Eyewitness Ocean

Today’s oceans started to take shape in the last 200 million years of Earth’s 4,600-million-year existence. But long before, as the early planet cooled, water vapor in the atmosphere condensed, clouds formed, and rain filled the oceans. Water also came from space in icy comets. As the land masses drifted, new oceans opened up, old oceans disappeared, and ocean life changed, too. Simple organisms first appeared in the oceans 3,500 million years ago, followed by ever more complex life forms.