The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago. The presence of land based plants permitted the accumulation of oxygen in the atmosphere as never before. When the concentration of oxygen rose about 13%, wildfire became possible. Wildfire is first recorded in the late Silurian fossil record, 420 million years ago, by fossils of charcoalified plants.
Source: Wikipedia