There'd been a time before witches, but Homura only knew about it because she'd asked the right question, once, back when the Incubator was still trying to lay out the rules of science and the universe.

"Entropy is only because of living things," the Incubator explained, "but once things live, the universe begins to decay; witches weren't necessary before that, but now they'll always be, forever after."

Homura wished she'd never asked, and so she went back to a time when she hadn't.