Large brains appear several times in the mammalian radiation. Example species are illustrated for each major mammalian group. The mammalian radiation is based on the findings of Murphy et al. (18) and Kaas (19). Brain images are from the University of Wisconsin and Michigan State Comparative Mammalian Brain Collections (www.brainmuseum.org).
Herculano-Houzel S. The remarkable, yet not extraordinary, human brain as a
scaled-up primate brain and its associated cost. Proc Natl Acad Sci U S A. 2012
Jun 26;109 Suppl 1:10661-8.
She review allometric scaling in mammals, noting first that it is more fine-grained than size alone by giving examples of animals with similar sized brains but different cognitive abilities. “The human cerebral cortex is 75.5% of the entire brain mass, other animals, primate and nonprimate, are not far behind: The cerebral cortex represents 73.0% of the entire brain mass in the chimpanzee (7), 74.5% in the horse, and 73.4% in the short-finned whale (3).” Both cows and chimps have brain mas of ~400g, and rhesus monkey and capybara have 70-80g.
She measures the proportionality between brain mass and number of brain neurons, for the whole brain and the cerebral cortex.
Brain mass to number of neurons:
- rodents: exponential (1.5)
- primates/insectivors: linear
i.e. 10x more neurons in a rat brain gives a 35x more massive brain.
Cerebral cortex to num neurons:
- rodents: exponential (1.7)
- insectivores: exponential (1.6)
- primates: linear.
All species had linear scaling between both cerebellar and for nonneuronal cell counts as a function of brain cell count.
“A decrease in long-range connectivity, favoring local connectivity, in larger primate brains is expected from the nearly linear increase in cortical size as the brain gains neurons …
Neuronal connectivity in the cerebral cortex has properties of a small-world network, with mostly local connectivity and only a relatively small number of long-range connections”
And finally, the human brain is not an outlier in terms of num neurons/size, but rather what one would expect from a primate.
Neuroscientists have become used to a number of “facts” about the human brain: It has 100 billion neurons and 10- to 50-fold more glial cells; it is the largest-than-expected for its body among primates and mammals in general, and therefore the most cognitively able; it consumes an outstanding 20% of the total body energy budget despite representing only 2% of body mass because of an increased metabolic need of its neurons; and it is endowed with an overdeveloped cerebral cortex, the largest compared with brain size. These facts led to the widespread notion that the human brain is literally extraordinary: an outlier among mammalian brains, defying evolutionary rules that apply to other species, with a uniqueness seemingly necessary to justify the superior cognitive abilities of humans over mammals with even larger brains. These facts, with deep implications for neurophysiology and evolutionary biology, are not grounded on solid evidence or sound assumptions, however. Our recent development of a method that allows rapid and reliable quantification of the numbers of cells that compose the whole brain has provided a means to verify these facts. Here, I review this recent evidence and argue that, with 86 billion neurons and just as many nonneuronal cells, the human brain is a scaled-up primate brain in its cellular composition and metabolic cost, with a relatively enlarged cerebral cortex that does not have a relatively larger number of brain neurons yet is remarkable in its cognitive abilities and metabolism simply because of its extremely large number of neurons.