Why the left hemisphere controls the right side of the body, and the right hemisphere controls the left, remains a longstanding question in neuroscience. A study in Frontiers in Neuroanatomy proposes that this crossed arrangement may have originated in the visual systems of ancient fish and been retained as other neural circuits evolved.
Researchers Martin Wessendorf and Daniel Wang examined sensory and movement pathways in four-limbed animals. They suggest that later circuits developed around an older visual system whose signals already crossed between the two sides of the nervous system.
In vertebrates, visual information passes through the optic chiasm, where signals from each visual field are directed to the opposite side of the brain. Some then reach the optic tectum, an ancient midbrain structure that helps coordinate rapid responses to visual events. The researchers note that this structure is found across vertebrate species and organizes information from the opposite visual field.
The study also describes crossed pathways that carry sensations such as touch, vibration, pain and limb position, as well as circuits involved in movement. Their distribution across different animal groups suggests that some of these connections emerged early and were maintained over evolutionary time. The corticospinal tract, which supports precise movements in humans, may also have adapted an older pathway, the researchers propose.
The authors suggest that the optic chiasm and optic tectum may have been present in primitive jawless fish during the Cambrian Period, around 538 to 486 million years ago. If so, the foundations of the brain’s crossed wiring could be more than half a billion years old. The idea could be investigated by looking for anatomical evidence in fossilized skulls of ancient jawless fish.
Rather than resulting from a specific advantage, the crossed organization may reflect evolution reusing an existing arrangement as new sensory and motor circuits developed. The study’s hypothesis is that the early visual system provided a structural foundation that later pathways followed.
Sources: ScienceDaily.