From Tina Hesman Saey at Science News:
Just as Rolex, Timex, Swatch and Seiko make their own versions of a wristwatch, organisms including cyanobacteria, fungi, plants and insects have all invented their own varieties of circadian clocks. The cycling proteins are as different among these organisms as digital watches are from precision quartz clockworks. But all of them mark days with the predictable ebb and flow of messenger RNA and protein production.
There’s no doubt that today’s circadian clocks are must-have accessories for most organisms living on Earth’s surface. But does the run-away-from-the-light origin story make sense?
A main piece of evidence in favor of the “flight from light” idea is that cells tend to replicate their DNA at night safely under cover of darkness and repair it during the day as damage from UV light accumulates. Some of the same protein cogs that drive the circadian clocks are also involved in DNA repair, further solidifying the connection.