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In order to see the world as clearly as we do, we process vision from each eyeball on both sides of our brain—a capability known as bilateral visual projection. For a long time, researchers thought this feature developed after fish transitioned to land, more than 375 million years ago. But does this theory hold water today?

In a new study, scientists injected fluorescent tracers into the eyes of 11 fish species to illuminate their visual systems. After examining their brains under a specialized 3D fluorescence microscope, they found ancient fish with genomes more similar to mammals can project vision on both the same and opposite side of their brain (see video, above). This suggests bilateral vision did not coincide with the transition from water to land, researchers report this week in Science. In the future, scientists plan to uncover the genes that drive same-sided visual projection to better understand how vision evolved.