The theory behind continental drift posits that the Earth's continents were once part of a single landmass, called Pangaea, which gradually broke apart and drifted to their current positions. Proposed by Alfred Wegener in the early 20th century, the theory is supported by evidence such as the alignment of coastlines, similar geological formations, and fossil correlations across continents. Continental drift is fundamental to the understanding of plate tectonics, explaining how tectonic plates move and interact, causing geological phenomena like earthquakes and volcanic activity.