PhD Dissertations: Carol Elizabeth MacLeod, 2000

The Cerebellum and Its Part in the Evolution of the Hominoid Brain

The evolution of intelligence is often equated with the expansion of the noecortices in human and primate evolution, but this ignores the potential contribution of the cerebellum. Recent experimental data have shown that the lateral hemispheres of the cerebellum are crucial to the planning of motor activities, the integration of the sensory with the motor, procedural learning, and cognitive dimensions of visuo-spatial and linguistic tasks. An increased contribution of the cerebellum to intelligence should be evident in the differential expansion of cerebellar structures within the primate order. This is not apparent from current knowledge of the evolution of the mammalian brain, in which brain parts show increase according to tight allometric scaling with overall brain volume.

In constructing a comparative model of cerebellar circuitry in monkeys, apes and humans, this study measures the volumes of the whole brain, the cerebellum (including its two divisions of vermis and hemisphere), and two nuclei specifically associated with the cerebellar hemispheres, the dentate and the principal inferior olive. It assembles the largest sample size of higher primates to date with in vivo magnetic resonance scans and post mortem preserved and sectioned brains. The two data bases cross-validate each other, and provide an indication of within-species variability, two unique aspects of this study. The collection of volumes reveals differential expansion of the cerebellar hemispheres in hominoids (apes and humans) over monkeys, and concomittant changes in the associated cerebellar nuclei, challenging the developmental constraint hypothesis of allometric expansion.

The magnitude and patterning of this lateral lobe expansion suggests that it occurred with the common ancestor to the hominoids, and would have conferred the advantage of more efficient processing of multimodal information as a basis for intelligent behavior. The specific neuropsychological functions of the cerebullar hemispheres would have been advantageous to the suspensory feeding patterns of the early apes, as well as contributing to a more complex social and ecological adapation characteristic of a grade shift.