The continents we know today were not always the same. They move continuously. Sometimes smaller continents join to form a larger one. Other times, a large continent breaks into smaller ones.
About 200 million years ago, all the landmasses were connected forming a large continent called Pangaea. This continent was "broken", creating smaller continents. One of these continents was Gondwana, which also broke into two fragments - today the South American and African continents.
On the newly formed coast of South America, the Serra do Mar, the coastal plains and beaches were built. So was formed our coastline!