Embryological development of the eye

There continues to bemuchdiscussion and controversy regarding the evolution of animal eyes: whether eyes evolved once, or at least once in invertebrates and again in vertebrates. Because of the morphological variety in eyes (135) and the different embryological beginnings of the numerous eye structures, the idea that animal eyes arose independently several times during evolution has gained some acceptance. This evolutionary debate is exemplified by the photoreceptor cells. While these cells differ dramatically between invertebrates and vertebrates, both use opsins to catch photons, even though the two opsins are fundamentally different proteins. On the other hand, Arendt et al. reported a surprising similarity between photoreceptors of a marine worm and humans, suggesting that eyes evolved only once (5). Their data show that, in addition to the usual invertebrate opsin, the worm has in the brain another opsin similar to the human protein. This suggests that photosensitive cells in the invertebrate brain could have evolved into vertebrate eyes.