The idea that the continents were once one large landmass is primarily supported by the evidence of similar fossils found on many continents. This fossil evidence, particularly the discovery of the same species of plants and animals on widely separated continents (such as the Mesosaurus found in both South America and Africa), suggested that these landmasses were once connected.
Additionally, the observation that continents seem to fit together, like pieces of a jigsaw puzzle, supported this hypothesis. Other evidence, such as glacial markings in warmer regions and geological similarities across continents, further reinforced the idea of continental drift, which was first notably proposed by Alfred Wegener in the early 20th century.
In summary, while all the factors listed contributed to the understanding of continental drift, the similarity of fossils across continents played a crucial role in prompting scientists to consider that the continents were once joined.