Proc. Natl Acad. Sci. USA (2017)


Try to picture our ancestors' emergence from the ocean's depths and you're guaranteed to imagine fins morphing into limbs. But research from Malcolm MacIver and colleagues indicates that a different change may have preceded our legged conversion. MacIver et al. examined the eye-socket sizes of animals spanning the transition from water to land, occurring some 385 million years ago. They found that a three-fold increase in socket size — which is highly correlated with eye and pupil size — predated the completion of the shift to limbs.

The authors computed four measures of visual function for the mean pupil size of finned and digited groups, estimated from their socket size. They calculated the visual ranges in aquatic and above-water environments using a simple model, and found that the tripling of eye size translated to a negligible performance increase for animals remaining underwater. But for animals capable of looking out to the horizon, the increase was found to be more than a million times larger. The information increase afforded by long-range vision may have prompted our evolution to terrestriality.