The more we learn about Earth and life, the clearer it becomes that their mutual feedbacks, not just their individual processes, are the true drivers of change. Consider how microbial genetic innovations once triggered biological reactions that reshaped Earth’s surface. In turn, environmental shifts driven by tectonics or other geological processes created new ecological opportunities, sometimes in entirely unexpected ways. The most striking example of this interplay is the coevolution of oxygenic photosynthesis and Earth’s redox history. Ancient cyanobacteria evolved the ability to split water, releasing oxygen that accumulated in the atmosphere and oceans. Rising oxygen levels enabled the ozone layer to form and aerobic respiration to emerge, while continental growth and cooling enhanced oxidative weathering, delivering trace nutrients that fueled primary production. Positive feedbacks reinforced oxygen release, but negative ones—such as sulfate-driven anoxia stripping metals from seawater—constrained it. Thus, life simultaneously drove and moderated atmospheric change, with cyanobacteria both architects and casualties of their own success. In this talk, I will explore the main forces that shaped cyanobacterial evolution and, by extension, Earth’s oxygenation. Recent studies reveal that the rise of oxygen was not a smooth, monotonic climb, but rather a dynamic and oscillatory process. Periods of oxygen abundance alternated with episodes of scarcity, creating cycles of opportunity and stress for cyanobacteria and their microbial competitors, including methanogens and photoferrotrophs. These ecological struggles and feedbacks not only determined who thrived, but also left lasting imprints on the redox evolution of our planet.