“The Human Brain in Numbers: a Linearly Scaled-Up Primate Brain”, 2009 (; backlinks; similar):
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.
[This suggests there is no major architectural innovation in human brains which is responsible for the extraordinary success of Homo sapiens, through an argument by absence: if there were major neurological architectural changes, you would expect to either observe them directly, or observe the brain economizing on (highly-expensive) neurons after having found some important trick or architectural innovation (eg. bird brains appear to economize on neurons by instead making them really dense and fast-connected). While the fact that we seem to just find a standard-looking-but-big primate brain suggests more like “the important thing was finding a niche which rewarded the smaller intelligence gains from scaling a primate brain even further, eventually unlocking novel benefits of intelligence like hunting tools & clothing & fire.” Further, the superior slope of primate brains vs non-bird-brains suggests that there may well have been “innovations” to our brains, but they are shared with the other primates and so not uniquely human.]