The Optic Chiasm is the part of the brain where the optic nerves cross over - i.e. the nerves from the right eye route to the left side of the brain and vice versa. It's not only found in all vertebrates, but also in fossils of the very earliest vertebrate ancestors.

A number of different (and conflicting) theories have attempted explanations for the evolutionary reasons behind the cross-over.

As an example, the 'Visual Map Theory' was proposed in 1989 - it suggests that "the function of the optic chiasm is to repair the retinal field image on the visual cortex". See Wikipedia

All theories proposed to date have self-evident weaknesses.

20th century experiments with specially constructed prism-based spectacles which swap left and right images (or even invert them upside-down) show that people can adjust to the swapped images in just a few days. Meaning the brain's processing power apparently eliminates any 'need' for the physical nerve crossover.