Why the Brain’s Right Side Controls the Left Side of the Body

How could vision shape later circuits?

The authors propose that the crossed visual map in the optic tectum supplied the reference pattern for later systems. The tectum also receives touch and hearing signals and issues motor commands, allowing other sensory and motor circuits to align with its organization.

Other crossed circuits probably arrived later. The touch and pain pathways to the thalamus (a central relay for sensory information), along with the tectospinal and rubrospinal tracts (motor routes from the midbrain to the spinal cord), were likely present in the last common ancestor of tetrapods, or four-limbed vertebrates.

The corticospinal tract (the main route from cortex to spinal cord) is unique to mammals, although the authors report a similar projection in some birds. When mammals appeared roughly 165 million years ago, many major pathways had already crossed.

An uncrossed neocortex (the cerebral cortex of mammals) would not have been impossible. It would have required either several additional crossings or rewiring existing circuits, likely raising wiring costs: slower function, higher energy demand, or both. The crossed connection was the most economical way to use the established network.

Does the pattern show a survival benefit?

The corticospinal tract varies among species. It has bilateral connections (links to both sides) in goats and elephants and remains mostly uncrossed in hedgehogs and moles. The authors take that variation as evidence that crossing is not strongly favored by natural selection.

The paper reviews six earlier explanations. Four broad ideas among them are:

• Crossing strengthens the nervous system structurally.
• Crossing helps animals survive injury.
• Crossing solves an engineering problem in steering toward goals.
• Crossing reflects an ancient twist in the body plan.

The authors identify weaknesses in each. For example, a topological argument (a geometric argument about continuity) says crossings preserve spatial maps, yet that logic would require every mapped pathway to cross. The observed circuits do not meet that requirement.

What could test the hypothesis?

Wessendorf and Wang said: “We propose that evolutionarily ancient visual pathways biased our central nervous systems towards being organized contralaterally — at least in tetrapods.”

They list three predictions:

• Cambrian fossils of early vertebrates preserve an optic tectum and optic chiasm.
• Crocodilians and a wide range of birds have crossed telencephalic tracts (pathways from the forebrain) connecting to the spinal cord.
• Crossed wiring proves more efficient than plausible uncrossed alternatives.

Because every living vertebrate has both an optic chiasm and a contralaterally organized optic tectum, they infer that Cambrian-era jawless fishes, which lived roughly 538 to 486 million years ago, had both. Examining their fossilized skulls could test that inference.

Like an engineer reusing an existing foundation, evolution often retains and builds on structures rather than inventing new ones. On that view, the visual system, and the optic tectum in particular, may have defined the organizational principles required by sensory and motor pathways that evolved later.

The proposal remains a hypothesis, not a confirmed reconstruction of the vertebrate brain. The comparative pattern supports it, but fossils and efficiency tests could still overturn it.

Their paper, “Why Is the Right Side of the Brain Responsible for the Left Side of the Body? The Evolutionary History of a Puzzling Trait,” appeared in 2026 in Frontiers in Neuroanatomy, volume 20. Its DOI is 10.3389/fnana.2026.1937941.

The left-right reversal may be less an adaptation than a legacy of how animals first organized vision.