Bacteria ruled the earth for two billion years and never got complex
They had energy. They had DNA. They had every biochemical trick life would ever use. And then they stopped.
For two billion years, bacteria dominated every ocean and landmass on this planet. They were metabolically brilliant: some breathed sulfur, others harvested light, others lived in boiling acid. They evolved every major biochemical pathway that life would ever deploy. And then they stopped. No true complexity emerged. No cells with internal compartments. No multicellular bodies.
Biologist Nick Lane argues the reason is energy. A bacterium powers its activities from membranes along its outer surface. To build more complex machinery—longer proteins, larger genomes, elaborate internal structures—requires more energy per gene than any bacterium can generate. They hit an energetic ceiling and could not break through it.
The exception came roughly two billion years ago, when one archaeal cell engulfed a bacterium and kept it alive. The captive bacterium eventually became mitochondria. This was not a gentle partnership—it was an accident with spectacular consequences. Mitochondria gave their host cell roughly 200,000 times more energy per gene than a free-living bacterium could manage. The host could now afford complexity: a nucleus, internal membranes, and eventually, after hundreds of millions more years, every plant, animal, and fungus that has ever existed.
The merger happened exactly once in four billion years of life on Earth. Everything we see in the living world descends from that single event.