The answer is that these continents were once joined together in one big continent.
All continents were once connected as one supercontinent Pangea. On that supercontinent, many species lived across all the supercontinent, such as mesosaurus. But after continents spread apart, some populations remained on the different parts of the supercontinent. So, some population ended up on South Aamerica, some on Africa, and some on Antartica. The geological theory that explains how continents move through Earth's surface is called continental drift.