Primitive deep-sea fish may have viewed the world in colour. The elephant shark, which evolved about 450 million years ago, is the oldest vertebrate to have "the colour vision system we know as humans", says David Hunt at University College London.
Recently, the genome of the elephant shark, Callorhinchus milii, a chimaerid holocephalan, has been sequenced and therefore becomes the first cartilaginous fish to be analyzed in this way. The chimaeras have been largely neglected and very little is known about the visual systems of these fishes.
By searching the elephant shark genome, Hunt's team have identified gene fragments encoding a rod visual pigment and three cone visual pigments. It also has two copies of the long-wavelength cone pigment gene, a duplication which may have given them trichromatic vision like primates.