Cyanobacteria are among the oldest photosynthetic organisms on Earth, playing a crucial role in shaping the planet’s biogeochemical cycles. Their evolutionary history dates back over 3.5 billion years, making them key contributors to the Great Oxygenation Event (GOE) around 2.4 billion years ago. Through oxygenic photosynthesis, cyanobacteria transformed Earth’s early anoxic atmosphere, leading to significant ecological and evolutionary changes, including the rise of aerobic life. Their ability to fix nitrogen further enhanced their impact on nutrient cycles, influencing primary productivity in both marine and terrestrial ecosystems. Over time, cyanobacteria diversified into various ecological niches, evolving specialized metabolic pathways to adapt to changing environmental conditions. They established symbiotic relationships with eukaryotes, ultimately giving rise to plastids in algae and plants through endosymbiosis. This evolutionary milestone had profound implications for the biosphere, leading to the expansion of oxygen-dependent life forms and the development of complex ecosystems. In modern biogeochemistry, cyanobacteria continue to play a vital role in carbon and nitrogen cycling, influencing global climate regulation. However, anthropogenic factors such as nutrient pollution and climate change have led to the proliferation of harmful cyanobacterial blooms, posing ecological and economic challenges. Understanding the evolutionary history and biogeochemical significance of cyanobacteria provides insight into their resilience and adaptability, offering potential applications in biotechnology, biofuel production, and environmental management. This chapter explores the evolutionary trajectory of cyanobacteria and their lasting influence on Earth’s biosphere, highlighting their fundamental role in shaping the planet’s atmospheric and ecological dynamics.