How many neurons make a human brain? For years, the answer has been (give or take) 100 billion. But neuroscientist Suzana Herculano-Houzel decided to count them herself. Her research approach involved dissolving four human brains (donated to science) into a homogeneous mixture — in her lab at the Institute of Biomedical Sciences in Rio de Janeiro, they call it “brain soup.” She then took a sample of the mix, counted the number of cell nuclei belonging to neurons, and scaled that up. Result: the human brain has about 86 billion neurons, 14 billion fewer than assumed — but intriguingly, far more than other animals, relative to brain size.
She suggests that it was the invention of cooking by our ancestors — which makes food yield much more metabolic energy — that allowed humans to develop the largest primate brain. She’s now working on elephant and whale brains to test her hypothesis.
Brain size can no longer be considered a proxy for numbers of neurons in the brain across species, contrary to what has been common practice so far under the assumption that different brains followed the same scaling rules (reviewed in Herculano-Houzel, 2011, 2011 and 2012). By comparing rodents, more rodents, primates, more primates, and insectivores, we showed that each order follows its own neuronal scaling rules – that is, has its own relationship between brain size and number of neurons. In contrast, the relationship between brain structure size and number of other cells (glial and endothelial cells) is shared across all orders and brain structures analyzed so far.
The metabolic cost of being human – and how cooking got us here
The human brain costs about 500 kCal per day, which is 20-25% of the energy consumed by the entire body. We have shown, however, that this seemingly extraordinary metabolic cost is actually just the expected amount of calories for the number of neurons in the human brain, given our finding that the metabolic cost of a brain is a simple linear function of its number of neurons, irrespective of brain size, at an average cost of 6 kCal per billion neurons per day.
The large metabolic cost of neurons shines new light on the humans vs. great apes paradox: We propose that great apes cannot afford a brain that is any bigger than it already is, due to a metabolic limitation imposed by their diet, which doesn’t offer enough calories to support both a huge body and the huge number of neurons that a larger brain would have. This metabolic limitation must also have applied to our ancestors, who we propose that were also limited by their diet to having about the same number of neurons that modern great apes have. We suggest that the evolution of modern humans, with the very fast increase in brain size in less than 2 million years since Homo erectus, was made possible by the invention of cooking, which made more calories available in less time per day, thus allowing large brains to go from being a major liability to being a major asset, subject to strong positive selection in evolution.