need. In the wild, other apes can’t evolve bigger brains unless they reduce their body size. Cooking allowed humans to circumvent the limitation on how much they could eat in a day.
Reference: “Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution” by Karina Fonseca-Azevedo and Suzana Herculano-Houzel, 3 October 2012, Proceedings of the National Academy of Sciences.