The network of nerves that connects 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 fish gar.
Ingo Braasch of Michigan State University helped an international research team show that this linking scheme was already present in ancient fish at least 450 million years ago. That makes it about 100 million years older than previously believed.
“It’s my first time that one of our publications is literally changing the textbook I’m teaching,” said Braasch, as an assistant professor in the Department of Integrative Biology at the College of Natural Sciences.
This paper was published in the journal Science April 8, also means that this type of eye-brain connection precedes animals that live on land. There was a theory that this connection first evolved in terrestrial beings and from there was transmitted to humans where scientists believe it helps in our perception of depth and 3D vision.
And this work, led by researchers from the French public research organization Inserm, more than reshapes 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 links to human conditions from these models can be a challenge.
For example, the zebrafish is a popular model of an animal, but their ocular-cerebral wiring is very different from that of a human. In fact, it helps explain why scientists thought the human connection first evolved in four-legged terrestrial creatures or tetrapods.
“Modern fish do not have this connection between the eye and the brain,” 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 on another type of fish known as gar. Gar evolved more slowly than zebras, meaning that gar are more similar to the last common ancestor shared by fish and humans. These similarities could make Gar a powerful animal model for health studies, which is why Braasch and his team are working to better understand Gar’s biology and genetics.
This, in turn, is why Inserm researchers sought Braasch for this study.
“Without his help, this project would not have been possible,” said Alain Chédotal, research director at Inserme and group leader at the Vision Institute in Paris. “We didn’t have access to the spotted woodpecker, a fish that doesn’t exist in Europe and occupies a key position on the tree of life.”
To conduct the study, Chédotal and his colleague Filippo Del Bene used a revolutionary technique to see the nerves that connect the eyes and brain in several different species of fish. This included well-studied zebras, but also rarer specimens such as Braasch clothing and Australian lung fish provided by an associate from the University of Queensland.
In the zebrafish, each eye has one 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 the other nerve connects its right eye to the left side of the brain.
Other, “older” fish do things differently. They have so-called ipsilateral or bilateral visual projections. Here each eye has two nerve connections, one that goes to either side of the brain, which is also what people have.
Armed with an understanding of genetics and evolution, the team could look back in time to assess when these bilateral projections first appeared. Looking forward, the team is excited to upgrade 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 extremely motivating to see Ingo’s enthusiastic reaction and warm support when we presented him with the first results. We can’t wait to continue the project with him.”
Both Braasch and Chédotal noticed how powerful this study was thanks to the strong collaboration that allowed the team to examine so many different animals, which Braasch said is a growing trend in the field.
The study also reminded Braasch of another trend.
“We’re increasingly discovering that many of the things we thought developed relatively late are actually very old,” Braasch said, making him feel a little connected to nature. “I learn something about myself when I look at these weird fish and understand how old the parts of our own body are. I’m excited to tell the story of the evolution of the eyes with a new shift this semester in our comparative anatomy class.”