LAUSR

Significance Many cetacean species can dive to extraordinary depths on a single breath, but the evolutionary origins of deep-sea foraging in ancestral cetaceans remain unclear. We present a resurrected ancestral cetacean visual protein (rhodopsin) to investigate this critical innovation from a visual ecological perspective. Our results indicate that the ancestor of modern cetaceans was a deeper diver; we discovered both a deep-sea spectral shift and accelerated retinal kinetics over the terrestrial-to-aquatic transition. These findings suggest that ancient whales were active at mesopelagic depths and had evolved a faster dark adaptation rate, a trait that allows diving mammals to rapidly adjust to dimming light. As such, our study provides compelling evidence for deep-diving behavior before the divergence of toothed and baleen whales.