The left side of the brain controls the right side of the body, and the right side controls the left. A new hypothesis traces that arrangement back to the visual systems of fish that lived more than 500 million years ago.
The study, published in the journal Frontiers in Neuroanatomy and written up by ScienceDaily, is the work of Dr Martin Wessendorf, a neuroscientist and associate professor emeritus at the University of Minnesota who studies how the brainstem regulates neurons in the spinal cord, with Daniel Wang, who specialises in cognitive neuroscience and neurodegenerative diseases.
The explanation begins at the eye. Light passing through the lens projects a reversed image on to the retina: objects appearing to the right are projected on to the left side of the retina, and objects on the left on to the right. At a structure called the optic chiasm, nerve fibres are organised so that information from the right visual field reaches the left side of the brain, and information from the left visual field the right.
Some of those signals reach the optic tectum, known in mammals as the superior colliculus, a midbrain structure that helps produce fast reactions to visual events. Each side of it contains a representation of the opposite visual field, with maps of physical sensation arranged to match. Because the optic tectum exists in every vertebrate species, it is considered one of the most ancient structures in the vertebrate nervous system.
The authors then examined the pathways that carry sensation. The dorsal column pathway, which conveys gentle touch, vibration and the position of body parts, crosses in the brainstem and is found in amphibians, reptiles, birds and mammals, though not in fish. The spinothalamic tract, which carries pain, itch and certain kinds of touch, crosses within the spinal cord itself and occurs in birds, mammals and reptiles as well as some cartilaginous fish, including sharks and rays.
Three motor pathways cross as well: the tectospinal pathway, which helps the body respond rapidly to its surroundings; the rubrospinal pathway, which begins in a midbrain structure called the red nucleus and is especially prominent in most mammals; and the corticospinal tract, which in primates supports the fine control of the fingers.
The researchers propose that the earliest visual circuits established a pattern in which sensory and movement pathways were organised across opposite sides, and that later circuits inherited the arrangement. Because every living vertebrate possesses an optic chiasm and an optic tectum organised around the opposite visual field, they suspect both structures were already present in primitive jawless fish during the Cambrian Period, approximately 538 to 486 million years ago. The hypothesis may be testable against fossilised skulls.
The broader idea challenges the assumption that every unusual biological feature must have evolved because it provided a direct survival benefit. Evolution often modifies structures that already exist rather than developing new ones, and the crossed arrangement may reflect the lasting influence of an older layout rather than a better one.
This is the part worth dwelling on. The reason the human nervous system crosses the midline is not that crossing works better. It is that crossing was already there.
Our analysts recognise the pattern immediately. The wiring was laid down to suit a reversed image on a retina, in an animal with no limbs to control, and every subsequent circuit was routed around it rather than through a redesign. Half a billion years later, a surgeon's fingers are operated from the opposite hemisphere because of a constraint on an eye.
The escalation is in the timescale. The dorsal column pathway inherited the layout. The motor pathways inherited it from them. The six-layered cerebral cortex, which mammals alone possess, was built on top of all of it and crosses as well, which means the most recent addition to the vertebrate brain is routed to suit the oldest.
The Department of Unnecessary Research has searched for the point at which the arrangement might have been reviewed and has not found one. There is no mechanism by which a species may issue a correction, and the only parties in a position to authorise one stopped existing in the Cambrian.