Human brain is like one of those good old times luxury Cadillac’s of the 50’s and 60’s. It is the most cognitively able of the animal kingdom, but it is large and expensive to maintain. With 86 billion neurons, it consumes an outstanding 20% of the total body energy budget despite representing only 2% of body mass because of the increased metabolic need of its neurons.
Humans are not the largest living primates. Gorillas can grow to be three times larger than humans, but their brains amount to about only one-third of the size of the human brain, making our comparatively larger brain size appear an oddity. Why are not the largest primates those endowed with the largest brains as well, if brain and body size are usually well-correlated across species?
Neuroscientist Suzana Herculano-Houzel and her graduate student Karina Fonseca-Azevedo think they have a plausible answer. They have studied how much it costs to have a large brain, and they have reached the conclusion that developing a very large body and a very large brain might have been mutually excluding strategies, very likely due to a simple metabolic constraint(1,2).
Growing a large body comes at a cost. Large animals require less energy per unit of body weight because total metabolic requirements, on average, scale with body mass raised to an exponent of ∼3/4 (Kleiber’s law). Adding neurons to the brain also comes at a sizeable cost; and in this case, the energetic cost of the brain is a linear function of its number of neurons, which amounts to 6 kCal/day per billion neurons.
Suzana and Karina also calculate that, given the existent raw diet of nonhuman primates, the average energy intake per hour increases with body mass varying from 8.9 kCal/h in the owl monkey or 10.3 kCal/h in the marmoset to 334.7 kCal/h in the gorilla. The number of hours available for feeding together with the low caloric yield of raw foods imposes a trade-off between body size and number of brain neurons. The larger the number of neurons, the higher is the total caloric cost of the brain, and therefore the more time required to be spent feeding to support the brain alone, and feeding can be very time-consuming!
Such a trade-off 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.
The additional number of neurons necessary (122 billion) for a gorilla to have a brain corresponding to 2% 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% of 12 h of day in feeding.
Based on the estimated body masses and numbers of brain neurons of extinct hominids, it is not difficult to estimate that Australopithecus afarensis, Paranthropus boisei, and Homo habilis, with 30 to 40 billion neurons, would have had similar feeding requirements of more than 7 hour/day if feeding on a raw diet equivalent to that of extant great apes. Homo erectus, with a predicted 62 billion neurons would have had to spend more than 8 hour/day feeding on raw foods; and Homo heidelbergensis, Homo neanderthalensis, and Homo sapiens would have had to spend consistently more than 9 hour/day feeding to afford their 76 to 86 billion neurons, longer than extant great apes spend feeding.
How did humans escape this apparently inescapable physiological curse? Very simple: they learnt to cook!
The idea was first proposed by Richard Wrangham who suggested that brain began to expand rapidly 1.6 million to 1.8 million years ago in our ancestor Homo erectus, because this early human learned how to roast meat and tuberous root vegetables over a fire. Cooking pre-digests the food, making it easier and more efficient for our guts to absorb calories more rapidly
Besides increasing the caloric yield and making previous metabolic limitations irrelevant, cooking would have also increased the time available for social, more complex more cognitively demanding “Machiavellian” activities, which in turn would impose a positive pressure for increased numbers of neurons, now affordable by the new diet.
Fast forward and here we are. It’s Christmas time and your grandma is complaining that she spends the whole day in the kitchen… Go tell her this funny story and convince her that cooking actually frees time!
(1) Karina Fonseca-Azevedo and Suzana Herculano-Houzel, “Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution”, PNAS vol. 109; no. 45; November 6, 2012
(2) Suzana first developed a method (isotropic fractionator) that allows a rapid and reliable quantification of the numbers of cells in a brain:
It took me a couple of months to make peace with this idea that I was going to take somebody’s brain or an animal’s brain and turn it into soup. … (Suzana Herculano-Houzel quoted by TED)
Using this method she was able to show that neuronal scaling (the proportionality between brain mass and number of brain neurons) is different across different brain structures in mammalian orders, and that, very likely, the cognitive abilities of the human brain are exclusively due to its large number of neurons. You can see the whole story here explained by Suzana herself.
Featured Image: Gorilla, Wildlife by Ruth Read