Scientists May Have Solved the Mystery of Why the Brain and Body Are Cross-Wired - Video

Why does the left side of your brain control the right side of your body, and vice versa? Scientists have proposed a fascinating explanation suggesting that this unusual arrangement may have originated more than 500 million years ago in the visual systems of ancient fish.

Published in the journal Frontiers in Neuroanatomy, the study by researchers Martin Wessendorf and Daniel Wang explored the evolutionary origins of the brain's crossed neural connections.

In humans, many nerve pathways cross from one side of the central nervous system to the other. This allows the left hemisphere of the brain to control movements on the right side of the body, while the right hemisphere controls the left.

Researchers suggested that this arrangement may have developed from the organisation of early visual pathways.

When light enters the eyes, images are projected onto the retina in a reversed orientation. Visual information is then transmitted through pathways that direct signals from each visual field to the opposite side of the brain.

The researchers focused on an ancient brain structure called the optic tectum, known as the superior colliculus in mammals, which helps coordinate rapid responses to visual information.

Because this structure is present across vertebrate species, scientists believe its organisation may have influenced the evolution of other neural pathways.

Importantly, the researchers examined sensory pathways responsible for touch, pain and body position, alongside motor pathways involved in movement.

They proposed that as these neural circuits evolved, they may have adopted the crossed organisation already established within the visual system.

The researchers also suggested that the human corticospinal tract, which supports precise movements such as finger coordination, may have developed by modifying older sensory pathways.

However, the proposed explanation remains an evolutionary hypothesis rather than an experimentally established fact.

Further comparative and evolutionary research is needed to confirm how these neural connections originally developed.

REFERENCE: Martin W. Wessendorf, Daniel Delie Wang. Why is the right side of the brain responsible for the left side of the body? The evolutionary history of a puzzling trait. Frontiers in Neuroanatomy, 2026; 20 DOI: 10.3389/fnana.2026.1937941