Look at this squid's eye. Just look at it. See anything eerily familiar?
Squid, along with the rest of the family Cephalopoda, haven't shared a common ancestor with us vertebrates in some 500 million years—long before the evolution of our camera-like eyes. And yet, there the cephalopods are, flagrantly swimming about with eyes that use a lens to project an image onto a retina. Call it Squid Eye for the Vertebrate Guy. So, how's it work?
Convergent evolution, my friends. Convergent evolution. We happened to hit on similar solutions to the same problem of sight, even though the eyes of vertebrates and cephalopods evolved separately, in very different ways, at different times. Today, we can see that legacy in cephalopod and vertebrate fetal development. With vertebrates, the eyes grow on stalks, reaching out from the brain. In cephalopods, the eyes start as a clumping of cells on the surface of the skin and reach backwards, into the head, to make brain contact. Similar destinations. Very different road maps.
This lovely illustration—featuring dissections of the head, funnel, mantle and eye of a Thaumatolampas diadema—comes from The Cephalopoda Part I: Oegopsida and Part II: Myopsida, Octopoda Atlas written in 1910 by zoologist Carl Chun following a German expedition to the Indian, Atlantic and Great Southern oceans.
You can see more of Chun's detailed, passionate illustrations at the BibliOdyssey blog.