Neural Decussation – Historical Perspectives and Evolutionary Concepts

Neural decussation and contralateral limb control by the human forebrain is a feature shared across many vertebrates, but not invertebrates. This review explores evolutionary theories behind this trait. Santiago Ramón y Cajal’s work in 1901 proposed that neural decussation in vertebrates, particularly optic nerve crossing, was favoured during evolution because it aligns visual inputs. However, such adaptive theories struggle to explain why nerve crossing is present in vertebrates but absent in invertebrates with well-developed nervous systems. Recent alternative spandrel theories, such as the ‘somatic’ and ‘axial’ twist hypotheses, proposed by Kinsbourne in 2013 and de Lussanet and Osse in 2012 respectively, suggest that decussation is a by-product of the evolutionary 180-degree rotation of the head’s rostral part, relative to the body. These spandrel theories imply that neural decussation is not itself adaptive, but rather a consequence of a different, primary evolutionary change. Although each theory explains important aspects of vertebrate neuroanatomy, none fully accounts for the diversity of decussation patterns observed across neural systems. We therefore propose the ‘Selective Decussation’ Hypothesis, raising the possibility that neural decussation accumulated through successive evolutionary stages in response to distinct selective pressures, accounting for different degrees of midline crossing across vertebrate neural systems.