For a long time, scientists thought that all mammal brains were made of the same material. But if that were true, then mammals with the same brain size should have similar cognitive abilities, and those with larger brains would be the most cognitively able. That simply isn’t true.

The brains of chimps and cows weigh about the same – 400 grams, and yet chimps are more cognitively able than cows. Elephants and whales have larger brains than that of the human, and yet humans conquer and train them.

Neuroscientist Suzana Herculano-Houzel has spent the last decade studying the human brain. She started out by trying to get an accurate count of the brain neurons. She came up with the idea of “brain soup’ which was made by dissolving donated brains, destroying the cell membranes but leaving the nuclei intact. This made a homogenous mixture that allowed her to count the neurons in a sample. As it turns out, the human brain really has 86 billion neurons, instead of 100 billion frequently quoted.

Human brains reveal a few oddities. For instance, Herculano-Houzel says, we have a larger cerebral cortex than it seems like we should have, given the size of our bodies. Meanwhile, human brains use a tremendous amount of energy. While the brain is 2% of the body, it uses 25% of the calories we need to function each day. Why should the rules of evolution not apply to humans?

Why would we have a much larger brain than the great ape that has a much larger body? The answer comes down to the extreme energy cost of the primate brain. For example, Guerrilla’s body size is approximately 75 kg and its brain contains 30 billion neurons. Guerrillas spend eight and a half hours feeding daily in order to keep the brain and the body function. Assuming nine hours a day is the practical limit for food hunting, for a primate with 53 billion neurons, it has to give up body size (to 25 kg) in order to provide enough energy for both the body and brain. For our 86 billion neurons and 60 to 70 kg body weight, we’d have to spend more than nine hours everyday on feeding. If we eat like primates, we shouldn’t be here. “There appears to be some kind of evolutionary trade-off between the size of the brain and the size of the body — there’s a metabolic limitation, and primates can only consume enough calories to support one or the other.”

So we humans either have to consume more food or get more out of the food we eat. That’s exactly what happened 1.5 million years ago when our ancestors invented cooking. “Cooking is essentially the act of using fire to pre-digest food, and thus to get more energy out of the same amount of food. In fact, cooking food makes it yield about three times as many calories. This is what allowed our brains to get bigger in a relatively short period of time while remaining a primate brain. Cooking also allowed us to support this large cerebral cortex, which in turn supports complex thought.” So our brain, as cognitively able and fascinating as it is, is still a primate brain, but larger. “It’s a great reminder of our place in evolution,” says Herculano-Houzel.

Our dog Joey is a 9-pound Maltese. His mind is on food constantly. Right after we give him his dinner on the deck, he comes in looking for food again. Now I wonder about his brain size and the number of neurons in that little head. For us humans, the same food that has fueled our brain growth is starting to become a problem. These days we can get all the calories we need for an entire day by eating one meal at a fast food joint. Obesity, diabetes, fatty livers are all diet related diseases. If we are smarter than our primate friends, we need to develop self-control and healthy diet to keep our body growth in line with the brain growth.

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