The Evolution of Night Vision in Primates

According to a story in e! Science News, researchers investigating the embryological development of Owl Monkeys and Capuchin monkeys have determined that subtle changes in the developmental timing of the growth of rods and cones result in large changes in night vision capability:

By comparing the timing of retinal cell proliferation in the two species, the researchers found evidence that an extended period of progenitor cell proliferation in the owl monkey gave rise to an increased number of rod and other associated cells that make its eyes adept at night vision; the eyes also evolved to be large, with bigger light-gathering and light-sensing structures needed for nocturnal sight.

"The beauty of the evolutionary mechanism we have identified is that it enables the eye to almost toggle back and forth between a nocturnal and a diurnal structure," said neurobiologist Michael Dyer of St. Jude's hospital. "It is an elegant system that gives the eye a lot of flexibility in terms of specialization."

More pieces of the puzzle.