Meet professor Suzana Herculano-Houzel. You may have seen her groundbreaking TED Talks conference on how we owe everything we have today to cooking — but her success story is much more complicated. Starting her journey as a prospective geneticist, Herculano-Houzel left her home country of Brazil for Case Western Reserve University, in Cleveland, Ohio, where she discovered the endless opportunities of neuroscience. Herculano-Houzel gladly moved away from the world of tubes with transparent liquids, refusing to be restricted to “a tiny bit of a chromosome.” Since her studies, she has come to realize that her main passion is finding out what brains are really made of — which in some degree involved disqualifying the prevailing statement suggesting that we humans only use 10 percent of our brains.
Brain World: Do we really know where this statement comes from?
Suzana Herculano-Houzel: I ran a quiz in Rio and found out that 60 percent of people with undergraduate degrees subscribed to it. So they must think that 90 percent of what they carry around in their head is useless — really?
It is of course not true. You use your entire brain the whole time, just in different ways. Still, I started digging around for where this myth came from, along with looking for whoever said that we had 100 billion neurons and 10 times more glial cells — which also could amount to using only 10 percent of your brain cells — than neurons (although that would also be wrong; all brain cells work together).
The short answer is that nobody, nobody I could find actually knew what the human brain is made of. Nobody has really counted, because there were no adequate methods to count cells in a whole brain. But from reading and from my undergraduate experience in a virology lab, I knew that you could harvest the nuclei out of brain cells. Each cell usually has one and only one nucleus. You can dissolve all cells and keep their nuclei, and if you do that, you get rid of the tissue and end up with a soup of free nuclei, which are very easy to count. I like to call this method “brain soup,” but it does not fly so well with editors.
They usually want something that sounds more serious. So, it has an official name: isotropic fractionator. But it really is soup. [Laughs.] You take fixed brains and turn them into soup, and that allows you to address these really basic questions that we really do not know the answers to. They boil down to what our brain is made of: What makes us human, how it is that we are the ones wondering about our brains and studying other species and not the other way around — or at least we don’t think so.
BW: What makes us human? How can we explain that neurologically? What makes us wonder about why we are here? A dog is not going to ask that question. So, what is happening inside the brain that makes the difference?
SHH: When I first became interested in these issues, there was the consensus that the human brain is special. It is much larger than it would apparently need to be for the size of the body that we have. Technically, if you have a larger body, then you need more brain matter with more neurons to take care of that body. I do not think there is really that strong a requirement. Orangutans and gorillas actually have brains that are smaller than they “should” be for the size of their body — which already means that this preconception about the human brain being larger than it “should” be is wrong, because orangutans and gorillas are doing fine, thank you. That is lesson one: Brain size cannot be what matters.
Then, I thought, maybe what really sets us apart from other species is something really simple. Maybe regardless of the size of the brain you are looking at, there could be different numbers of neurons inside. If you consider that neurons are the basic information-processing units of the brain, then it would make sense that the more brain neurons in a species, especially in the cerebral cortex, the more cognitive capabilities it has.
SHH: Yes, it is a very important word: capabilities — and I do not mean abilities. Cognitive capabilities are biological; abilities are what you may or may not turn your capabilities into.
BW: How do we acquire any ability?
SHH: It turns out that in the cerebral cortex, we have by far the largest number of neurons of any species: 16 billion neurons on average. But having that many neurons is not enough — it’s no guarantee that you will have great abilities. To do that, you have to cultivate those neurons, put them to good use. Yes, the human brain is wonderful and does all of these amazing things, but we seem to forget that we are born capable of everything, but able to do nearly nothing. You have to learn everything you will become able to do, in a sort of recapitulation of everything that happened in the last 200,000 years. Every single healthy human brain has the biological capabilities, but these capabilities have to be turned into abilities through life. We have yet to become aware of how dependent we are on technology, and of how cultural transmission is important for that. We are at that point where human abilities have by far transcended what the individual human brain can do. No individual person is able to learn everything we have created collectively — which is why training in science and technology is so important.
In one single generation, we could lose all these abilities — although our cognitive capabilities will still be there. That is why knowledge is so important, that is why science is so important, that is why we really have to prize not only our ability to generate new knowledge but also take care of what we have already generated in the past and pass that on.
BW: When does the importance of cooking come into picture?
SHH: The more neurons you have, the more energy your brain costs: 6 kilocalories per billion neurons, per day. In our case, the human brain costs on average some 500 kilocalories a day — which in our modern terms is not even a whole hamburger. That is all you need in a modern household, but we forget that we come from the wild, a world with no food-distribution chains, grocery stores, and refrigerators.
That was when I realized that getting so many calories should not be an easy job, especially with a large body or a larger body than humans. We could calculate that if we still fed like other primates do — which must have been how our ancestors got hold of calories — we would have to eat 9.5 hours every day. Imagine your entire working day, and all you can do with your hours is look for food and eat, constantly.
Without cooking, I would expect us to still be where our ancestors were 1.5 million years ago, with bodies a bit smaller than ours and the brains almost the same size of a modern gorilla. I would expect them to have the same capabilities as the modern gorilla, with the technology to make stone tools — probably a bit more than that.
We would really not be here if our ancestors had not come up with a way to trick the system and jump over this energy wall by getting more energy from the food they ate. Cooking does that, and the fossil records show that it was there at least 1 million, or probably even 1.5 million, years ago. This is the same time you see the size of the human brain take off in evolution.