The zebrafish is a popular model animal, for example, but its eye-brain wiring is very different from that of a human. Modern fish do not have this type of eye-brain connection. This has allowed us to discover something that we did not know until now.
The prevailing theory had been that this connection first developed in terrestrial creatures and, from there, carried over to humans, where scientists believe it helps with our depth perception and 3D vision. But it seems that this is not what happened .
Previous connection to land animals
According to this new study , published in Science , this type of eye-brain connection predates animals that live on land. To conduct the study, the researchers used an innovative technique to view the nerves that connect the eyes to the brain in several different species of fish .
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 your left eye to the right hemisphere of your brain, and another nerve connects your right eye to the left side of your brain. The other, more "ancient" fish do things differently . They have what are called ipsilateral or bilateral visual projections. Here, each eye has two nerve connections, one on each side of the brain, which is also what humans have.
Next, the brain of the gar fish . In this microscope image, the left hemisphere of the brain glows green and the right hemisphere magenta. However, at the bottom of the image, ribs of both colors can be seen connecting to both hemispheres. This shows that both eyes of the gar are connected to both sides of its brain, as are the eyes of a human.
According to the authors:
We are increasingly finding that many things that we think evolved relatively late are actually very old. We learn something about ourselves when we look at these strange fish and understand how old the parts of our own bodies are.