by Jamie Hale
College students are taught that the human brain consists of 100 billion neurons. This claim can be found in numerous textbooks. College instructors often promote the 100 billion neuron claim. This claim is also promoted by widespread media sources. If you have read much about the brain or engaged in dialogue regarding the human brain there is a good chance you have encountered this statement: seemingly, this is general neuroscience- basic stuff. When I was a graduate student this number was accepted without question. What is the original source for this number?
Another claim that is often made regarding brain science is that there is "ten times more glia (glia often referred to as neuron support cells) than neurons in the human brain." Is there an original source for this number? Is there evidence for 1 trillion glia in the human brain?
Herculano-Houzel addresses both of these topics in her book The Human Advantage: A New Understanding of How Our Brain Became Remarkable (2016). She provides evidence to refute the 100 billion neuron and the 1 trillion glia claim. Her research has led to a change in teaching neuroscience and has driven pop and scholarly publications to make changes.
The Human Advantage: In Review
Suzana Herculano-Houzel, is the author of The Human Advantage. She is a former associate professor and head of the Laboratory of Comparative Anatomy at the Federal University of Rio de Janeiro. She is the author of six books on the neuroscience of everyday life. She is a former writer and presenter of the TV series Neurologica. Currently, she resides at Vanderbilt University.
How do humans have such tremendous cognitive abilities? Herculano-Houzel argues that human are remarkable, but they are not special in light of evolution. Human brains follow the rules of primate evolution. Primates have an advantage over other mammals regarding brain structure; primates brains have evolved in a way that allows neurons to be added to the brain, without the large increases in average cell sizes seen in other mammals. Primate brains have evolved differently than those brains of other animals. As an example, cows and chimpanzees have brains that are similar in mass, but the chimpanzee can be expected to have at least twice as many neurons as a cow. Human brains are scaled up primate brains. Contrary to the popular claim that the human brain is larger than can be expected for body type (expressed as encephalization quotient), the author argues the number of brain neurons as a function of body mass is what can be expected for a non-great ape primate.
Neuroscientists, in the past, thought that the human brain is large relative to the size of the body that contains it, when directly comparing to brain and body size of great apes. If our body is smaller, then our brain should be smaller, and yet it is three time larger in terms of mass. However, Herculano-Houzel's data show that when great apes are excluded humans show the same relationship between their body mass and number of brain neurons as that of other primates. In the first decade of the twenty-first century, systematic comparisons relative to the encephalization quotient, started being made of cognitive abilities among nonhuman primates, and of self control abilities among birds and mammals. The general finding was "simple absolute brain size was a much better correlate of cognitive capabilities than the encephalization quotient. It was back to square one. If the human brain is not the largest, then how can it be the most capable of them all?" (Hercualno-Houzel, 2016, pp. 16-17.). The human brain is just what can be expected for a primate brain that has evolved to adapt to human conditions. The primary mechanism responsible for human cognitive abilities is the number of neurons in the cerebral cortex. The human brain has more neurons in the cerebral cortex (16 billion) than any other animal, even when the animal (African elephant) has 257 billion brain neurons.
What is the original source for the - 100 billion neurons in the human brain assertion? Herculano-Houzel asked senior neuroscientists and no one was able to point her to the original source. After an extensive search through the scientific literature she wasn't able to find a single source supporting the 100 billion neuron claim. According to Herculano-Houzel, Eric Kandel (Nobel Laureate), co-author of Principles of Neural Science, couldn't provide an original source for the claim, even though the claim was made in Principles of Neural Science (a book Kandel co-authored). When asked about the claim, Kandel responded saying he wasn't responsible for the chapter containing the 100 billion neuron claim.
Is there evidence for 1 trillion glia in the human brain? Herculano- Houzel reports she couldn't locate any research to support the claim- 1 trillion glia in the human brain. Both of these claims (100 billion neurons and ten times more glia) are often taken as fact. Accepting information as fact, even though it is not supported by evidence is problematic; specifically problematic as it is odds with a central tenet of science; scientific data is based on evidence. A paper published by Herculano-Houzel and colleagues titled "Equal Numbers of Neuronal and Non-Neuronal Cells Make the Human Brain an Isometrically Scaled-Up Primate Brain," which is now a heavily cited paper, was rejected by high ranking journals including Nature, Proceedings of the National Academy of Sciences of the U.S.A., Neuron and the Journal of Neuroscience. The paper was eventually published in the Journal of Comparative Neurology.
Houzel developed a method called the "isotropic fractionator" that allowed her to create what she calls brain soup? The method allows dissolving only cell membranes, but not nuclear membranes (each neuron consists of one nuclear membrane), therefore producing brain soup with free-floating nuclei. These nuclei are relatively easy to count by sampling tiny amounts of the soup. All the nuclei from all of the cells are stained blue, collected and counted. In the book she provides a description of what went into developing the technique. The first attempts to use the method led to the destruction of some of the nuclei. Early attempts involved testing the preparation after a few hours of fixation. In order for the all of the nuclei to remain intact longer preparation times were required. It was finally established that after approximately two weeks of fixation the nuclei would all stay in place during testing. Other researchers have used this method. Christopher von Bartheld, from the University of from the University of Reno, and Jon Kaas, from Vanderbilt University have shown this method to be faster, more reliable and easier to apply than stereology, which was commonly used in the past.
Results, after using the isotropic fractionator, indicate the human brain has an average of 86 billion neurons and 85 billion non-neuronal cells (glia and endothelia- cells composing blood vessels) For people who like to point out that "86 is close to 100" and who claim the 100 billion is reasonable as an order-of-magnitude estimate, Houzel asserts, an entire baboon brain contains 11 billion neurons. Fourteen billion is not a small number of neurons.
The author concludes that the human brain is remarkable due to the number of neurons in the cerebral cortex (approximately 16 billion) and secondly it is remarkable thanks to cooking, which allowed humans to escape the energetic limitations of a raw food diet, that limits other animals to less cortical neurons. Chapter 11 provides detailed information on how cooking contributed to the human brain.
The book appeals to a large audience. Even though sections of the book might be difficult for some to read, with the appropriate effort the information is accessible for most people. The author points out that some of her earlier work was met with resistant. It shouldn't be surprising that some may have a problem accepting views that challenge what they thought to be neuroscience fact for so many years. I highly recommend this book. Herculano-Houzel is a major player in neuroscience.
Title: The human advantage: a new understanding of how our brain became remarkable / Suzana Herculano-Houzel.
Description: Cambride, MA: The MIT Press, 2016