Final answer:
Alfred Wegener proposed the continental drift theory, suggesting all continents were once part of a supercontinent called Pangaea, based on evidence like the fit of continents and fossil records. His idea was only widely accepted after the development of plate tectonics theory, which provided the mechanism for continents shifting.
Step-by-step explanation:
The theory proposed by Alfred Wegener that explained the continents were once joined as one supercontinent is called continental drift. Wegener suggested that all continents were part of a single supercontinent named Pangaea around 200 million years ago. His hypothesis was supported by several pieces of evidence, such as the jigsaw-puzzle-like fit of the continents, matching fossils on continents now separated by oceans, similar mountain ranges across different continents, and paleoclimate data that indicated past geographical proximity to the poles or the equator. Wegener published his groundbreaking book in 1915, presenting these ideas which were initially met with skepticism due to the absence of a proposed mechanism for the drifting of continents.
The concept of plate tectonics, which provides the mechanism for Wegener's theory, wasn't developed until the 1960s, thereby confirming and expanding upon Wegener's initial ideas. This theory describes the Earth's lithosphere being divided into tectonic plates that move over the asthenosphere due to various geological forces.