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ELife digest The brains of mammals consist of the same basic structures, but each of these structures varies from one species to the next. A given structure may be larger in one species than another, for example. It may contain different numbers or sizes of cells. It may even have different connections to other brain regions. By comparing individual brain structures between species, we can map how the mammalian brain has evolved. Smaers et al. have now done this for the cerebellum, a structure at the back of the brain. The mammalian cerebellum consists of three main areas: the vermis, paravermis, and the lateral hemispheres. Smaers et al. show that in apes, dolphins and seals, the lateral hemispheres are unusually large relative to the cerebellum as a whole. This could indicate that these three groups of animals share a common ancestor with enlarged lateral hemispheres. Yet, genetic studies suggest that this is not the case. Another possibility is that apes, dolphins and seals independently evolved enlarged lateral hemispheres. This may have given rise to a trait that proved beneficial for each of them. But what might this be? Studies in people suggest that the lateral hemispheres help to support some forms of learning. Apes, dolphins and seals are among only a few species of mammal with the ability to learn new calls and vocalizations. The expansion of the lateral cerebellum may therefore have contributed to the evolution of vocal learning, and this may have occurred independently on at least three separate occasions. Future work should extend this analysis to other cognitive skills, as well as to other species. Bats, for example, would be of particular interest because of their ability to echolocate. Finally, the lateral hemispheres consist of several subregions that play different roles in learning and information processing. Further experiments should explore whether different subregions have increased in size in different species.
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... Mammals from four orders showed the same coordinated scaling of neuron number in both the cerebral and cerebellar cortices, supporting arguments for integrated functions and selective pressures [88]. Indeed, some researchers argue that the cerebellum has been neglected in studies of vertebrate brain evolution and cognition [89][90][91]. ...
... Thus, one constraint on an individual's quantitative cognition is the biological capacity for information processing in their brain, as determined genetically and developmentally for each species [74][75][76][77][78][79][80][81]. Additionally, quantitative precision was related to neuron density in the cerebellum, a brain structure that has been overlooked in studies of vertebrate brain evolution and cognition [89][90][91]. We found that density of neurons was a more accurate proxy for quantitative ability than brain volume when comparing species across taxa. ...
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... Considerable attention has been paid to primate brain evolution (e.g., [14,20,90,91]), perhaps the result of the anthropocentricism and since there are substantial data available on this taxonomic group making comparative tests easy to implement. Likewise, carnivorans are also now receiving attention (e.g., [19,88,92,93]) since variation in their brain and body size, and ranging social and physical environments, makes them excellent models for these tests too. Indeed, most of the literature surrounding brain size hypotheses is based on analyses of these two groups. ...
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... The finding that cerebellar neurons account for most of the difference in neuron numbers between reptiles and endotherms is interesting in light of the mounting evidence that the cerebellum plays a key role in the evolution of sensorimotor and cognitive control of complex behaviors (38)(39)(40)(41). ...
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... Lastly, the elephant's cerebellum has deviated markedly in evolution. It is the relatively largest compared to the rest of the brain size of all other mammals, and the lateral cerebellar hemispheres are expanded compared to the vermis (Maseko et al., 2012), although there are mammals that have greater lateral cerebellar relative enlargement (Smaers et al., 2018). The elephant cerebellum is specialized such that it has increased neurons relative to the cerebral cortex [97.5% of the 257 billion neurons in the elephant brain are found in the cerebellum (Herculano-Houzel et al., 2014)], and the neurons are packed more compactly than in other afrotherians (Herculano-Houzel et al., 2014). ...
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(Current Biology 27, 714–720; March 6, 2017) In this article, we unintentionally omitted to expand on a citation of previously published results. In the caption of Figure 2, we stated that “F and p values indicate the significance of a phylogenetic ANCOVA testing for intercept differences between humans and other primates (see also Smaers and Rohlf [9], Supplemental Information…, and Table S2 for more detailed results)” (p. 716). We would like to clarify that in this statement, “see also Smaers and Rohlf” refers, specifically and exclusively, to the phylogenetic ANCOVA of primate prefrontal cortex to primary visual cortex and frontal motor areas using the Smaers dataset in [9]. These results were depicted in a subsection of our Figure 2 (the two top left regression plots) and were numerically presented in a subsection of our Table S2. Smaers and Rohlf presented these results as an empirical example when describing the least-squares solution of phylogenetic ANCOVA and did not discuss the wider biological implications of these results for primate brain evolution. The presentation of the previous results was discussed openly during the review process of this manuscript. The authors apologize for any confusion this oversight may have caused.
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