Continental drift is the earliest version of what is now known as the theory of plate tectonics. It was first postulated in the late 16th century as world maps were becoming more accurate and the shapes of opposing coastlines on different continents looked somewhat complimentary in shape. This view was further bolstered in later centuries when similar fossils were discovered on different continents separated by vast ocean. Plate tectonics tells us that the different land masses have come together and broken apart several times in Earth's past, with the most recent super-continent being Pangaea. The Earth's crust, or lithosphere, is composed of large plates that float on top of the asthenosphere. These plates move very, very slowly over the course of geological time, thus changing their positions relative to each other (or drifting).