At first glance, the eyes of an octopus bear a striking resemblance to our own. A closer look reveals profound differences. Illustration by Alison Schroeer
Eyes are a primary organ of the human experience: Not only are we profoundly visual animals, but eyes are organs of expression, they are indicators of health and beauty, and we have a cultural tradition of regarding them as windows into the mind. We value eyes in other animals, too. If you have a pet, you know that one of the signs of its domesticity and your friendliness toward one another is a willingness to look into each other's eyes and acknowledging each other's gaze, while looking into the eyes of a wild animal is a challenge and a threat. The eyes that we like most are those most similar to our own, that share an evolutionary affinity with us, and the farther we move from our own lineage, the more disquieting the face and eyes of an animal can be. Eyes are actually remarkably diverse and, although one might think that visual function is fairly straightforward, and that there wouldn't be that many ways to put together a visual sensor, nature has done it.
While eyes are common in larger animal species, about a third of all animal phyla lack eyes altogether; sea urchins do not bother with them, nor do many worms. Another third have eyes that look rudimentary to us; spots and patches and pits that can sense whether it's night or day or whether a shadow is passing overhead, but that do not form any kind of image. The final third have true image-forming eyes that can capture a picture of what's going on around them and pass that on to some kind of brain or nerve net. The phyla that have true eyes are a diverse subset of the multicellular animals, including jellyfish and sea anemones, molluscs, annelid worms, onychophora (velvet worms), arthropods, and us chordates, which is a strange distribution. It's as if eyes popped up in scattered lineages interspersed with groups that lack them. For a long time, one of the hypotheses to explain all these eyes was that they evolved independently, multiple times within the animal kingdom.
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If you look at the deep structure of eyes and their cellular components, that impression is reinforced. For instance, from the outside, octopus eyes look remarkably human. They are fluid-filled eyeballs with an iris, a pupil, and a lens. On the inside, they are fundamentally different. The light-sensitive layer, the retina, is inside out in humans relative to the octopus. Our photoreceptor cells all point to the back of the eye, while theirs point forward toward the lens. Octopus eyes also have all the nervous wiring exiting out the back, rather than being draped over the photoreceptors. They have an organization that is visually superior to ours, but the important point is that the structural details are so radically different that we know cephalopod eyes did not evolve from chordate eyes or vice versa. They evolved independently from simpler precursors along different lines, and the external similarities are a result of convergence, not evolutionary relationship.