The evolution of the single-celled organism, cyanobacteria, almost wiped out life on earth. Cyanobacteria were the primary cause of the first mass extinction on Earth more than 2 billion years ago. By developing the capability to convert sunlight and carbon dioxide into sugar and oxygen gas, the early development of photosynthesis, it was able to become its own powerhouse.
The cascade of outcomes that occurred from this one event almost caused all anaerobic species on earth to die off due to excess oxygen in the air, and eventually paved the way for complex life. The Oxygenation Event changed the path of life on earth from mainly anaerobic life forms to a growing number of constantly evolving aerobic beings. It turns out we might just owe the modern world to these little guys!