What if another creature lives inside your cells?

That time, that place

A young American biologist in the 1960s. Her eye kept returning to one small part inside the cell: the mitochondrion, the grain a cell uses to breathe.

It was strange. The grain sat inside the cell yet carried its own genes. And it multiplied by splitting in two, like a bacterium.

Why this question

If it looks like a bacterium, multiplies like one, and keeps its own genes like one — was it once a bacterium?

Long ago, she proposed, a large cell swallowed a small bacterium and failed to digest it. The bacterium settled inside and took over the job of breathing. Two became one.

What was found

Her paper was rejected more than fifteen times. Common sense said a cell was one organism, not several joined together.

Then in the 1970s genes could be read, and the answer came. The mitochondrion's genes **looked more like a bacterium's** than like the genes of the cell around it. The chloroplasts of plants told the same story.

Inside every cell of your body live the descendants of a bacterium that moved in billions of years ago. Every breath you take, they are working.