“In larger rodent brains, the average size of the neuron increases, so the brain inflates very rapidly and gains size much faster than it gains neurons. But primate brains gain neurons without the average neuron becoming any larger,which is a very economical way to add neurons to your brain. The result is that a primate brain will always have more neurons than a rodent brain of the same size, and the larger the brain, the larger this difference will be.
Well, what about our brain then? We found that we have, on average, 86 billion neurons,16 billion of which are in the cerebral cortex, and if you consider that the cerebral cortex is the seat of functions like awareness and logical and abstract reasoning, and that 16 billion is the most neurons that any cortex has, I think this is the simplest explanation for our remarkable cognitive abilities.”
Suzana Herculano-Houzel
However, this increase in size came with [its] own problems. Neurons require energy — our brains alone demand 25% of our bodily energy — not such a problem with our current selection of energy-rich food sources, but back when we were traversing unknown lands as hunter gatherers, spending long hours chewing raw foods, that 25% was hard to come by.
“…a primate that eats eight hours per day can afford at most 53 billion neurons,but then its body cannot be any bigger than 25 kilos. To weigh any more than that, it has to give up neurons. So it’s either a large body or a large number of neurons. When you eat like a primate, you can’t afford both.”
Yuval Noah Harari also talks of the cognitive revolution that begun around 70,000 years ago in his book Sapiens: A Brief History of Humankind:
“Archaic humans paid for their large brains in two ways. Firstly, they spent more time in search of food. Secondly, their muscles atrophied. Like a government diverting money from defence to education, humans diverted energy from biceps to neurons. It’s hardly a foregone conclusion that this is a good strategy for survival on the savannah. A chimpanzee can’t win an argument with a Homo sapiens, but the ape can rip the man apart like a rag doll.”
[…]
“The advent of cooking enabled humans to eat more kinds of food, to devote less time to eating, and to make do with smaller teeth and shorter intestines. Some scholars believe there is a direct link between the advent of cooking, the shortening of the human intestinal track, and the growth of the human brain. Since long intestines and large brains are both massive energy consumers, it’s hard to have both. By shortening the intestines and decreasing their energy consumption, cooking inadvertently opened the way to the jumbo brains of Neanderthals and Sapiens.”
It’s also interesting that something that gave rise to such an explosion in cognitive abilities would later lead to obesity and heart problems — but that’s a discussion for another day.
