Origin of Earth and Circadian Clocks

Four billion years ago, shortly after the origin of the Earth-Moon System, it is predicted that the length of day was close to four hours. Life on Earth originated not long after that. Since then the length of day changed continuously to the 24 hour day as we know it today. One group of prokaryotes that lived through all these changes are cyanobacteria, which are among the oldest organisms on Earth. Cyanobacteria are a unique group of bacteria as they are the only known prokaryotes to perform oxygenic photosynthesis as well as the only known prokaryotes to have a circadian clock.