Endosymbiosis

The most extreme form of symbiosis is endosymbiosis - when one organism lives inside another, and the relationship becomes so fundamental they can't separate.

Your cells contain two types of endosymbionts: mitochondria and (if you're a plant) chloroplasts. Both were once independent bacteria. Between 1.5 and 2 billion years ago, based on molecular clock estimates, an ancestral cell engulfed a bacterium capable of aerobic respiration - burning oxygen to generate energy. Instead of digesting it, the cell kept it alive. The bacterium generated far more energy than the host cell could produce alone. The host provided a stable environment and nutrients. Over time, the bacterium lost genes for independent survival, and the host became dependent on the bacterium's energy production.

Right now, in every cell of your body except red blood cells, dozens to thousands of mitochondria are burning oxygen. They're technically separate organisms living inside your cells. Bacterial immigrants that became so essential we can't imagine life without them. Every breath you take feeds these ancient invaders. Without them, you'd die in minutes.

The symbiosis became irreversible.