OXYGEN is life. That's true not just for us: all animals and plants need oxygen to unleash the energy they scavenge from their environment. Take away oxygen and organisms cannot produce enough energy to support an active lifestyle, or even make them worth eating. Predation, an essential driver of evolutionary change, becomes impossible.

It is easy to picture a planet without oxygen. It looks like Mars. Our nearest planetary neighbour was probably once a water world too, primed for life to evolve.