Look at a mitochondrion — the power plant inside your cells — under a microscope, and something's off. It has its own DNA, separate from the DNA in the nucleus. It has its own ribosomes, which build proteins using a slightly different genetic code. It reproduces independently, on its own schedule, splitting when it wants to. It has a double membrane, the inner one looking suspiciously like a bacterial cell wall.

That's because, two billion years ago, a mitochondrion was a bacterium. A free-living, oxygen-using prokaryote, swimming around in ancient oceans. Then one day it got swallowed by a larger single-celled organism — probably a host that expected to digest it. Instead, for reasons we still debate, the bacterium survived inside. It kept burning oxygen. It kept producing energy. And it started sharing that energy with its host in exchange for shelter and food.

This is the endosymbiotic theory, proposed by Lynn Margulis in 1967 and dismissed for a decade before genetic evidence forced the scientific establishment to accept it. Mitochondrial DNA is bacterial DNA, slightly modified. Your mitochondria are descendants of a bacterium that made a deal.

Every single complex cell on Earth carries this same ghost. Plants have two of them — mitochondria plus chloroplasts (descended from photosynthetic bacteria). The oxygen you breathe is processed by what is, biologically, a captured organism living in your cells.

You inherited yours from your mother. Mitochondrial DNA is passed maternally — fathers contribute nothing. Which means you can trace an unbroken line of mitochondrial ancestors back through every mother in your family tree, for billions of years, to that single ancient swallowed cell.