A discovery that ‘literally changes the textbook’

The network of nerves that connect our eyes to our brains is sophisticated and researchers have now shown that it evolved much earlier than we previously thought, thanks to an unexpected source: the garvis.

Ingo Braasch, Michigan State University, helped an international research team show that this compounding scheme has been present in ancient fish for at least 450 million years. This makes it about 100 million years older than previously believed.

“This is the first time for me that one of our publications has literally changed the textbook I teach,” said Braasch, an assistant professor in the Department of Integrative Biology at the College of Science.

This work, published in the journal Science on April 8, it also means that this type of eye-brain connection precedes animals that live on land. The existing theory was that this connection first developed in terrestrial beings and from there was passed on to humans where scientists believe it helps with our depth perception and 3D vision.

And this work, led by researchers from the French Public Research Organization Inserm, shapes more than just our understanding of the past. It also has implications for future health examinations.

Studying animal models is a valuable way for researchers to learn about health and disease, but it can be challenging to draw connections with human conditions from these models.

Zebrafish, for example, are a popular model animal, but their brain-brain wiring is very different from a human. In fact, it helps to explain why scientists thought that human connection first came into being in four limbs, or four-legged ones.

“Modern fish, they do not have this type of eye-brain connection,” Braasch said. “This is one of the reasons why people thought it was a new thing in tetrapods.”

Braasch is one of the world’s leading experts in another type of fish, known as gar. Gar evolved more slowly than zebrafish, meaning that gar more closely resembled the last common ancestor that fish and humans shared. These similarities can make a powerful animal model for health studies, which is why Braasch and his team are working to better understand garbiology and genetics.

This is again the reason why Inserm’s researchers sought Braasch for this study.

“Without his help, this project would not have been possible,” said Alain Chédotal, director of research at Inserm and a group leader at the Vision Institute in Paris. “We did not have access to spotted yarn, a fish that does not exist in Europe and occupies a key position in the tree of life.”

To do the study, Chédotal and his colleague, Filippo Del Bene, used a groundbreaking technique to see the nerves that connect eyes to brains in different fish species. These include the well-studied zebrafish, but also rarer specimens such as Braasch’s gar and Australian lungfish produced by a collaborator at the University of Queensland.

In a zebrafish, each eye has one nerve that connects it to the other side of the fish’s brain. That is, one nerve connects the left eye to the right hemisphere of the brain and another nerve connects its right eye to the left side of its brain.

The other, more “ancient” fish do things differently. They have so-called ipsilateral or bilateral visual projections. Here, each eye has two nerve connections, one of which goes to either side of the brain, which humans also have.

Armed with an understanding of genetics and evolution, the team can look back in time to estimate when these bilateral projections first appeared. The team looks forward to building on this work to better understand and explore the biology of visual systems.

“What we found in this study was just the tip of the iceberg,” Chédotal said. “It was very motivating to see Ingo’s enthusiastic response and heartfelt support when we offered him the first results. We can not wait to continue the project with him.”

Both Braasch and Chédotal noted how powerful this study was thanks to a strong collaboration that enabled the team to investigate so many different animals, which according to Braasch is a growing trend in the field.

The study also reminded Braasch of another trend.

“We find more and more that a lot of things we thought relatively late are actually very old,” said Braasch, which actually makes him feel a little more connected to nature. “I learn something from myself when I look at these strange fish and understand how old parts of our own bodies are. I’m excited to tell the story of the evolution of the eye this semester with a new twist in our class for comparative anatomy. ”