From History: ~1.3 Billion years ago (+/- 200 million)
By 1.3 billion years ago, as Earth’s chemistry shifted, Bacteria split into major phyla like the hardy, spore-forming Firmicutes, the chemical-producing Actinobacteria, and the fiber-digesting Bacteroidota. This massive diversification filled every niche from deep-sea vents to the first soils, establishing the complex microbial networks that would eventually allow complex life to survive.