Unique to mammals and elaborated in Primates (most notably humans), neocortex is the predominant brain locus for sensory data processing, conscious motor control, cognition and volitional communication. Its basic constituent is the cortical column. Classical studies reported the number of neurons in a cortical column (`radial neuron number`) to be remarkably constant across most species and neocortical regions, with Primate primary visual cortex containing double the number of radial neurons found elsewhere. However, recent evidence suggests reduction in radial neuron number in marine mammals, as well as variation in radial neuron number in different cortical areas of monkeys, cats, and rats. This investigation aims (1) to provide `unbiased stereological` measurements of radial neuron number, neuron size, and cortical thickness across a diverse sample (15 orders) of mammals, (2) to assess contributions of phylogeny, ecology, and life history to the observed variation, and (3) to quantify primary-visual and primary-motor cortex in Primates so as to compare radial neuron number across species with that in different regions of the same brains. Neocortical development is directly observable only in species lacking the variance in radial neuron number characteristic of mammals as a class. Broader data on radial neuron number will thus provide a more naturalistic context for interpreting normal developmental and pathological events in `laboratory models,` with mechanisms implicated in neurological health and disease presumably recruited during brain evolution to achieve specific adaptive ends.
