premise: Water is part of the History of Earth.
hypothesis: History of Earth . Volcanic outgassing probably created the primordial atmosphere and then the ocean ; but the atmosphere contained almost no oxygen and so would have been toxic to most modern life including humans . Over time , the Earth cooled , causing the formation of a solid crust , and allowing liquid water to exist on the surface . The process of plate tectonics continues to play a dominant role in the shaping of Earth 's oceans and continents and the living species they harbor . Changes in the biosphere -- now dominated by human activity -- continue , in turn , to produce significant effects on the atmosphere and other systems of the Earth 's surface , such as the integrity of the ozone layer , the proliferation of greenhouse gases , the conditions of productive soils and clean air and water , and others . Water . Water covers 71 % of the Earth 's surface .