Could photoperiodic responses have evolved before the emergence of circadian clocks?

Summary Plants use photoperiod (i.e. day length) as a seasonal cue for timing when to flower. This ability, known as photoperiodism, also underlies phenomena such as migration, seasonal reproduction, and hibernation in animals. Because a circadian (daily) clock underlies the day/night length timing mechanism in most organisms, it has been generally assumed that circadian rhythms evolved before the ability to measure the photoperiod. Our recent discovery that adaptive photoperiodic responses extend as far back as bacteria, with cyanobacteria showing a photoperiodic response remarkably similar to those of eukaryotes, has led us to question this assumption. In this Tansley insight, we put forward a new hypothesis for how photoperiodism might have evolved which is based on the evolutionary implications of bacteria being capable of photoperiodic responses.