The theory that proposes the continents were once part of a single landmass is known as continental drift. Proposed by Alfred Wegener in the early 20th century, this theory suggests that all continents were once joined in a supercontinent called Pangaea before slowly drifting apart. Evidence supporting this includes similar rock formations, fossil records, and climatic patterns found across continents, indicating they were once contiguous. This theory laid the groundwork for modern plate tectonics.