The distinctiveness of their eyes has played a major role in debates about snake origins and early evolution, having been interpreted as providing evidence for nocturnal and/or fossorial (and to a lesser degree, aquatic) origins. Much of this evidence came from anatomical studies of snake retinas in the 1900s. More recent morphological and molecular studies have provided further evidence for the distinctness of the snake eye that lacks many of the traits present in lizards. Data remain patchy and are particularly sparse for extant lineages (scolecophidians and non-caenophidian alethinophidians) that bear special importance for inferring traits of the ancestral snake. However, evidence is strong for: (1) the ancestral snake having lost multiple anatomical and molecular genetic components present in the eyes of the ancestral squamate and retained by most lizards; (2) the eye of the ancestral snake being adapted for low-light environments and/or activity cycles but being notably less regressed than that of extant scolecophidians; (3) an elaboration and diversification of the eye within endoglyptodont caenophidian snakes.