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Front. Hum. Neurosci., 09 November 2009 | http://dx.doi.org/10.3389/neuro.09.031.2009

The human brain in numbers: a linearly scaled-up primate brain

Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brasil
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. 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.
brain scaling, number of neurons, human, encephalization
Herculano-Houzel S (2009). The human brain in numbers: a linearly scaled-up primate brain. Front. Hum. Neurosci. 3:31. doi: 10.3389/neuro.09.031.2009
15 July 2009;
 Paper pending published:
05 August 2009;
29 September 2009;
 Published online:
09 November 2009.

Edited by:

Andreas Jeromin, Allen Institute for Brain Science, USA

Reviewed by:

Karl Herrup, Case Western University, USA
Robert Barton, University of Durham, UK
© 2009 Herculano-Houzel. This is an open-access article subject to an exclusive license agreement between the authors and the Frontiers Research Foundation, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are credited.
Suzana Herculano-Houzel, Laboratório de Neuroanatomia Comparada, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rua Carlos Chagas Filho 373, 21950-902 Rio de Janeiro, Rio de Janeiro, Brazil. e-mail: suzanahh@gmail.com