Cells merge
One bacterium moves into another and becomes a power plant
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The endosymbiosis theory, popularised by Lynn Margulis, explains where mitochondria came from: an archaeal host cell took up an oxygen-breathing bacterium, and the two became inseparable. Mitochondria still carry their own DNA. It happened again later with cyanobacteria, which became the chloroplasts of plants. The extra energy budget per gene that mitochondria supply is probably why eukaryotic cells could become large and complex — and why multicellular life became possible at all.
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One cell swallowed another — but instead of digesting it, it let the guest stay. The guest became a tiny power plant making energy for them both. Every animal, plant and fungus is built from cells like these.
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Part of Terra Saga — the story of Earth on a timeline. terrasaga.org