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.

四十亿年前，在地月系统形成后不久，据推测当时的白昼时长接近四小时。地球生命在此后不久便得以起源。自彼时起，白昼时长持续演变，最终成为我们今日熟知的24小时制一天。历经所有上述演化变迁的原核生物（prokaryotes）类群之一便是蓝细菌（cyanobacteria），它们属于地球上最古老的生命体之一。蓝细菌是一类极为独特的细菌类群：它们是目前已知唯一能够进行产氧光合作用（oxygenic photosynthesis）的原核生物，同时也是目前已知唯一拥有昼夜节律钟（circadian clock）的原核生物。