Humans are far and away the smartest animal on the planet. We’ve always known this, but we’s struggled with how to prove it.

We don’t have the largest brains. That would be the sperm whale.

Well, then maybe we have the largest brain/body size ratio, scientists must have thought. Wrong again! that honor seems to go to small ants.

Well then maybe we have the largest brain/brain expected for our body size ratio? Bingo! What most distinguishes the human brain from that of all other animals is that it’s 7.6 times larger than statistically expected for a mammal of our size and probably well over 8 times larger than for an animal of our size.

No other creature comes close.

Our huge encephalization quotient allowed humans to conquer the World, accumulating more biomass than any other species except for cattle whose body-mass we’ve deliberately grown as our food supply.

Of course there’s far more to intelligence than just brain size adjusted for body size, but when comparing vastly different species who differ enormously on this metric, it seems to be a strong indicator.

Enter Brazilian neuroscientist Suzana Herculano-Houzel who commenter Race Realist has been blathering on about for years. While she agrees that humans are the smartest animal on the planet, she felt she had a more direct way of measuring it than brain size adjusted for body size. Indeed, she felt we could just ignore body size completely, and simply count the number of neurons in the cerebral cortex, and she came up with a very innovative way of doing so: using detergent to dissolve brains into a homogeneous soup of cell nuclei.

At first this method looked promising. By simply counting the number of cerebral neurons and ignoring body size completely, it seemed Herculano-Houzel had found a simple brain property that ranked humans in first place. In 2017 she wrote:

the human cortex, with an average 16 billion neurons…, has almost three times as many neurons as the twice-larger elephant cerebral cortex, with not even six. The second-largest primate brain, the gorilla, has about nine billion. Even the largest whales don’t have more than three or four billion. Most mammals have less than one billion.

Thus, the human brain has by far the most neurons in the cerebral cortex – that part of the brain responsible for personality, temperament, pattern finding, logic reasoning and planning for the future, making behaviour more than simply reacting to stimuli. That, I believe, is the simplest explanation for our remarkable cognitive abilities.

Herculano-Houzel’s star began to rise. Seeing a Brazilian woman make such a monumental scientific discovery was inspiring. She gave a TED talk and commenter Race Realist began drinking the kool-aid (or should I say brain soup).

However a 2014 study found ” the long-finned pilot whale neocortex has approximately 37.2 × 10⁹ neurons, which is almost twice as many as humans”.

Thus despite Herculano-Houzel’s Herculean efforts, encephalization quotient remains the single best neurological proxy for intelligence at the inter-species level, or at least the only one that puts us humans where we belong: at the top.