That's actually not even necessarily true. You can just leave flour around in water and the natural yeast ferments. Then you can throw that in with some more water and flour and it's sourdough. Would be better with some salt though.
Or you can make super depressing unleavened flatbread.
you don't even need yeast, in sweden we have "stick bread" which is just flour, water, baking powder, and salt which you wrap around a stick and grill over a campfire. Perfectly tasty.