Convergent evolution
Eyes: One of the most famous examples of convergent evolution is the camera eye of cephalopods (e.g. squid), vertebrates (e.g. mammals) and cnidaria (e.g. box jellies). Their last common ancestor had a simple photoreceptive spot, but there was progressive refinement of its structure to the advanced camera eye. The similarities of such a complex structure shows how there may be some challenges which have an optimal solution.