Fossil fuels are so named because they are derived from the remains of ancient living organisms. Over millions of years, the decayed organic matter from plants and animals was subjected to immense heat and pressure beneath the Earth's surface. This process transformed the material into energy-rich substances like coal, oil, and natural gas. The term "fossil" refers to this ancient biological origin, not to the presence of actual fossils within the fuel.

Final answer: derived from the remains of living organisms.