A team from The University of Manchester has found that mammalian sight must have something missing compared with other vertebrates, like birds, fishes and amphibians.

The finding seems to confirm the longterm hypothesis that early mammals were nocturnal. "The classical view of how the eye sees is through photoreceptive cells in the retina called rods and cones," explained Dr Jim Bellingham.

"But, recently, a third photoreceptor was discovered that is activated by a gene called melanopsin. This melanopsin photoreceptor is not linked to sight but uses light for non-visual processes, such as regulating our day-night rhythms and pupil constriction."

The gene encoding melanopsin is found in all vertebrates, but the mammalian version was very peculiar compared to those found in fish, amphibians and birds. "At first, we put this genetic anomaly between mammals and other vertebrates down to evolutionary differences," said Bellingham.

"But we have now learnt that other vertebrates have a second melanopsin gene -- one that matches the one found earlier in mammals and humans. The first melanopsin gene found in the other classes of vertebrates does not exist in mammals."

Scientists are curious about how these two melanopsins differ in their function and which are the implications of having just one melanopsin gene but they guess they allow vertebrates to detect different wavelengths of light and see more colors.

The two melanopsin genes in non-mammals might have the same role or bear different functions in non-visual light detection. "The two genes and their associated proteins have been maintained in vertebrates for hundreds of millions of years, only for one of them to be lost in mammals."

"We are keen to discover why this might have happened -- perhaps the early mammals were at one stage nocturnal and had no need for the second gene, for instance. We also want to find out what losing one of these genes means for humans."