Comparative analysis of the structure and functioning of sensory systems provides key information about natural history and evolution. During their origins from lizard ancestors, snakes lost many eye structures and genes, possibly as adaptation to life in low light conditions. Remarkably, living snakes have an extraordinary diversity of retinal and photoreceptor types, but they remain very understudied compared with other vertebrate groups. A lack of studies on the detailed anatomy of snake retinas and the patterns of expression of key visual genes, especially in ‘lower’ snakes, precludes accurate and precise interpretations of the origins, early evolution, diversity and function of snake visual systems. SNAKE-EYES will substantially populate this knowledge gap by generating the biggest dataset to date on the ultrastructure and spatial patterns of gene expression of snakes, using cutting-edge electron microscopy and immunohistochemistry imaging techniques to achieve three main research goals. 1) By sampling ‘lower’ snakes, SNAKE-EYES will use groundbreaking new information about the structure of the retina, ultrastructure of photoreceptors and expression of opsin genes, to newly infer the retina of the ancestral snake to test hypotheses of snake origins. 2) This action will generate new data on retinal structure and gene expression in fossorial lizards and snakes, to assess the extent of determinism in visual system evolution during multiple independent adaptations to fossoriality. 3) This project will provide, for the first time, data on the circuitry of snake retinas, by analyzing connectivity patterns of photoreceptors and bipolar cells in retinas with only rods and with rods and cones. In addition to advancing knowledge of vertebrate vision, this action will improve the fellow professional skills, and help her establishment as an innovative leader in evolutionary neuroscience, promoting important future collaborations among institutes in Brazil and Europe.