> What environment existed 4 billion years ago that is totally gone today? One with no complex lifeforms but sufficient resources to support life seems like th

> What environment existed 4 billion years ago that is totally gone today? One with no complex lifeforms but sufficient resources to support life seems like the obvious answer. Also, you can't really ignore the transformative effect that life has had on all environments (underwater and on land) in terms of chemical composition. Less water, less oxygen, etc. Presumably the evolution of cellular life was synergistic with the evolution of the environments on the planet?