Evolution of the cerebellum
This article provides basic information on the anatomy of the cerebellum, its development, and the genetic regulation thereof, information which can be used to detect adaptations of the cerebellum to changes in functional systems in which it is involved. Bolk's (1906) common plan for the gross anatomy of the mammalian cerebellum and Larsell's (1952) Roman numeral nomenclature makes it possible to describe variations in external form in terms of lobes and lobules. These variations consist of a local increase in length and/or width of the folial chain. The uniform histology of the cerebellar cortex and its microcircuitry do not present essential differences between different mammalian species. The output of the cerebellum is organized as a set of parallel modules, each consisting of a longitudinal zone of Purkinje cells, their cerebellar or vestibular target nucleus, and the innervation of these Purkinje cells and the target nucleus by a subset of climbing fibers from a particular subnucleus of the contralateral olive. One subset of modules shows little variation among mammals. It is concerned with spinal and brainstem motor systems, such as the vestibulospinal, rubrospinal, and vestibulo-oculomotor systems. Other modules, especially the modules of the hemisphere, connected with the dentate nucleus, show a local increase in width and length in primates. These modules are preferentially connected with motor, premotor, and frontal association areas of the cerebral cortex and their size may reflect the growing dependency of movement on the cerebral cortex in primates. The spectacular increase in size of still another module (C2) in cetacea, and the presence of a species-specific module (A2) in rodents remains unexplained. Most variations in cerebellar circuitry probably occur in the mossy fiber afferent systems. Again, this is exemplified by the increase in size and differentiation of the connections of the cerebral cortex with the cerebellum in primates. This increase mainly, but not exclusively, concerns the cerebropontocerebellar connections of visual association and visuomotor areas, raising the question of whether the acquirement of foveal vision in primates is another important factor in the evolution of their cerebellum. Sections on the morphogenesis of the cerebellum and its genetic regulation focus on the isthmic organizer at the metencephalic/mesencephalic boundary, which determines the presence of a cerebellum, and on the factors determining the modular pattern and the size of the individual modules. Experiments emphasizing the position of the progenitor cells in the cerebellar primordium or their time of orgin as determining factors in the genesis of the modular pattern are discussed. The role of recognition molecules in the shaping of the Purkinje cell zones and their efferent and afferent climbing fiber connections is reviewed. Factors determining the distribution of mossy fibers have not received much attention in the literature. It is concluded that mutations in genes regulating temporal and spatial gradients in the cerebellar primordium are a likely substrate for natural selection, leading to adaptations of cerebellar modules. Adaptations to new or extended functions may also occur as changes in mossy fiber afferent systems, which use the conserved, modular output system of the cortex for a new purpose. In the final section of this article, the functional aspects of the cerebellum are discussed. Adaptations of the cerebellum during evolution can only be understood when the contribution of the cerebellum to the different functional systems in which it is involved is known. Unfortunately, this is not the case. Observations and theories throwing some light on this problem are discussed.