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We have long deemed ourselves to be at the pinnacle of cognitive abilities among animals. But that is different from being at the pinnacle of evolution in a number of very important ways. As Mark Twain pointed out in 1903, to presume that evolution has been a long path leading to humans as its crowning achievement is just as preposterous as presuming that the whole purpose of building the Eiffel Tower was to put that final coat of paint on its tip. Moreover, evolution is not synonymous with progress, but simply change over time. And humans aren’t even the youngest, most recently evolved species. For example, more than 500 new species of cichlid fish in Lake Victoria, the youngest of the great African lakes, have appeared since it filled with water some 14,500 years ago.
Still, there is something unique about our brain that makes it cognitively able to ponder even its own constitution and the reasons for its own presumption that it reigns over all other brains. If we are the ones putting other animals under the microscope, and not the other way around,1 then the human brain must have something that no other brain has.
Sheer mass would be the obvious candidate: If the brain is what generates conscious cognition, having more brain should only mean more cognitive abilities. But here the elephant in the room is, well, the elephant—a species that is larger-brained than humans, but not equipped with behaviors as complex and flexible as ours. Besides, equating larger brain size with greater cognitive capabilities presupposes that all brains are made the same way, starting with a similar relationship between brain size and number of neurons. But my colleagues and I already knew that all brains were not made the same. Primates have a clear advantage over other mammals, which lies in an evolutionary turn of events that resulted in the economical way in which neurons are added to their brain, without the massive increases in average cell size seen in other mammals.
We also knew how many neurons different brains were made of, and so we could rephrase “more brain” and test it. Sheer number of neurons would be the obvious candidate, regardless of brain size, because if neurons are what generates conscious cognition, then having more neurons should mean more cognitive capabilities. Indeed, even though cognitive differences among species were once thought to be qualitative, with a number of cognitive capabilities once believed to be exclusive to humans, it is now recognized that the cognitive differences between humans and other animals are a matter of degree. That is, they are quantitative, not qualitative, differences.
Did the African elephant brain, more than three times as heavy as ours, really have more neurons than our brain?
Our tool use is impressively complex, and we even design tools to make other tools—but chimpanzees use twigs as tools to dig for termites, monkeys learn to use rakes to reach for food that is out of sight, and crows not only shape wires to use as tools to get food, but also keep them safe for later reuse. Alex, the African gray parrot owned by psychologist Irene Pepperberg, learned to produce words that symbolize objects, and chimpanzees and gorillas, though they cannot vocalize for anatomical reasons, learn to communicate with sign language. Chimpanzees can learn hierarchical sequences: They play games where they must touch squares in the ascending order of the numbers previously shown, and they do it as well and as fast as highly trained humans. Chimpanzees and elephants cooperate to secure food that is distant and can’t be reached by their efforts alone. Chimpanzees, but also other primates, appear to infer others’ mental state, a requirement for showing deceitful behavior. Even birds seem to have knowledge of other individuals’ mental state, as magpies will overtly cache food in the presence of onlookers and then retrieve and move it to a secret location as soon as the onlookers are gone. Chimpanzees and gorillas, elephants, dolphins, and also magpies appear to recognize themselves in the mirror, using it to inspect a visible mark placed on their heads.
These are fundamental discoveries that attest to the cognitive capacities of nonhuman species—but such one-of-a-kind observations do not serve the types of cross-species comparisons we need to make if we are to find out what it is about the brain that allows some species to achieve cognitive feats that are outside the reach of others. And here we run into another problem, the biggest one at this point: how to measure cognitive capabilities in a large number of species and in a way that generates measurements that are comparable across all those species.
A 2014 study tested for self-control, a cognitive ability that relies on the prefrontal, associative part of the cerebral cortex, among a number of animal species—mostly primates, but also small rodents, doglike carnivores, the Asian elephant, and a variety of bird species. They found that the best correlate with correct performance in the test of self-control was absolute brain volume—except for the Asian elephant, which, despite being the largest-brained in the set, failed miserably at the task. A number of reasons come to mind, from “It did not care about the food or the task” to “It enjoyed annoying its caretakers by not performing.” (I like to think that the reason why it’s so hard to train monkeys to do things that are easily learned by humans is their dismay at the obviousness of the task: “C’mon, you want me to move to do just that? Gimme something more challenging to do! Gimme videogames!”)
