
Roughly two billion years ago, one single-celled organism swallowed another and instead of digesting it, ended up living with it permanently inside — and the bacterium that almost became a meal became the mitochondria that now power nearly every cell in nearly every plant and animal on Earth, still carrying a small remnant of their own ancient bacterial DNA, separate from ours.
Roughly two billion years ago, one single-celled organism ended up living permanently inside another.














