Among mammals, those species with the largest brains, such as cetaceans and primates, have a greater range and versatility of behavior than those with the smallest brains, such as insectivores ( Jerison, 1985 Curiously, the widespread concept that neurons represent about 10% of all cells in the human brain might be one of the arguments behind the popular, but mistaken, notion that we only use 10% of our brain ( Herculano-Houzel, 2002Īcross species, the number of neurons and their relative abundance in different parts of the brain is widely considered to be a determinant of neural function and, consequently, of behavior ( Williams and Herrup, 1988 Indeed, an informal survey with senior neuroscientists that we ran in 2007 showed that most believed that the number of cells in the human brain was indeed already known: that we have about 100 billion neurons, outnumbered by about 10 times more glial cells – but none of the consulted scientists could cite an original reference for these numbers (Herculano-Houzel and Lent, unpublished observations). This leaves the reader with the impression that the cellular composition of the human brain has long been determined. How many neurons does the human brain have, and how does that compare to other species? Many original articles, reviews and textbooks affirm that we have 100 billion neurons and 10 times more glial cells ( Kandel et al., 2000 ), as well as cellular particularities such as the presence and distribution of Von Economo neurons ( Nimchinsky et al., 1999 Recent efforts to support this uniqueness have focused on finding genetic differences between humans and other primates (reviewed in Vallender, 2008 Emphasis is thus placed on an exceptionality that is, curiously, not brain-centered, but rather body-centered: With a smaller body but a larger brain than great apes, the human species deviates from the relationship between body and brain size that applies to other primates, great apes included, boasting a brain that is 5–7× too large for its body size ( Jerison, 1973 Since ours is obviously not the largest brain on Earth, our superior cognitive abilities cannot be accounted for by something as simple as brain size, the most readily measurable parameter regarding the brain. Most specialists seem to agree (for example, Marino, 1998 What makes us human? Is our brain, the only one known to study other brains, special in any way? According to a recent popular account of what makes us unique, “we have brains that are bigger than expected for an ape, we have a neocortex that is three times bigger than predicted for our body size, we have some areas of the neocortex and the cerebellum that are larger than expected, we have more white matter” – and the list goes on ( Gazzaniga, 2008 These findings argue in favor of a view of cognitive abilities that is centered on absolute numbers of neurons, rather than on body size or encephalization, and call for a re-examination of several concepts related to the exceptionality of the human brain. In what regards absolute numbers of neurons, however, the human brain does have two advantages compared to other mammalian brains: compared to rodents, and probably to whales and elephants as well, it is built according to the very economical, space-saving scaling rules that apply to other primates and, among economically built primate brains, it is the largest, hence containing the most neurons. Additionally, the so-called overdeveloped human cerebral cortex holds only 19% of all brain neurons, a fraction that is similar to that found in other mammals. These studies also showed that the human brain is not exceptional in its cellular composition, as it was found to contain as many neuronal and non-neuronal cells as would be expected of a primate brain of its size. However, our recent studies using a novel method to determine the cellular composition of the brain of humans and other primates as well as of rodents and insectivores show that, since different cellular scaling rules apply to the brains within these orders, brain size can no longer be considered a proxy for the number of neurons in the brain. Such uniqueness was seemingly necessary to justify the superior cognitive abilities of humans over larger-brained mammals such as elephants and whales. The human brain has often been viewed as outstanding among mammalian brains: the most cognitively able, the largest-than-expected from body size, endowed with an overdeveloped cerebral cortex that represents over 80% of brain mass, and purportedly containing 100 billion neurons and 10× more glial cells.
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