Early mammals are thought to have lived mainly nocturnal lives

Although the details remain up for debate, understanding the evolution of mammalian vision during an extended period of nocturnality could also help scientists to predict how modern mammals might adapt as increasing light pollution and human activity pushes some diurnal animals to reclaim the night. In his 1942 monograph, The Vertebrate Eye and its Adaptive Radiation, Gordon Walls, a biologist at the then Wayne University in Detroit, Michigan, proposed that there had been a prolonged period of nocturnal living during the early evolution of mammals, to explain why mammals’ eyes tend to differ from those of other vertebrates. And the eyes of most mammals — although notably not those of humans or certain other primates — lack a fovea, an area of the retina rich in cone cells that provides sharp and detailed vision to fish, birds and reptiles that hunt during the day.

Source: www.nature.com