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

Herculano-Houzel S (2009) Front Human Neurosci 3, 31.

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
fi ndings 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.

The PDF for this paper is freely available and can be downloaded here.