There have been many ideas about the origin and nature of the Earth, many them with religious origins. It was widely believed in Europe until the Middle Ages that the Earth was flat, and many in India believed that the Earth was carried on the back of an elephant.
Some Greeks reasoned that the Earth was round. Aristotle reasoned this, since when a ship sailed away, the mast was the last part to disappear over the horizon. The Greeks were also the first to find a value for the radius of the Earth.
In 1600 William Gilbert showed that the Earth had a magnetic field that would be produced by a giant magnet inside it. This meant there had to be a magnetic North pole and a magnetic South pole, and suggested that the Earth had a finite size.
From about 1800, studies of earthquakes suggested that the Earth had structure and testable scientific theories were developed about the birth of the Solar Systen and the planets, including the Earth.
In 1915, the theory of continental drift was put forward by Alfred Wegener. This suggested that all the continents were all part of a single supercontinent, which was broken up. Wegener originally supposed the continents slid over the ocean floor and the idea of continental drift only gained supporters when tectonic plates were discovered.
In 1928 it was proposed that the decay of radioactive materials inside the Earth kept the interior molten and mean that the Earths magnetic field could shift.
In the 1950s observations of deep sea ocean ridges showed that the ocean floor was spreading and supported by the discovery of magnetic 'stripes' in the rock. Later this led to the theory of tectonic plates.