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The Human Brain in Numbers: A Linearly Scaled-Up Primate Brain

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The Human Brain in Numbers: A Linearly Scaled-Up Primate Brain

Suzana Herculano-Houzel. Front Hum Neurosci.

Abstract

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 10x more glial cells. Such uniqueness was seemingly necessary to justify the superior cognitive abilities of humans over larger-brained mammals such as elephants and whales. 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. 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. 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. 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. 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.

Keywords: brain scaling; encephalization; human; number of neurons.

Figures

Figure 1
Figure 1
The human brain is not the largest. Brains of a human and of an African elephant are depicted here at the same scale. Drawings by Lorena Kaz based on images freely available from the University of Wisconsin and Michigan State Comparative Mammalian Brain Collections (www.brainmuseum.org).
Figure 2
Figure 2
Relative size of the cerebral cortex does not inform about the relative number of neurons in the cortex compared to the whole brain. Each point indicates, for a given species, the average relative cortical mass as a percentage of total brain mass (X-axis) and the average relative number of cortical neurons as a percentage of the total number of neurons in the brain (Y-axis). Data from Herculano-Houzel et al. (2006, 2007); Azevedo et al. (2009); and Sarko et al. (2009).
Figure 3
Figure 3
Brain mass and total number of neurons for the mammalian species examined so far with the isotropic fractionator. Brains are arranged from left to right, top to bottom, in order of increasing number of neurons according to average species values from Herculano-Houzel et al., (rodents), Herculano-Houzel et al., (non-human primates), Sarko et al., (insectivores) and Azevedo et al., (human brain). Rodent brains face right, primate brains face left, insectivore brains can be identified in the figure by their bluish hue (due to illumination conditions). All images shown to the same scale. Primate images, except for the capuchin monkey and human brain, from the University of Wisconsin and Michigan State Comparative Mammalian Brain Collections (www.brainmuseum.org). Insectivore images kindly provided by Diana Sarko, and human brain image by Roberto Lent. Rodent images from the author. Notice that some rodent brains, such as the agouti and the capybara, contain fewer neurons than primate brains that are smaller than them.
Figure 4
Figure 4
Brain size is not a reliable indicator of number of neurons across orders. Because of the different cellular scaling rules that apply to rodent and primate brains, primates always concentrate larger numbers of neurons in the brain than rodents of a similar, or even larger, brain size. Data from Herculano-Houzel et al. (2006, 2007). Illustration by Lorena Kaz.
Figure 5
Figure 5
Numbers of neurons increase faster in the cerebral cortex and cerebellum than in the remaining brain areas (the combined brainstem, diencephalon and basal ganglia). Data points indicate average values for individual species of rodents (Herculano-Houzel et al., 2006), primates (Herculano-Houzel et al., 2007), including humans (Azevedo et al., 2009), and insectivores (Sarko et al., 2009).

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