The network of nerves connecting our eyes and brain is complex. However, researchers recently found that it arose much earlier than previously thought. In this, they were helped by the study of cayman fish.
A team of scientists found that the network of nerves connecting the eyes and the human brain was already present in ancient fish at least 450 million years ago. This means that such a system is about 100 million years older than previously thought.
Studying animal models is invaluable for researchers to learn more about health and disease, but linking to human conditions from these models can be challenging. For example, zebrafish are popular model organisms, but their wiring between the eye and the brain is very different from that of humans.
Modern fish do not have this type of connection between the eye and the brain. This is one of the reasons why people thought such a network was something new for four-legged animals.
Carapace, or carapace pike, or cayman fish, is a family of ray-finned fish, the only modern one in the carapace-like order. The group includes seven living species of fish that live in fresh, brackish, and occasionally even in sea waters.
Such fish evolved more slowly than zebrafish. This means that the carapaces are more like the last common ancestor between fish and humans. These similarities could make the cayman fish a useful animal model for human health research. Therefore, scientists are working to understand their biology and genetics better.
To conduct the study, the scientists used an innovative technique to see the nerves connecting the eyes and the brain in various fish species. Among them are the well-studied zebrafish, as well as more rare specimens – carapace and Australian lungfish.
Each eye has one nerve connecting it to the opposite side of the fish’s brain in zebrafish. One nerve connects the left eye to the right hemisphere of the brain, and the other – the right eye to the left hemisphere of the brain. Another, more “ancient” fish functions differently. Armored pikes have so-called ipsilateral or bilateral visual projections. Here, each eye has two nerve connections, one on each side of the brain, just like humans.