The evolution of distributed association networks in the human brain

•The human brain is triple the size of ancestors that lived 3 million years ago.•Widely distributed cortical association regions are disproportionately expanded in humans compared with other primates.•The separate association regions are connected by multiple large-scale networks that mature late in development.•Cortical expansion may have caused critical properties of association networks to evolve as a spandrel.•Many cortical features may be consequences of expansion in the context of conserved developmental programs.

The human cerebral cortex is vastly expanded relative to other primates and disproportionately occupied by distributed association regions. Here we offer a hypothesis about how association networks evolved their prominence and came to possess circuit properties vital to human cognition. The rapid expansion of the cortical mantle may have untethered large portions of the cortex from strong constraints of molecular gradients and early activity cascades that lead to sensory hierarchies. What fill the gaps between these hierarchies are densely interconnected networks that widely span the cortex and mature late into development. Limitations of the tethering hypothesis are discussed as well as its broad implications for understanding critical features of the human brain as a byproduct of size scaling.