Asked by: Caroline Ellis, Edinburgh

Mitochondria are tiny chemical factories inside our cells. They have several roles, including extracting and storing energy from digested food. But they also have their own DNA, hinting at a once-independent existence. It’s thought that this reflects their origin as bacteria, which formed a symbiotic relationship with organisms about 2.4 billion years ago and then evolved into us.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.


Robert is a science writer and visiting professor of science at Aston University.