The sophisticated network of nerves that connect the eyes to the brain evolved 100 million years earlier than previously thought

The zebrafish are a popular model animal, for example, but their eye-brain wiring is very different from that of a human. Modern fish do not have this type of eye-brain connection. It allowed us to discover something that we did not know until now.

The dominant theory was that this connection first developed in terrestrial creatures and, from there, passed on to humans, where scientists believe it helps our depth perception and 3D vision. But it seems that 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 living on earth. To conduct the study, the researchers used an innovative technique to visualize the nerves that connect the eyes to the brain in several different fish species.

In a zebrafish, each eye has a nerve that connects it to the opposite side of the fish’s brain. Simply put, 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 “old” fish do things differently. They have what are called ipsilateral or bilateral visual projections. Here, each eye has two nerve connections, one on either side of the brain, which is what humans also have.

Then the brain of the gar fish. In this microscopic image, the left hemisphere of the brain glows green and the right hemisphere glows magenta. However, at the bottom of the image, ribs of both colors can be seen connecting to both hemispheres. This shows that the boy’s two eyes are connected to both sides of his brain, just like a human’s eyes.

According to the authors:

We’re finding more and more that a lot of things that we think evolved relatively late are actually very old. We learn something about ourselves by watching these strange fish and understanding the age of parts of our own body.

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