Suzana Herculano-Houzel

If only more scientists were like Harry Jerison

NOC_012 If only more scientists were like Harry Jerison

I was nobody. I was this Brazilian woman who trained with nobody to be a quantitative neuroanatomist, who did this outrageous thing of turning brains into soup, and had come out of left field from counting neurons with her brain soup to claim that bigger brains were not necessarily made of more neurons; that larger bodies did not need to have neither bigger brains nor more neurons; and, more recently, that the human brain had the most cortical neurons of any species, yes, but encephalization – the fact that our brain is seemingly “too large for our body” – was essentially meaningless.

All of that made me the person saying that Harry Jerison Was Wrong. Jerison, the father of the encephalization quotient, the man who finally offered a reasonable answer to the question of How come humans are the species who studies other species when we are not the owners of the largest brains around (elephants and many whales outrank us). Jerison, the paleontologist who scanned skulls and figured out that measuring the endocast (the markings left by the brain on the inside of the skull) could be used to infer brain size and then established that human evolution was the story of human brain enlargement, in absolute and, most importantly, relative terms.

And we were attending the same meeting.

The year was 2010, and we were at the Karger meeting in San Diego that precedes the annual meeting of the Society for Neuroscience. Jon Kaas had organized the Karger meeting that year and made me the Invited Speaker. Jerison, to my surprise, was in attendance. He was well in his 90s, a giant in the field, a small bald man with a nice playful smile – and he was sitting right in front of me. I was ready for him to hate me.

So I introduced myself – because what else am I going to do? “It is an honor to meet you, sir. I am of course aware of your work and I read your book The Evolution of Intelligence, I wish I had it with me so I could ask you to sign it for me, though I will understand if you are not my fan. I’m Suzana Herculano-Houzel and I just published that paper on the number of the neurons in the human brain showing that we are not special after all”. Or something like that. I’m paraphrasing from memory.

But he was more than a gentleman: he was a true Scientist. His face immediately lit up as he gave me a firm handshake and answered jovially, “Are you kidding? I wish I had the data that you generated! I could have used your data! I did what I did exactly because we couldn’t tell how many neurons brains were made of”.

Indeed, and he made it very clear. Absent direct estimates of numbers of neurons in different brains, humans included, he had to make three assumptions to compare humans to other species and make not quite testable predictions, because anything that concerns telling the true story of evolution would require a time machine, but at least come up with a narrative that made sense. Assumption number one: that all brains are made the same way, such that brains of a similar size have similar numbers of neurons, and larger brains have more neurons than smaller brains. Assumption number two: that larger animals need more neurons to operate their bodies, and thus must have larger brains with those more neurons. Assumption number three: that any neurons, or brain mass, beyond the “necessary” to run the body were “excess” and therefore contributed to making its owner more intelligent. Hence, if human evolution was the story of the human brain becoming three times as big inside a body that didn’t grow all that much, then human evolution was the story of humans, and humans alone, becoming more intelligent.

I had by then already shown that assumption #1 was wrong, and in time would show that all three were flawed. And if the assumptions are flawed, the whole intellectual edifice turns into a house of cards that crumbles and must be rebuilt from the ground up.

Which is why it is so important that every scientific attempt at a new theory make its assumptions crystal clear: so that everybody is aware of what exactly the whole structure relies on. There are the facts, the numbers, the data; there are the assumptions that we make about how those facts relate to what we want to understand; and then there is the narrative, or theory, that we create about what we want to understand, in which we interpret the facts in the light of those assumptions. Evolution is a fact; evolution through adaptation by means of the survival of the fittest is a theory. Brain enlargement in human evolution is a fact; brain enlargement relative to the size of the body as the source of our intelligence was a theory based on three key assumptions – and Jerison made his assumptions very clear in his book, The Evolution of Intelligence. He was a very good scientist. So all was good.

And I was relieved that I had done my part, too, in being a good scientist. I had generated new facts; I had disproven the assumption; I had challenged the narrative – but I never attacked the scientist. Jerison did the best that he could with what evidence he had. As did Darwin, for that matter. But that is a whole other story.

I do wish I had been much more daring and had asked Jerison to tell the story of his life work. But I was too green in the field, too young in life to appreciate that how we think, what we do, who we are is the sum of our stories, that every little story counts, and that sometimes the most savory ones, the stories that explain how we came up with an idea, a realization, a suspicion, are narratives that only come into being when we are asked to retrace our steps, and these are narratives that often don’t make it into books because the question never got asked.

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