Context: What drove circadian rhythms to evolve has been an enigmatic question. Previous hypotheses emphasized that photosensitive proteins and circadian rhythms may have originated together in the earliest cells, with the purpose of protecting replicating DNA from high levels of damaging ultraviolet radiation during the daytime. As a result, replication was relegated to the dark. However, evidence for this is lacking, since the simplest organisms with a circadian rhythm, the cyanobacteria, do the opposite of this - they divide more in the daytime. Recent studies instead highlight the importance of co-evolution of redox proteins with circadian oscillators in all three kingdoms of life following the Great Oxidation Event approximately 2.3 billion years ago. The current view is that circadian changes in environmental oxygen levels and the production of reactive oxygen species (ROS) in the presence of daylight are likely to have driven a need to evolve circadian rhythms to preempt, and therefore counteract, damaging redox reactions on a daily basis.

Question: What is theorized to have evolved with circadian rhythms?
Answer: photosensitive proteins

Question: What is thought that circadian rhythm evolved to protect?
Answer: replicating DNA

Question: From what did  DNA need to be protected in the earliest cells? 
Answer: ultraviolet radiation

Question: One possible reason for the development of the circadian system is the need to counteract what ?
Answer: redox reactions

Question: What environmental event occurred 2.3 million years ago?
Answer: Great Oxidation Event

Question: What type of proteins developed as a result of circadian rhythms?
Answer: Unanswerable

Question: What developed to protect replicating DNA from ultraviolet radiation and night?
Answer: Unanswerable

Question: What type of replication is only done during the day?
Answer: Unanswerable

Question: What simple organism provides evidence the DNA replicates at night?
Answer: Unanswerable

Question: What event occurred approximately 2.3 million years ago?
Answer: Unanswerable