Why does the left side of the brain run the right side of the body? A neuroscientist points to ancient fish eyes

Martin Wessendorf, an associate professor emeritus at the University of Minnesota, thinks the crossed wiring of the vertebrate brain started with eyes. In a paper written with Daniel Wang, he proposes that the habit of each hemisphere serving the opposite side of the body may have originated more than 500 million years ago in the visual systems of ancient fish. The idea, published in Frontiers in Neuroanatomy, treats the arrangement as a leftover of history instead of a design with a payoff. The clinical consequence is familiar: after a stroke in the left hemisphere, movement on the right side of the body is the part that suffers.

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A crossing that stroke doctors see every day

MedlinePlus, the U.S. National Library of Medicine’s health reference, states that the left cerebral hemisphere controls movement of the right side of the body, and that a stroke in that hemisphere may impair motor skills on the right side, as described in its stroke illustration entry. The crossing shows up in many vertebrate circuits rather than only in people, and it has long lacked a convincing evolutionary account. Wessendorf and Wang start from the observation that nothing obviously useful comes from sending left-brain commands to a right-side limb.

Their answer is that no advantage was needed. Evolution reuses what already exists, and the crossing may be a structural inheritance. In the authors’ phrasing, cited in the Frontiers news release, contralateral organization need not have evolved as an adaptation, since evolution builds with the foundation and materials it has at hand.

The optic chiasm and the optic tectum as the founding circuit

The paper’s anchor is vision. Visual fibers cross at the optic chiasm so that each side of the brain receives the opposite visual field, and a midbrain structure called the optic tectum maps that opposite field. In mammals the tectum goes by the name superior colliculus, and it drives fast reactions. The authors reason that because every living vertebrate has an optic chiasm and a contralaterally organized optic tectum, the earliest jawless fishes of the Cambrian Period, roughly 538 to 486 million years ago, probably had both. A standard neuroscience textbook, Neuroscience from Sinauer Associates, covers these visual projections in its chapter on central visual pathways. Once the tectum was organized this way, Wessendorf and Wang argue, it supplied a structural foundation that later pathways followed. A newer circuit built alongside an old one tends to inherit its layout, much as a renovated house keeps the plumbing where the original builders put it.

The dating rests on inference from living animals rather than on a fossil brain. Soft tissue seldom survives half a billion years, so the authors work backward: a feature found across all living vertebrates was most likely present in their last common ancestors. That logic places the optic chiasm and tectum in the Cambrian jawless fishes, long before any animal walked on land, and it makes the visual crossing the oldest in the set. The clinical side of the same wiring is laid out in MedlinePlus’s stroke illustration entry, where damage to one hemisphere shows up as weakness on the opposite side of the body.

Touch, pain and movement pathways that cross the midline

The authors walk through several later pathways. The dorsal column pathway, which carries light touch, vibration and limb position, crosses in the brainstem and is absent in fishes but present in amphibians, reptiles, birds and mammals. The spinothalamic tract, which carries pain, itch and some touch, crosses in the spinal cord and appears in birds, mammals, reptiles and some cartilaginous fishes such as sharks and rays.

Motor routes cross as well. The tectospinal tract links the optic tectum to the spinal cord and drives rapid orienting to sights, sounds and touch. The rubrospinal tract, which arises in the midbrain’s red nucleus and supports limb control, is large in most mammals. The corticospinal tract, which feeds fine motor control in primates, depends on the six-layered cortex that only mammals have. The authors suggest human dexterity may have come from co-opting a circuit whose original job was handling incoming sensation, giving an existing pathway a new role.

The full argument appears in the Frontiers in Neuroanatomy paper, and ScienceDaily’s write-up of the University of Minnesota release carries the summary quoted here.

The proposal also sets itself against a more common instinct, which is to look for a benefit. If crossing improved survival, natural selection would explain it directly. Wessendorf and Wang argue instead that contralateral organization arose as a byproduct, reinforced as each new pathway attached itself to circuitry that already crossed. Under that view the corticospinal tract, which gives primates their fine motor control, is a case of reuse, an existing sensory-handling pathway pressed into a motor role.

The authors say their proposal can be tested. Fossilized skulls of early jawless fishes might show whether the structures were present, and the answer would either support the idea that visual wiring set the template or leave the crossing without a historical anchor. Until such a fossil is examined, the hypothesis stays what its authors call it: a proposal built on comparative anatomy of living animals.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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