When evolution gets weird...

[Odin2005]

9. If you stare at a clear blue sky you can see the white blood cells in your retinal blood vessels.

It's really unnerving.

And speaking of tetrachromacy, the common ancestor of all land vertebrates and bony fish was a tertrachromat, but we mammals were reduced to dichromacy when our shrew-like ancestors became nocturnal burrowers. Then our primate ancestors evolved trichromacy.

In fact, the higher primates are the only mammals that have color vision better than dichromacy.