Final answer:
The first line of evidence that led early investigators to suspect that the continents were once connected is the remarkable "jigsaw puzzle" like fit of South America and Africa. Glacial evidence and Glossopteris fossils found on multiple continents also supported the theory of continental drift.
Step-by-step explanation:
The first line of evidence that led early investigators to suspect that the continents were once connected is the remarkable "jigsaw puzzle" like fit of South America and Africa. Geologists noticed the similarities in the shape of the coastlines of these two continents, which made them question whether they were once joined together.
Glacial evidence is another key piece of evidence. When the continents were connected, they had a single supercontinent called Pangaea. Glacial scratches and markings found on rocks in different continents indicate the presence of ancient glaciers, supporting the theory of continental drift.
Fossils of Glossopteris, a type of fern, were found on multiple continents. This discovery suggested that these continents were once connected because it is unlikely for ancient plants to have dispersed across vast oceans. Glossopteris fossils provided further evidence for the theory of continental drift.
Learn more about Plate Tectonics