Crossed Brain Wiring May Trace Back 500 Million Years To Ancient Fish Eyes

A stroke on the right side of the brain can weaken or numb the left side of the body. Wiggling the toes on a left foot sends a command from the right side of the brain, a crossover seen in humans and most other mammals. A new paper in Frontiers in Neuroanatomy traces the origins of this setup back roughly 500 million years, long before mammals or even limbs existed, and proposes that it has less to do with better performance than with the brain’s evolutionary history.

Martin W. Wessendorf and Daniel Delie Wang of the University of Minnesota reviewed decades of animal studies and concluded that crossed wiring probably began with vision in ancient jawless fish. In their account, the left-right swap was inherited piece by piece from systems that crossed hundreds of millions of years before the first mammal brain developed, not adopted mainly because it performs better.

By the time mammals evolved the neocortex, the outer brain layer involved in higher-level thinking, most major pathways for touch, movement, and orientation were already crossed over, the researchers argue.

Evidence Points to Vision as the Origin of Crossed Brain Wiring

Wessendorf and Wang ran no experiments and worked more like detectives on a very old case. Their paper reviews earlier brain anatomy research and proposes a new explanation, so there were no participants and no sample size to report. They combed databases such as PubMed and Google Scholar for studies on fish, amphibians, reptiles, birds, and mammals. They also tracked down an 1898 paper in Spanish by the neuroscientist Santiago Ramón y Cajal, ran it through text-recognition software and Google Translate, and corrected the errors by hand.

Their reasoning rests on a simple rule: if a trait shows up in most members of a related group, such as all mammals or all vertebrates (animals with backbones), the likeliest explanation is that their last shared ancestor had it too.

Vision came first. In every backboned animal reported, nerve fibers from the eyes are sorted at the optic chiasm so the right half of the visual world lands on the left side of the brain, whichever eye took it in. Lampreys and hagfish, jawless fish still living today, have this crossing, so the last common ancestor of all backboned animals likely did too.

Cajal proposed in 1898 that vision shaped the crossed arrangement, and the new paper agrees. Its added idea involves the optic tectum, a midbrain structure that combines sight, touch, and hearing into one map of surrounding space. Because the tectum’s visual map was crossed, the authors propose, its other senses and movement outputs lined up with it.

Touch and Movement Pathways Crossed After Vision Did

Touch, pain, and body-position pathways were likely crossed by the time of the last common ancestor of four-limbed animals such as amphibians, and circuits that steer the body toward or away from sights and sounds appear to predate limbs. A pathway linking the neocortex to the spinal cord, the system behind voluntary movement in mammals, came last.

Plenty of circuits break the pattern. Smell, taste, and balance control stay mostly on the same side, as does the cerebellum, a movement-coordination region. Even the neocortex’s link to the spinal cord varies: it crosses in most mammals but runs on both sides in goats and elephants and mostly the same side in hedgehogs and moles. That variety, the authors say, hints that crossing in this pathway is not strongly favored by natural selection.

Crossed Brain Wiring Likely Persisted Because Rewiring Would Have Cost Too Much

Mammals appeared about 165 million years ago, and the crossed touch and movement systems they inherited were already almost 200 million years old. An uncrossed neocortex, the authors reason, would have required either several extra crossings to bring signals to the same side or a rewiring of existing circuits from scratch.

Either path, they argue, would likely have carried what they call a “wiring cost,” which could leave mammals less energy efficient, less intelligent, or both. Cited research describes a trade-off between performance and energy use that mammal brains appear to balance well.

Wessendorf and Wang present this as a hypothesis, not a conclusion proven by new experiments. Earlier explanations, such as crossing helping animals survive injury to one side, remain untested in living animals, and crossing may still have offered some benefit.

Future work could test it. Fossils of early jawless fish might preserve the optic tectum and optic chiasm, which would show vision crossed that early. More bird species could be checked for a crossed brain-to-spinal-cord pathway, and crocodilians, which appear never to have been examined, are predicted to have one. The authors also predict that crossed wiring will prove more efficient than any plausible uncrossed alternative, a claim still untested.

That the right brain commands the left foot may owe less to smart design than to inheritance. Vision crossed first in ancient fish, and building every later system around that layout was likely easier than starting over. Whether Wessendorf and Wang are right will depend on what those tests turn up.

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Disclaimer: This article is based on a peer-reviewed scientific paper but is intended for informational purposes only and is not intended to serve as medical advice, diagnosis, or treatment. Always consult a licensed physician or qualified healthcare provider with any questions regarding a medical condition.

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