Despite the abundant connectivity between the cerebral and cerebellar cortices, the increase in relative size of the cerebral cortex in the face of a constant relative size of the cerebellum across species has been used as an argument against the functional coordination of these structures and their joint evolution (Clark et al., 2001
). The rationale for considering relative size as a proxy for functionality or even “functional evolution” of a brain structure, however, is questionable. First, even though the function of any given structure may depend on how much information it receives from others, its capabilities should ultimately reflect its own number of processing units, or neurons and their synapses, regardless of how large the remaining structures are – and, hence, regardless of the relative size of the structure at issue. Furthermore, the use of relative structure size as a proxy for function across species is based on the assumption that the relative size of a structure reflects the relative number of brain neurons it contains – and this assumption, which could now be tested directly, does not hold.
Rather, the direct comparison of the numbers of neurons in the cerebral and cerebellar cortices across species presented here indicates that these are not only correlated, but vary together in the same way across mammalian orders with a relatively stable numerical preponderance of 3–4 neurons in the cerebellum to every neuron in the cerebral cortex, even though these structures change in size following different cellular scaling rules across rodents, primates, and Eulipotyphla (insectivores). These results are consistent with the findings that, in primates, the cerebellum, neocortex, vestibular nuclei and relays between them exhibit concerted volumetric evolution, even after removing the effects of change in other structures (Whiting and Barton, 2003
), and increased size of the prefrontal cerebral cortex is accompanied by an increased prefrontal cortico-pontine system and prefrontal-projecting cerebellar lobules (Ramnani et al., 2006
; Balsters et al., 2010
). If extensive to all mammals, such a universal numerical relationship as observed here would account for why, across mammalian species, the number of neurons seems to be always larger in the cerebellum than in the cerebral cortex, even when cortical mass is relatively large, such as in humans (Azevedo et al., 2009
The numbers of neurons analyzed here encompass all neurons (interneurons and pyramidal projection neurons) in the cerebral cortex, and, in the cerebellum, consist of granule cells and interneurons. As observed in the Methods, Purkinje cells are not labeled with NeuN, and are therefore not included in the count; however, since they are a very small proportion of all cerebellar neurons (Lange, 1975
), their absence is unlikely to affect significantly the final neuron count of cerebellar neurons. In this way, the coordinate scaling of the cerebellum and cerebral cortex does not refer to the projection neurons that interconnect these structures, but rather to the ensemble of neurons that process information in them, and therefore determine how they function and generate the information that will be, as a result, communicated to other structures.
Remarkably, our quantitative analysis shows that the cerebral cortex can become enlarged, coming to represent over 80% of total brain mass, without the correlated increase in the percentage of neurons that it contains relative to the whole brain that would be required to give support to the presumed trend towards neocorticalization in evolution. Rather, for a variation in brain size of five orders of magnitude, the present analysis indicates that the ratio between numbers of cerebral cortical and cerebellar neurons varies relatively little, averaging 3.6 across all 19 species analyzed, and does not correlate with brain size. This is a strong argument against neocorticalization (in what concerns numbers of neurons) and, rather, in favor of the coordinated increase in numbers of neurons across the cortex and cerebellum, as brain size increases. The finding that such coordinated increase occurs with similar rates across insectivore, rodent, and primate species, including humans, suggests that it reflects a general principle in mammalian brain evolution, rather than a particularity of primates.
