More like Sky Mice, surely?

I do have a biology degree, but it's been ten years since I've used it, but some of you want a tl;Dr, so here goes.

A long time ago, the ancestors of Multicellular organisms ate a small bacteria-like thing. For whatever reason, this bacteria like thing didn't die, and instead survived and reproduced. Multicellular organisms got a pretty sweet deal out of this; if we provided these little guys with sugar, they would do the hard work of turning the sugar in the energy. They'd keep a little for themselves to keep the lights on, and give the rest of the energy to us.

This relationship has continued for a long time. And while our mitochondria have their own DNA (you often find people talking about 'mDNA' - that's the mitochondria's DNA), our DNA has been slowly taking over mitochondrial functions probably since our ancestors first ate them - that's what this paper is telling us.