The developmental principles that establish the columnar edifice of the cerebral cortex underlie its evolution and dictate its physiological operations and cognitive capacity. This article contrasts the initial discoveries made by Ramón y Cajal and his contemporaries, based on the ingenious interpretation of neuronal shapes and their relationships using the Golgi method, with new insights based on the application of the most advanced methods of molecular biology and genetics. We can now propose a realistic model of how the sequence of gene expression, cascade of multiple molecular pathways and cell-cell interactions establish the number of neurons, guide their migration and allocation into proper regions and determine their differentiation into specific phenotypes that establish specific synaptic connections. The findings obtained from different levels of analyses sustain the radial unit hypothesis as a useful framework for understanding the mechanisms of cortical development and its evolution as an organ of thought.
