Developmental mechanisms channeling cortical evolution

• Across species, disparity in neuron number per column grows with cortical size.
• A rostrocaudal gradient in neurogenesis duration generates this disparity in neuron number.
• Anisotropies in early axon extension further organize cortical area hierarchy.
• We suggest that the cortex has two zones of contrasting intrinsic topology.
• These mechanisms may progressively amplify hierarchical processing in larger brains.

Increase in the area and neuron number of the cerebral cortex over evolutionary time systematically changes its computational properties. One of the fundamental developmental mechanisms generating the cortex is a conserved rostrocaudal gradient in duration of neuron production, coupled with distinct asymmetries in the patterns of axon extension and synaptogenesis on the same axis. A small set of conserved sensorimotor areas with well-defined thalamic input anchors the rostrocaudal axis. These core mechanisms organize the cortex into two contrasting topographic zones, while systematically amplifying hierarchical organization on the rostrocaudal axis in larger brains. Recent work has shown that variation in ‘cognitive control’ in multiple species correlates best with absolute brain size, and this may be the behavioral outcome of this progressive organizational change.