Biologists have argued for decades over whether the two basic eye plans of vertebrates and invertebrates evolved independently or originated from a common ancestor. New data showing unexpected similarities between the eyes of a marine worm and those of humans suggest a shared origin.
Despite incredible variation in size and shape, eyes come in just two basic models. The vertebrates' photoreceptor cells, typified by rods and cones, are quite distinctive from the invertebrates'. And although both use light-sensing pigments called opsins, these differ in their amino acid makeup. Some scientists insist that eyes evolved only once, despite this modern difference, while others argue that eyes evolved at least once in invertebrates and again in vertebrates.
Detlev Arendt and Joachim Wittbrodt, developmental biologists at the European Molecular Biology Laboratory in Heidelberg, Germany, jumped into the fray after Arendt noticed some vertebrate-like photoreceptor cells in the brains of ragworms, a marine species that hasn't changed much for 500 million years. Arendt and his colleagues then looked for genes and proteins that say something about the origin and function of the cells.
The ragworm's brain photoreceptor contains an opsin pigment that is almost identical to the human version. Arendt and Wittbrodt also found retinal homeobox proteins in the ragworm's brain but not its eye, the researchers report in the 29 October issue of Science. This is the first time these proteins, which are key to building the nascent retina in vertebrates, have been found in invertebrates.
Their findings suggest that even the earliest animals had the makings of both vertebrate and invertebrate visual systems, and that some of the photoreceptor cells in the invertebrate brain were transformed through a series of steps into vertebrate eyes. Arendt and Wittbrodt propose that both optical systems existed in an extinct common ancestor called Urbilateria and likely arose in Urbilateria's predecessor. One probably sensed the light needed to set up a circadian rhythm, and the other might have been a primitive prelude to the eye. "We think both photoreceptor cells track back to one cell type," notes Wittbrodt.
The results drive home the antiquity of invertebrate and vertebrate photoreceptors and opsins. "Not only the morphology but also the molecular biology of the two types of receptors was already set in our common ancestor," says Alain Ghysen, a neuroscientist at the University of Montpelier, France. But whether or not this points to a single origin of the eye remains a subject for debate.