Cyanobacteria were among the 1st organisms to evolve on earth. The molecular circadian clock proteins of cyanobacteria and their phylogenetics have recently been elucidated. This allows for a conjecture on the evolution of 1 of the 1st circadian clocks. A scenario has now been created by combining known in vitro and in vivo properties of the 3 clock proteins of cyanobacteria (KaiA, KaiB, and KaiC). This scenario describes the evolution of the cyanobacterial clock in gradual steps: evolving from a masking mechanism, toward an hourglass, into a clock.