The coordinated scaling of the number of neurons in the cerebral cortex and cerebellum stresses the importance of analyzing numbers of neurons directly instead of using absolute or relative mass as proxies in comparative studies, specially across animal orders, whose brain structures may scale differently in size as a function of their numbers of neurons. The coordinated scaling of numbers of neurons across the cortex and cerebellum also argues strongly in favor of the integrated function of these two structures, the conservation of their functional relationship across mammalian orders, and their coordinated subjection to selective pressures in evolution. In light of the present findings, the traditional focus on the enlarged neocortex as the main event in brain evolution seems excessive and should be reevaluated across all mammals, as proposed by Whiting and Barton (2003
) based on primate data, with more attention now dedicated to the concerted evolution of cortico-cerebellar circuits and of the behavioral and cognitive (not only sensorimotor) functions they
The seemingly paradoxical relative enlargement of the cerebral cortex concurrent to a coordinated increase in numbers of neurons in the cerebral and cerebellar cortices across species may be explained by a combination of at least three factors: the faster increase in volume of the cerebral subcortical white matter than of the cerebellar white matter in larger brains (Zhang and Sejnowski, 2000
; Bush and Allman, 2003
); the small relative size of the cerebellum (about 10–14% of brain size), because of which variations of even 50% in its relative size may fail to reach significance across species; and a faster increase in overall neuronal size (including dendrites and axons) in the cerebral cortex than in the cerebellum predicted to occur in rodents and insectivores of increasing brain size (Herculano-Houzel et al., 2006
; Sarko et al., 2009
). The faster increase in neocortical than in cerebellar white matter in larger brains probably reflects the functional importance of long-range connectivity through the subcortical white matter for the operation of associative networks in the cerebral cortex (Wen and Chklovskii, 2005
), while associative connections in the cerebellum, in contrast, consist mostly of shorter-range connections within the gray matter (Bush and Allman, 2003
It is important to bear in mind, however, that the interspecific numeric relationships described here, both between numbers of neurons in the cerebral cortex and in the cerebellum, and between each structure's mass and its number of neurons, do not necessarily apply to variation across individuals within the same species. First, while structure mass and numbers of neurons vary across species in the present sample by about 10,000-fold, variation within each of these species is typically of less than 0.3-fold (Herculano-Houzel et al., 2006
). Such comparatively small intraspecific variations in brain size may not be correlated with variations in number of neurons (P. Morterá and S. Herculano-Houzel, unpublished observations), but rather reflect individual variations in the size of neuronal arborizations and in numbers of synapses. Therefore, while the number of neurons in the cerebral cortex is today considered a good correlate of cognitive abilities across species (Roth and Dicke, 2005
), and absolute brain size has been found to be the best predictor of cognitive abilities across non-human primate species (Deaner et al., 2007
), a larger brain or structure size or even number of neurons is not necessarily accompanied by better cognitive abilities within a species. Across these individuals, other factors such as variations in number and identity of synaptic connections within and across structures, building on a statistically normal, albeit variable, number of neurons, and depending on genetics and life experiences such as learning, are more likely to be determinant of the individual cognitive abilities (see, for instance, Mollgaard et al., 1971
; Black et al., 1990
; Irwin et al., 2000
; Draganski et al., 2004
Finally, the coordinated increase in numbers of cortical and cerebellar neurons across adult brains of different sizes in evolution raises the possibility that a direct mechanism may be in place that adjusts the number of neurons in the cerebellum and cerebral cortex to one another during development, such that, when the number of neurons in the cerebral cortex undergoes a major evolutionary change, the number of neurons in the cerebellum is changed accordingly in a self-organizing fashion, although probably not through direct coupling of neurogenesis, since the two structures originate from different progenitor populations (reviewed in Goldowitz and Hamre, 1998
; Jones, 2009
). It is well established that the number of granule cells is regulated by the number of Purkinje cells, while the number of Purkinje cells themselves seems to be subject to regulation by cell death (reviewed in Goldowitz and Hamre, 1998
), which might in turn be subject to afferent-dependent regulation (reviewed in Linden, 1994
; Sherrard and Bower, 1998
). The neuronal populations in the cerebral cortex and in the cerebellum might therefore be matched numerically through afferent- or target-dependent regulation via their thalamic and pontine relay nuclei, even though they are not directly connected with each other. Given that the adult number of cerebellar neurons is only established well into postnatal development, and after the adult complement of neurons in the cerebral cortex has been reached, the cortico-ponto-cerebellar projection is a likely candidate to mediate the numerical matching of cerebral and cerebellar cortical neurons. In this manner, even if small variations in the number of neurons in the cerebral cortex turn out not to correlate with variations in the number of neurons in the cerebellum across normal individuals of a same species, larger changes resulting from genetic alterations in evolution that affect the size of the neuronal population of the cerebral cortex alone might conceivably result in a coordinated increase in numbers of neurons in the cerebellum. Circumstantial support for this hypothesis is provided by several reports of crossed cerebrocerebellar atrophy in humans, considered to reflect secondary degeneration of the cerebellar hemisphere caused by massive disconnection with the contralateral cerebral hemisphere (Tien and Ashdown, 1992
; Kozic and Kostic, 2001
) and accompanied by pontine atrophy (Tan and Urich, 1984
). We are currently testing this hypothesis experimentally.