The discovery that species go extinct was made relatively recently in human history: it dates from the early 19th century when Georges Cuvier reconstructured whole skeletons from bone fragments. The most convincing cases of extinction were for gigantic forms like mastodons: it was hardly plausible that they were still alive, undiscovered by explorers.
Extinction needs to be distinguished from pseudoextinction. This occurs when a lineage evolves so that later forms may look sufficiently different from earlier ones that a taxonomist with an incomplete fossil record will classify them as different species, even though there is a continuous breeding lineage.
Many species have gone extinct during the course of human history: the dodo is one of the earliest and most famous examples. Today rates of extinction have reached dramatic levels with many species under threat.
Why do species go extinct?