The most interesting possibility to me, however, is that the African elephant might not have all the prefrontal neurons in the cerebral cortex that it takes to solve self-control decision tasks like the ones in the study. Once we had recognized that primate and rodent brains are made differently, with different numbers of neurons for their size, we had predicted that the African elephant brain might have as few as 3 billion neurons in the cerebral cortex and 21 billion neurons in the cerebellum, compared to our 16 billion and 69 billion, despite its much larger size—if it was built like a rodent brain.
On the other hand, if it was built like a primate brain, then the African elephant brain might have a whopping 62 billion neurons in the cerebral cortex and 159 billion neurons in the cerebellum. But elephants are neither rodents nor primates, of course; they belong to the superorder Afrotheria, as do a number of small animals like the elephant shrew and the golden mole we had already studied—and determined that their brains did, in fact, scale very much like rodent brains.
Here was a very important test, then: Did the African elephant brain, more than three times as heavy as ours, really have more neurons than our brain? If it did, then my hypothesis that cognitive powers come with sheer absolute numbers of neurons would be disproved. But if the human brain still had many more neurons than the much larger African elephant brain, then that would support my hypothesis that the simplest explanation for the remarkable cognitive abilities of the human species is the remarkable number of its brain neurons, equaled by none other, regardless of the size of the brain. In particular, I expected the number of neurons to be larger in the human than in the African elephant cerebral cortex.
The logic behind my expectation was the cognitive literature that had long hailed the cerebral cortex (or, more precisely, the prefrontal part of the cerebral cortex) as the sole seat of higher cognition—abstract reasoning, complex decision making, and planning for the future. However, nearly all of the cerebral cortex is connected to the cerebellum through loops that tie cortical and cerebellar information processing to each other, and more and more studies have been implicating the cerebellum in the cognitive functions of the cerebral cortex, with the two structures working in tandem. And, because these two structures together accounted for the vast majority of all neurons in the brain, cognitive capabilities should correlate equally well with the number of neurons in the whole brain, in the cerebral cortex, and in the cerebellum.
Which is why our findings for the African elephant brain turned out to be better than expected.
Brain Soup by the Gallon
The brain hemisphere of an African elephant weighs more than 2.5 kilograms, which meant that it would obviously have to be cut into hundreds of smaller pieces for processing and counting since turning brains into soup to determine the number of neurons inside works with chunks of no more than 3 to 5 grams of tissue at a time. I wanted the cutting to be systematic, instead of haphazard. We had previously used a deli slicer to turn a human brain hemisphere into one such full series of thin cuts. The slicer was wonderful for separating cortical gyri—but it had one major drawback: Too much of the human brain matter stayed on its circular blade, precluding estimates of the total number of cells in the hemisphere. If we wanted to know the total number of neurons in the elephant brain hemisphere, we had to cut it by hand, and in thicker slices, to minimize eventual losses to the point of making them negligible.
Why spend $100,000 when a handheld butcher knife would do the job well enough?
And so the day started at the hardware store, where my daughter and I (school vacation having just started) went looking for L-brackets to serve as solid, flat, regular frames for cutting the elephant hemisphere, plus the longest knife I could hold in one hand. (Here was an opportunity not to be missed for a young teenager, who years later could say, “Hey, Mom, remember the day we sliced up an elephant brain?”) We first sawed off the structural reinforcements of the L-brackets then made the elephant brain fit inside. Sure, there are fancy $100,000 machines that would do the job to perfection, but why spend that much money when a handheld butcher knife would do the job well enough?
I laid the hemisphere flat on the bench top, framed inside the two L-brackets. A student held the frames in position while I held the hemisphere down with my left hand and sliced firmly but gently through the brain with the right, in back-and-forth movements. Several cuts later, also into the back half as well as the cerebellum, and we had a completely sliced elephant brain “loaf” lying flat on our benchtop: 16 sections through the cortical hemisphere, eight through the cerebellum, plus the entire brainstem and the gigantic, 20-gram olfactory bulb (10 times the mass of a rat brain) lying separately.
