The first endogenous circadian oscillation was observed in the 1700s by the French scientist Jean-Jacques d'Ortous de Mairan who noticed that 24-hour patterns in the movement of plant leaves continued even when the plants were isolated from external stimuli. Circadian rhythms may be defined by three criteria:
- The rhythm persists in constant conditions (for example constant dark) with a period of about 24 hours.
- The rhythm period can be reset by exposure to a light or dark pulse.
- The rhythm is temperature compensated, meaning that it proceeds at the same rate within a range of temperatures.
Circadian rhythms are believed to have originated in the earliest cells, with the purpose of protecting replicating DNA from high ultraviolet radiation during the daytime. As a result, replication was relegated to the dark. The fungus Neurospora, which exists today, retains this clock-regulated mechanism.
The simplest known circadian clock is that of the prokaryotic cyanobacteria. Recent research has demonstrated that the circadian clock of
Synechococcus elongatus can be reconstituted in vitro with just the three proteins of their central oscillator. This clock has been shown to sustain a 22 hour rhythm over several days upon the addition of ATP. Previous explanations of the prokaryotic circadian timekeeper were dependent upon a DNA transcription / translation feedback mechanism, and although this has not been shown to be the case, it is still believed to hold true for eukaryotic organisms. Indeed, although the circadian systems of eukaryotes and prokaryotes have the same basic architecture: input - central oscillator - output, they do not share any homology. This implies probable independent origins.
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