Built-in sunscreen for your eyes
We can only see light between red and violet. Outside those ranges, the rods and cones in our eyes aren’t going to do much. But this isn’t true of all animals, or even all mammals. A massive review has found a number of other mammals that might be able to see ultraviolet (UV) light.
Starting with eyeballs, the researched looked for protective layers that might block UV light from even entering the iris. Humans were noted as having such a layer that prevents most UV light from even hitting our light receptors, much less allowing that to be sent on to our brain (where it would likely be interpreted as part of the visual spectrum, since that’s what the human brain expects.) But a number of familiar mammals were added to the ranks of animals that likely use UV light.
Cats and dogs, for starters, mostly lack the UV-blocking layer, which may aid their night vision. Ferrets, hedgehogs and okapis also fell into this category. Their eyes let in a wider range of light allowing sight in darker conditions. So why would human evolution favor the opposite?
One possible reason is that blocking the UV light may allow us to better focus our red-green-blue-based color vision. Humans have strikingly high resolution vision, and blocking the UV may be allowing us to capture more detail that would otherwise be getting washed out.
Source: Cats and Dogs May See in Ultraviolet by Tanya Lewis, Live Science.com