Lizards and several other lower vertebrates have a third, or parietal, eye. Whereas their other two eyes provide high-level visual functions such as image processing, this evolutionary vestige essentially tells time. At dawn, the composition of wavelengths in the sunlight differs considerably from that in the middle of the day. The same is true at dusk. Thus, by comparing the relative numbers of these wavelengths, the parietal eye can mark the passing of each day. The precise mechanisms by which this occurs long remained unknown, however. In a 1993 Nature paper, researchers at Syracuse University in New York described the parietal eye. They found that, although the photoreceptors involved in image processing in conventional vision hyperpolarize when exposed to light — that is, they cause the electrical potential inside the cell to turn more negative — the photoreceptor of the parietal eye can either hyperpolarize or depolarize, depending on the wavelength. Furthermore, they noted different light-signaling pathways for these mechanisms. To date, no other photoreceptor has shown this sort of “chromatic antagonism.” Scientists have described the “third eye” that can be found in lizards and in some other lower vertebrates. Unusual photoreceptors, which mark the passing of the day rather than provide high-level visual function, may be the missing link (with respect to seeing) between vertebrates and our invertebrate evolutionary forebears. King-Wai Yau, a researcher at Johns Hopkins University in Baltimore, took up the question in 1998: Specifically, he wanted to determine which molecular components are involved in the pathways. He and colleagues at Kyoto University in Japan and at Rockefeller University in New York reported their findings. Their study showed that each of the pathways consists of a pigment, a G-protein, an enzyme and an ion channel. The pigment is different in each. The blue-sensitive pigment is called pinopsin and also can be found in chickens. The green-sensitive pigment turned out to be completely new. Named parietopsin by the researchers, it does not bear a strong resemblance to any other pigments. In fact, Yau said, “it seems to be a rather ancient pigment.” The G-protein also is different in each of the pathways. In the blue-sensitive pathway, pinopsin interacts with a G-protein called gustducin, closing the ion channel and causing hyperpolarization. In the green-sensitive pathway, it interacts with a Go (the o stands for other), opening the ion channel and causing depolarization. “What is really interesting,” Yau noted, “is that the parietopsin-Go signaling pair has a parallel in the scallop,” an invertebrate. As vertebrates emerged from invertebrates, the question has always been: How did they evolve the novel components of the image processing eye? “There must be a transition somewhere,” he continued. “In fact, this parieto-receptor represents the missing link,” as it embodies the lower animal pathway as well as a precursor to the pathway currently found in higher animals. Early vertebrates likely had both pathways. As the latter continued to evolve, the Go pathway was, in most cases, dropped. Although Yau’s work is primarily in the area of biomedicine — he describes the current study as a “naturalist adventure” — he plans to continue his investigations of the parietal eye. He and his colleagues have identified the components of the light-signaling pathway, but many of the dynamics of the signaling process remain unknown. Understanding these dynamics would satisfy a general “biological curiosity” and could shed light on signaling in the retina, one of the major emphases of his overall research. Science, March 2006, pp. 1617-1621.