How night vision was developed in early mammals?

Night vision evolved millions of years ago in mammals to help them avoid being eaten by dinosaurs.

A recent research solved the mystery, suggesting that rods in the mammalian eye, extremely sensitive to light was developed from color-detecting cone cells during this time to give mammals an edge in low-light conditions.

Cone cells are specialised for certain wavelengths of light to help animals detect color, while rods can detect even a single photon and are specialised for low-light vision.

"The majority of mammals have rod-dominant retinas, but if you look at fish, frogs, or birds, the vast majority are cone-dominated, so the evolutionary question has always been, 'What happened?'" says Anand Swaroop, a retina biologist at the National Eye Institute, part of the National Institutes of Health.

Previous work done by Swaroop and his colleagues showed that a transcription factor called NRL pushes cells in the retina toward maturing into rods by suppressing genes involved in cone development.

To investigate the origin of rods in mammals, Swaroop and his team examined rod and cone cells taken from mice at different stages of development.

Details of an organism's embryonic development often reveal traits carried by its evolutionary ancestors; consider, for instance, how human embryos initially develop gill-like slits and a tail.

The researchers saw that in early stages, two days after the mice were born, developing rod cells expressed genes normally seen in mature short-wavelength cones (which are used in other animals to detect ultraviolet light).

When the researchers examined the epigenetics of purified rod cells from mice, they saw that these aspects became repressed by histone and DNA methylations later in development, ten days after the mice were born.

The team concluded that in mammals, the transcription factor NRL became restricted to the photoreceptors in the eye, forcing the cells to change from cones to rods and giving early mammals the edge they needed to take up an active nighttime lifestyle.

The study has been published in Developmental Cell.