Final answer:
Alfred Wegener is the scientist who theorized that the continents were once part of a large landmass called Pangaea. His theory of continental drift laid the groundwork for the later understanding of plate tectonics.
Step-by-step explanation:
The scientist who came up with the theory that continents were once one large landmass is Alfred Wegener. Wegener proposed the scientific theory for the slow shifting of continents, which is known as continental drift. Fascinated by the world's geology from a young age, he went on to propose the existence of a supercontinent he called Pangaea, based on evidence such as the jigsaw-like fit of the continents, matching fossils, similar mountain ranges across oceans, and paleoclimate data. Despite bringing forward four pieces of evidence to support his theory in 1915, Wegener's proposal was initially met with skepticism because he could not provide a mechanism for how continents moved. His pioneering theory served as the foundation for modern plate tectonics, which was more widely accepted after the 1960s.