Why is the right side of the brain responsible for the left side of the body? The evolutionary history of a puzzling trait

In humans and most other mammals, the right motor cortex controls the spinal motoneurons that cause the limbs on the left sides of our bodies to move. Similarly, the right side of the cerebral cortex predominantly receives sensory information about the left side of our bodies and the left side of the outside world. Although there have been numerous papers (dating back at least to Cajal) speculating on the adaptive value of this contralateral organization, to date none appears to have examined its evolutionary history. Our paper examines the major contralaterally organized circuits found in extant vertebrate brains and proposes a novel hypothesis regarding their evolution. We found that vision is contralaterally organized in all vertebrates reported: input from each visual field is carried by the optic tract to the contralateral optic tectum (i.e., the superior colliculus in mammals) and is retinotopically mapped there. In addition, in most tetrapods, somatosensation and hearing are carried rostrally to forebrain by contralaterally organized circuits. Stem amniotes (−-the predecessors of reptiles, birds, and mammals) appear to have had contralaterally organized motor systems including the tectospinal tract and rubrospinal tract. Finally, the last common ancestor of mammals itself appears to have had a contralaterally organized neocortex, including a corticospinal tract. We conclude that the mammalian neocortex evolved in brains in which many major motor and sensory circuits were already contralaterally organized. By adapting contralateral organization itself, the neocortex was able to integrate directly with those circuits without the “wiring cost” incurred by altering them.