Next, we had to separate the internal structures—striatum, thalamus, hippocampus—from the cortex, then cut the cortex into smaller pieces for processing, then separate each of these pieces into gray and white matter. In all, we had 381 pieces of tissue, most of which were still several times larger than the 5 grams we could process at one time. It was by far the most tissue we had processed. One person working alone and processing one piece of tissue per day would need well over one year—nonstop—to finish the job. This clearly had to be a team effort, especially if I wanted to have the results in no more than six months. But, even with a small army of undergraduates, it was taking too long: two months went by and only one-tenth of the brain hemisphere had been processed. Something had to be done.
Capitalism came to the rescue. I did some math and realized I had some $2,500 to spare—roughly $1 per gram of tissue to be processed. I gathered the team and made them an offer: Anybody could help, and everyone would be rewarded financially by the same amount. Small partnerships quickly formed, with one student doing the grinding, the other doing the counting, and both sharing the proceeds. It worked wonders. My husband would visit the lab and comment, in awe, on the crowd of students at the bench, chatting animatedly while working away (until then, they mostly worked in shifts, it being a small lab). Jairo Porfírio took over the large batches of antibody stains, I did all the neuron counts at the microscope—and in just under six months we had the entire African elephant brain hemisphere processed, as planned.
And the Winner Is …
Lo and behold, the African elephant brain had more neurons than the human brain. And not just a few more: a full three times the number of neurons, 257 billion to our 86 billion neurons. But—and this was a huge, immense “but”—a whopping 98 percent of those neurons were located in the cerebellum, at the back of the brain. In every other mammal we had examined so far, the cerebellum concentrated most of the brain neurons, but never much more than 80 percent of them. The exceptional distribution of neurons within the elephant brain left a relatively meager 5.6 billion neurons in the whole cerebral cortex itself. Despite the size of the African elephant cerebral cortex, the 5.6 billion neurons in it paled in comparison to the average 16 billion neurons concentrated in the much smaller human cerebral cortex.
So here was our answer. No, the human brain does not have more neurons than the much larger elephant brain—but the human cerebral cortex has nearly three times as many neurons as the over twice as large cerebral cortex of the elephant. Unless we were ready to concede that the elephant, with three times more neurons in its cerebellum (and, therefore, in its brain), must be more cognitively capable than we humans, we could rule out the hypothesis that total number of neurons in the cerebellum was in any way limiting or sufficient to determine the cognitive capabilities of a brain.
Only the cerebral cortex remained, then. Nature had done the experiment that we needed, dissociating numbers of neurons in the cerebral cortex from the number of neurons in the cerebellum. The superior cognitive capabilities of the human brain over the elephant brain can simply—and only—be attributed to the remarkably large number of neurons in its cerebral cortex.
While we don’t have the measurements of cognitive capabilities required to compare all mammalian species, or at least those for which we have numbers of cortical neurons, we can already make a testable prediction based on those numbers. If the absolute number of neurons in the cerebral cortex is the main limitation to the cognitive capabilities of a species, then my predicted ranking of species by cognitive abilities based on number of neurons in the cerebral cortex would look like this:
which is more intuitively reasonable than the current ranking based on brain mass, which places animals such as the giraffe above many primate species, like this:
As it turns out, there is a simple explanation for how the human brain, and it alone, can be at the same time similar to others in its evolutionary constraints, and yet so different to the point of endowing us with the ability to ponder our own material and metaphysical origins. First, we are primates, and this bestows upon humans the advantage of a large number of neurons packed into a small cerebral cortex. And second, thanks to a technological innovation introduced by our ancestors, we escaped the energetic constraint that limits all other animals to the smaller number of cortical neurons that can be afforded by a raw diet in the wild.
So what do we have that no other animal has? A remarkable number of neurons in the cerebral cortex, the largest around, attainable by no other species, I say. And what do we do that absolutely no other animal does, and which I believe allowed us to amass that remarkable number of neurons in the first place? We cook our food. The rest—all the technological innovations made possible by that outstanding number of neurons in our cerebral cortex, and the ensuing cultural transmission of those innovations that has kept the spiral that turns capacities into abilities moving upward—is history.
Suzana Herculano-Houzel is a Brazilian neuroscientist. She is an associate professor and the head of the Laboratory of Comparative Anatomy, Institute of Biomedical Sciences, Federal University of Rio de Janeiro.
Excerpted from The Human Advantage: A New Understanding of How Our Brain Became Remarkable by Suzana Herculano-Houzel published this month by The MIT Press. All rights reserved.
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