Repost Wednesday is a short series of guest-posts that will be featured on our blog periodically, highlighting some of the wonderful posts MIT Press authors have written about their books on other platforms. We hope you enjoy this series. In this post neuroscientist Suzana Herculano-Houzel turns brains into soup to understand cognitive ability.
I've turned human brains into soup. Actually, I've made soup out of all sorts of brains - elephant, whale, lion, monkey, cat and many more. I love the symmetry that my ability to slice, dice and dissolve brains into soup, to find out how our brain came to be, turns out to be much the same ability that may have allowed us to become humans in the first place.
I am all for technology, especially now that I've realised how important a role it must have played in our evolutionary history. But it turns out that turning brains into soup is a highly efficient and inexpensive, low-tech way to obtain accurate estimates of the numbers of neurons that compose them.
Why bother knowing how many neurons different brains are made of? Because neurons are the fundamental information-processing units of brains. Whatever animal had the most neurons should be the most cognitively capable. But 12 years ago, I realised that no-one knew how many neurons are in different brains, or how their numbers compared and scaled across differently sized brains.
At that time, the (incorrect) intuition was that brains of similar size must be built of similar numbers of neurons. By extension, then, cows and chimpanzees should be equally cognitively capable, since they have brains of comparable size. Likewise, larger brains should always have more neurons than smaller brains - which meant that elephants and whales, with their three- to six-fold larger brains than ours, should have more neurons than us humans.
Since humans were nowhere near the top in terms of brain size, it seemed that the only explanation for the cognitive prowess of our species was that the human brain was somehow extraordinary. And so began the quest for special genes, special brain connectivity or synapses or human-exclusive cells, anything that could distinguish us from other species.
But what if larger brains did not always have more neurons than smaller brains? That's why I wanted to find out how many were in different brains. There might be a simple explanation for the superior cognitive abilities of the human brain.
I figured that instead of the standard high-tech solution of counting cells through laborious and expensive sampling of brain sections under a microscope, I could use detergent to dissolve the brains and their heterogeneously distributed neurons into a soup of free cell nuclei. Since each brain cell has one, and only one, cell nucleus, counting the free nuclei would be as good as counting cells - and I could easily and quickly do that under a microscope, by sampling drops of the now-homogeneous soup of cell nuclei.
It turned out that primate brains are made differently to others, with more smaller neurons fitting in primates than, say, rodent cortices of a similar size - a baboon cerebral cortex has ten times more neurons than the similarly sized cortex of an antelope.
For the same reason - because it scales like a generic primate brain - the human cortex, with an average 16 billion neurons 2, 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, given that the human brain has an overall connectivity and distribution of functions that is not far from typical for primates in general.
With an energy "price tag" of about six kilocalories per billion neurons per day, that many cortical neurons would have forced our ancestors to spend ten hours a day looking for food and eating like other primates do in order to support brain and body 3. That is, however, not feasible; orangutans and gorillas, the largest primates, can at most do eight. Without access to enough calories, we should not be here.
And yet we are. So how did we, and we alone, come by so many cortical neurons? The answer seems to lie in what may have been one of the first technologies invented by our kind: cooking.
The stone tools of our smaller-brained ancestors of three million years ago could be used to cut and mince meat and pound and puree roots - that is, to process food before it was eaten. Cooking with fire, introduced about one million years later, helped increase even further the amount of energy that can be obtained from the same foods. Our ancestors cooked, and because of that, they alone among primates could obtain the calories required to feed both body and a larger brain. Homo culinarius, I like to call them - maybe that would be a better name for our modern selves, rather than the presumptuous sapiens that implies that no other species thinks or knows.
As humbling as recognising that we never stopped being primates is acknowledging that biology alone does not suffice. Our brain reached its current size 400,000 years ago, but if now we are born with the cognitive capabilities that come with 16 billion cortical neurons, our wondrous abilities have to be developed de novo in each individual, a process that takes a lifetime and is enabled by culture and technology.
Once biologically capable, the achievements of our brain, and the body of knowledge and know-how that it can hold, have transcended the grasp of the individual. That is why science (the knowledge) and engineering (the crafts) must be carefully cultivated, documented and passed on: to ensure that the biological capabilities of our species will keep giving rise to the abilities of future generations.
Suzana Herculano-Houzel is a neuroscientist at Vanderbilt University and the author of The Human Advantage (MIT Press).