Neural Network From Brain to Eyes Evolves Much Earlier, Fish Genetics Study Shows

In a new discovery, the complex neural network that connects the brain to the eyes might have developed far earlier than previously thought, according to a new study.

This new insight on the development of the nerve connections was kickstarted by an unexpected source: the gar fish. Ingo Braasch, from the Department of Integrative Biology and Program in Ecology, Evolution, and Behavior at Michigan State University, was a part of an international team that established a similar connection of nerves between the brain and the eyes in an ancient fish at least 450 million years ago - about 100 million years earlier than previously believed.

Researchers present their findings in the article "Bilateral visual projections exist in non-teleost bony fish and predate the emergence of tetrapods," appearing in the latest journal Science, April 9.

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Rewriting Science Textbooks

"It's the first time for me that one of our publications literally changes the textbook that I am teaching with," Braasch said in a news release from Michigan State. The findings reported in the new study also strongly suggests that the neural network between the brain and the eye also existed in terrestrial creatures. Previous theories suggested that this network of nerves first evolved in terrestrial creatures, which carried on into humans, further evolving to arrive at the depth perception and three-dimensional vision we now have.

Researchers led the work from France's Inserm public research organization, which changes our understanding of the past and created a variety of implications for future studies on biology and, more importantly, health.

Animal models guide the human understanding of health and diseases. However, establishing human connections from these zoologic models is not always straightforward. One of the most commonly used animals for scientific studies is the zebrafish. These marine species possess a very different setup connecting their brain to the eyes. This difference also supported the previous theory of human nerve connection evolving from quadruped terrestrial animals or tetrapods.

"Modern fish, they don't have this type of eye-brain connection," Braasch notes. "That's one of the reasons that people thought it was a new thing in tetrapods."

Tapping into the Closest Descendant of Fish and Human Ancestors

Braasch was tapped for the study because of his expertise in gar fishes. Evolving a lot slower than other fishes like zebrafish, the gar is more similar to the last common ancestor of both fishes and humans than any other species. Understanding an animal model based on the gar could provide a more accurate guide for health studies, leading researchers to better understand gar genetics and biology.

"Without his help, this project wouldn't have been possible," says Alain Chédotal, Inserm research director and a group leader at Paris' Vision Institute, regarding Braasch's unique expertise. Chédotal adds that spotted gar is not endemic in Europe and thus, was not accessible to them. They believe that this fish has a "key position in the tree of life."

Researchers used a groundbreaking method to examine the neural network between the brain and the eyes in different species of fish. While zebrafishes have one of their eyes connected to the opposite hemisphere of their brain, the more "ancient" gar is differently wired. Each eye has two nerve connections going to each side of the brain - the same as humans.