Interesting! My pet theory about the crossover in the optic nerve is that it's the simplest way to get goal-seeking behaviour. Proto-eye activates, activates muscles on opposite side of body, organism turns towards activated proto-eye.

Fun speculation: Maybe we started with no crossover (which gives avoidance behaviour, keeping the organism free-swimming). This still works for a while as the axial angle between eyes and muscle groups increases, so there's no real penalty for having a bit of a twist. As the twist increases, it starts acting a bit like a discriminator, where we avoid small things less than large things, which seems good if we want to eat small things. Past 90°, we start spiraling towards things instead of away from them, which admittedly makes us crash into large things more, but we can chase moving things. Hunting has evolved!

That’s interesting! My mind always just assumes the reason is some “core” biological reason. I think contralateral wiring helps the brain manage signal sequencing while preserving evolutionary symmetry.

The spinal cord handles rapid reflexes (pulling away from a hot stove), leaving the brain for slower non-immediate tasks. By crossing nerves before they reach the brain a standardized delay is introduced, giving the brain a predictable offset to filter against. The optic chiasm follows the same logic.

And I think this is necessary to keep same-side brain-body pairs from over-optimizing (direct nerve connection from the right hand to the right brain hemisphere) their paths at the expense of balance, preserving biological symmetry.

Invertebrates seem to have had no problems evolving hunting even though they don’t have a twisted body plan. And hunting almost certainly evolved before this change occurred in the vertebrate lineage.

Ive always wondered if it had something to do with up being the direction of the sun and light and down being the direction of gravity, depth, darkness.