We hypothesize, based on geology, that there were prior supercontinents.
The Earth's crust is constantly in motion, rearranging and reshaping itself. Based on our current understanding of the age of the Earth and the motion of its tectonic plates, we presume that this motion follows a supercontinent cycle. This means that the land which is above sea level cycles between two extremes; a dispersed geographic arrangement, like the one we have today, and a condensed one, with virtually all of the exposed landmasses physically connected.
Direct evidence of ancient supercontinents comes in the form of geologic and paleomagnetic data. This is to say that rocks from two locations are compared; if they are found to have formed at the same time, share the same composition and chemistry, and are aligned to the same magnetic polarities, then we presume that they were physically connected when they formed. We can also look at fossil assemblages, isotopes, and volcanic ash deposits. By following the patterns of these connections, we can reconstruct the relationships that describe the physical layout of the supercontinent.
There are competing theories as to exactly how many supercontinents existed. Scientists generally agree that Pangaea did exist and was a supercontinent, but prior to that, things become a bit more murky. It becomes harder to discern associations between rocks because they've been around for so long, and exposed to so many physical and chemical changes. Our current models are accurate to around 200 million years ago, but we know that rocks and land existed at least as far back as a billion years ago, and earlier.
In one model, the supercontinent cycle was initiated in the Ediacaran Period due to changes in the tectonic nature of the Earth. Prior to the beginning of this cycle, all land was concentrated in a supercontinent called Protopangaea-Paleopangaea. In the second model, supercontinents existed prior to the Ediacaran, called Superia, Sclavia, Vaalbara and Kenorland. Both models posit that subsequent supercontinents formed prior to Pangaea, called Columbia (or Nuna) and Rodinia. The exact relationships and compositions of these supercontinents are not known with certainty.