The brain’s wiring is famously cross-connected: the right hemisphere controls the left side of the body, while the left hemisphere controls the right. In a new paper, University of Minnesota neuroscientists Martin W. Wessendorf and Daniel Derry Wang argue that this arrangement may not have evolved because it gave mammals a specific survival advantage. Instead, they propose that the mammalian cortex inherited a crossed organization from much older visual pathways.
“In humans, the right motor cortex controls spinal motor neurons to move limbs on the left side of the body,” the researchers wrote.
“Similarly, the right side of the cerebral cortex primarily receives sensory information about the left side of the body and the left side of the outside world.”
“Many papers have speculated about the adaptive value of this contralateral tissue, going back at least to the anatomist Santiago Ramón y Cajal, but to date no papers seem to have examined its evolutionary history.”
Vision may have established the pattern
Wessendorf and Wang reviewed decades of comparative research on brain circuits in fish, amphibians, reptiles, birds and mammals. They looked for patterns in the traits shared by these groups and identified vision as the oldest known system in which neural pathways cross from one side to the other.
Every reported vertebrate species, from lampreys and hagfish to humans, has an optic chiasm, where optic nerve fibers cross. Signals from each visual field are sent to the opposite side’s optic tectum, a midbrain structure known as the superior colliculus in mammals.
The researchers conclude that the last common ancestor of vertebrates probably had both an optic chiasm and a contralaterally organized optic tectum.
Other crossed circuits appear to have emerged later. According to the paper, pathways carrying touch and pain information to the thalamus, along with the tectospinal and rubrospinal tracts, were likely present in tetrapods or in the last common ancestors of tetrapods. The tectospinal and rubrospinal tracts are motor pathways that run from the midbrain toward the spinal cord.
The corticospinal tract, the principal pathway from the cerebral cortex to the spinal cord, is unique to mammals, although similar projections have been reported in some birds.
An inherited arrangement
The researchers propose that the crossed visual map in the optic tectum provided a template for the later organization of other systems. The operculum also receives tactile and auditory information and sends motor commands, potentially linking the visual arrangement to additional sensory and motor circuits.
By the time mammals appeared roughly 165 million years ago, many major neural pathways had already crossed. A neocortex with no crossovers would not have been impossible, but creating one would have required adding new crossings or rewiring existing circuits.
Such changes could have increased what the authors call “wiring costs,” potentially reducing functionality, increasing energy demands or doing both. Given the circuits already in place, connecting the neocortex to them through a crossed arrangement may have been the “most economical way” to build the system.
The researchers also point to differences in corticospinal tracts among species. In goats and elephants, the tract is bilateral, while in hedgehogs and moles there is almost no crossing. They argue that this variation suggests the crossed arrangement was not strongly maintained by natural selection.
Earlier explanations and possible tests
Wessendorf and Wang examine six earlier hypotheses about why neural pathways cross. These proposals include the ideas that crossings strengthen the nervous system, help animals withstand injury, solve engineering problems involved in steering toward a goal, or reflect an ancient twist in the body’s developmental plan.
The authors argue that each explanation has weaknesses. For example, a topological proposal holds that crossings are needed to preserve spatial maps. However, the researchers note that this would imply that all mapped pathways must intersect, which is not the case.
They emphasize that their own explanation is a hypothesis and identify predictions that could be tested. These include determining whether the optic tectum and optic chiasm are preserved in Cambrian fossils of early vertebrates, investigating whether crocodilians and various birds reach the spinal cord through telencephalic tracts, and testing whether crossed wiring is more efficient than uncrossed wiring.
“We propose that an evolutionarily ancient visual pathway, at least in quadrupeds, biased our central nervous system toward organizing on opposite sides,” the researchers said.
“Given that all modern vertebrates have both an optic chiasm and a contralaterally organized optic tectum, it is likely that the Cambrian jawless fishes, which lived approximately 538 million to 486 million years ago, had both an optic chiasm and an optic tectum.”
“We may be able to test this hypothesis by examining their fossilized skulls.”
“Because evolution is conservative in innovation, often retaining and building on existing structures, pathways, and mechanisms rather than inventing new ones, the organization of the visual system (particularly the optic tectum) may have defined the organizational principles needed for later evolved sensory and motor pathways,” the researchers added.
“We propose that contralateral tissues did not necessarily evolve as an adaptation, but because evolution is an engineer who builds with the foundations and materials at hand.”
Their paper was published in Frontiers in Neuroanatomy.
Martin W. Wessendorf and Daniel Derry Wang. 2026. “Why is the right hemisphere responsible for the left side of the body? Evolutionary history of puzzling traits.” Frontiers in Neuroanatomy 20. doi: 10.3389/fnana.2026.1937941
Source: www.sci.news
