The cells in the retina that enable night vision may have evolved from those that sense colour.
Typically, most of the light-sensing cells in mammalian retinas are rod cells, which are sensitive in low light. However, vertebrate ancestors only had cells resembling cones, which function under bright light and can discriminate colour. Ted Allison at the University of Alberta in Edmonton, Canada, Anand Swaroop at the US National Eye Institute in Bethesda, Maryland, and their co-workers studied mouse rod and cone cells, and monitored these cells in the developing mouse retina. They found that early in rod cells' development the cells expressed key genes that are normally active in 'S' (blue) cones. Zebrafish rods, however, did not.
The adaptation of cone cells to function under low light may have allowed mammals to adopt nocturnal lifestyles during mammalian evolution.