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.
I have recently wondered if the invention of fire also provided a tangential stimulus for increased brain complexity. Fire would have provided light during otherwise dark hours. It would have allowed humans to interact socially at night but would not likely have helped much in nighttime hunting or gathering. It would have enabled the development of storytelling, acting, music, and assorted crafts that beforehand would have not been possible because of lack of light. Those skills, which are competitive, have become valuable in traditional societies and their adepts by and large get respect and the social status – and sex and children – that come with it. So it’s all self-reinforcing: the heat from the fire allows more efficient food processing and bigger brains, light from the fire leads to activities which reinforce complex mental activities which, in turn, lead to higher status and more gene tranfer. I’m sure it wasn’t quite as simple as that, but it could have been part of the bootstrap that got us here.
Why is it that an increased caloric intake resulted in a bigger brain rather than obese hominids? Why does a high calorie intake today just make us fatter, not smarter?
Size of the brain is directly proportional to the number of neurons in the brain. Compared to the body size humans have got bigger head than gorillas or for that matter any primates. We know pretty well that one gets drowsy after a heavy meal. This is because more blood is sent to the intestines to digest and less blood to the brain causing drowsiness. If we apply the same rule to animals which eat raw uncooked food, the apportioning of blood to their brains will always be little compared to the blood supply to their guts to digest their raw meal. Have we not come across a snake which engulfs a full animal and remain silent in hibernated state for days to digest the same. Their blood supply to the brain will always be in short supply. Hence their size of the head will remain comparatively smaller and so are neurons in the brain. Humans can only take three times or more their meal in a day. Other animals are either diurnal or nocturnal taking just one time a day excepting some exceptions. This explains how man`s cooked food is responsible for development of brain neurons with constant higher supply of blood compared with animals. Calorie intake will necessarily make one obese but the point of importance in this article is the proportion of calories sent to the brain to develop neurons better than over sized gorillas. Thank You.
The idea that cooking enabled our ancestors to consume more and higher-quality calories is very plausible. It is convincingly explored in Richard Wrangham’s excellent 2009 book Catching Fire, which I recommend to anyone who has an interest in the topic. (I am very surprised this article does not mention Wrangham.)
The only problem is that there is little or no evidence that those ancestors actually used fire back at the time their brains started expanding so substantially. Perhaps such evidence will eventually be discovered, but for now its lack is a major stumbling block for the cooking hypothesis.