The network of nerves that connect our eyes to our brains is sophisticated, and researchers have now shown that it evolved much earlier than previously thought thanks to an unexpected source: the garfish.
Michigan State University Ingo Braasch has helped an international research team show that this connection scheme was already present in ancient fish at least 450 million years ago. That makes it about 100 million years older than previously thought.
“This is the first time for me that one of our publications is literally changing the textbook I teach,” said Braasch, an assistant professor at the Department of Integrative Biology at the College of Natural Science.
This work, published in the journal Science April 8 also means that this type of eye-brain connection precedes land-dwelling animals. The existing theory had been that this connection first evolved into terrestrial creatures and from there was passed on to humans, where scientists believe it helps with our perception of depth and 3D vision.
And this work, led by researchers at France’s Inserm public research organization, does more than reshape our understanding of the past. It also has implications for future health research.
Studying animal models is an invaluable way for researchers to learn about health and disease, but drawing connections to human conditions from these models can be challenging.
Zebrafish are e.g. A popular model animal, but their eye-brain wires are very different from a human’s. In fact, it helps explain why scientists believed that the human connection first evolved into four-legged terrestrial creatures or tetrapods.
“Modern fish, they do not have this type of eye-brain connection,” Braasch said. “That’s one of the reasons 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 has evolved more slowly than zebrafish, meaning that gar is more like the last common ancestor shared by fish and humans. These similarities could make gar a strong animal model for health research, which is why Braasch and his team are working to better understand garbiology and genetics.
That is again why Inserms 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 group leader of the Vision Institute in Paris. “We did not have access to spotted yarns, a fish not found in Europe and occupying a key position in the tree of life.”
To conduct the study, Chédotal and his colleague, Filippo Del Bene, used a groundbreaking technique to see the nerves that connect the eyes to the brain in several different fish species. This included the well-studied zebrafish, but also rarer specimens such as Braasch’s gar and Australian lungfish provided by a partner at the University of Queensland.
In a 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 its right eye to the left side of the brain.
The other, more “old” fish does things differently. They have what are called ipsilateral or bilateral visual projections. Here, each eye has two nerve connections, one going to each side of the brain, which is also what humans have.
Armed with an understanding of genetics and evolution, the team could look back in time to estimate when these bilateral projections first appeared. Looking ahead, 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 response and warm support as we presented him with 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 a robust collaboration that enabled the team to study so many different animals that Braasch said 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 that we thought evolved relatively late are actually very old,” Braasch said, actually making him feel a little more connected to nature. “I learn something about myself when I look at these weird fish and understand how old parts of our own bodies are. I’m excited to tell the story of eye evolution with a new twist this semester in our comparative anatomy class. ”
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“Bilateral visual protrusions are found in non-teleostatic bonefish and precede the emergence of tetrapods” Science (2021). science.sciencemag.org/cgi/doi… 1126 / science.abe7790
Provided by Michigan State University
Citation: A discovery that ‘literally changes the textbook’ (2021, April 8) retrieved April 8, 2021 from https://phys.org/news/2021-04-discovery-literally-textbook.html
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