The network of nerves that connect our eyes to our brains has been perfected, and researchers have now shown that it evolved much earlier than previously thought, thanks to an unexpected source: the fish.
Ingo Braash of Michigan State University helped an international research team show that this linking scheme was present in ancient fish at least 450 million years ago. This makes it about 100 million years older than previously thought.
“For the first time, one of our publications is literally changing the textbook I teach,” said Braash as an assistant professor in the Department of Integrative Biology at the College of Natural Sciences.
This work published in the journal Science on April 8, also means that this type of eye-brain connection precedes animals living on land. The existing theory was that this connection first evolved into terrestrial beings and from there spread to humans, where scientists believe it helps our deep perception and 3D vision.
And this work, led by researchers from the French public research organization Inserm, is more than a change in our understanding of the past. This also has implications for future health research.
Studying animal models is an invaluable way for researchers to learn about health and disease, but extracting connections to human conditions from these models can be a challenge.
Zebras are a popular animal model, for example, but their wiring between the eye and the brain is very different from that of humans. In fact, it helps explain why scientists believe that the human connection first evolved into four-legged terrestrial beings or tetrapods.
“Modern fish don’t have that kind of eye-brain connection,” Braash said. “That’s one of the reasons people think it’s something new in tetrapods.”
Braasch is one of the world’s leading experts in various types of fish, known as gar. Gars evolve more slowly than zebras, which means that gars are more like the last common ancestor shared by fish and humans. These similarities could make gar a powerful animal model for health research, which is why Braash and his team are working to better understand the biology and genetics of gar.
This, in turn, is why Inserm researchers sought Braasch for this study.
“Without his help, this project would not have been possible,” said Alain Shedotal, director of research at Inserm and head of the Vision Institute group in Paris. “We did not have access to a spotted woodpecker, 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 an innovative technique to see the nerves that connect the eyes to the brain in several different species of fish. This included well-studied zebrafish, but also rarer specimens such as the Braasch range and Australian lungs provided by a fellow at the University of Queensland.
In the zebrafish, each eye has a nerve that connects it to the opposite 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 his right eye to the left side of his brain.
Other, more “ancient” fish do things differently. They have so-called ipsilateral or bilateral visual projections. Here, each eye has two nerve connections, one goes on either side of the brain, which is also what people have.
Armed with an understanding of genetics and evolution, the team can look back in time to assess when these bilateral predictions first appear. Looking forward, the team is excited to build 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 reaction and warm support when we presented the first results. We can’t wait to continue the project with him.”
Both Braasch and Chédotal noted how powerful this study was thanks to the stable collaboration that allowed the team to study so many different animals, which Braasch said is a growing trend in this area.
The study also reminded Braasch of another trend.
“We’re increasingly finding that a lot of things that we think have evolved relatively late are actually very old,” Brush said, which actually makes him feel a little more connected to nature. “I learn something about myself when I look at these strange fish and understand how old parts of our own bodies are. I’m happy to tell the story of eye evolution with a new twist this semester in our comparative anatomy class.”
Fish reveal secrets for limb regeneration
“Bilateral visual projections exist in disembodied bony fish and precede the appearance of tetrapods” Science (2021). science.sciencemag.org/cgi/doi … 1126 / science.abe7790
Provided by Michigan State University
Quote: A discovery that “literally changed the textbook” (2021, April 8), retrieved on April 8, 2021 from https://phys.org/news/2021-04-discovery-literally-textbook.html
This document is subject to copyright. Except for any fair transaction for the purpose of private examination or research, no part may be reproduced without written permission. The content is provided for informational purposes only.