What is the powerhouse of the cell?

Mitochondria take the food you have eaten and the oxygen you have breathed and use them to make ATP, the molecule almost every process in your body actually spends. A liver cell might hold a couple of thousand of them; a heart muscle cell is roughly a third mitochondria by volume.

The interesting part is where they came from. A mitochondrion has its own circular DNA, its own ribosomes, a double membrane, and divides by splitting in two - all the signatures of a bacterium. The accepted explanation is that around two billion years ago one cell engulfed another and, instead of digesting it, kept it. Every complex organism on Earth descends from that single act.

Two consequences follow. Because sperm contribute essentially no mitochondria, you inherit yours from your mother alone, which makes mitochondrial DNA a clean maternal line back through the generations and a standard tool for tracing ancient migrations. And the phrase itself has had an odd afterlife: powerhouse of the cell is one of the few pieces of biology almost everyone remembers, mostly as a joke about the only thing anyone retained from school.