Hmm… quite a thought-provoking question. I agree with Tim, that it depends a bit on what you mean by “earth”. Certainly, we are inside the atmosphere and, also important, inside the magnetosphere. But we live “on” the surface, which means we are outside of the earth’s crust and oceans.
Difficult one. I was going to say on it, but we also often go into it (like for mines or to build the foundations of houses or in submarines, etc. Maybe we should just say we co-exist with it and so we need to make sure we take care of it.