Fire is one of the most important forces on Earth. The use of fire by humans has long been considered as a defining property of intelligence, separating us from other animals. The exact timing of the discovery and use of fire by humans has been a subject of continuing research, yet perhaps two questions have, until now, received little attention: What was fire on Earth like before humans appeared? And what experience of fire could early humans have had?
Three main components are needed for fire. First, there must be a fuel to burn. Second, oxygen must be available — after all, combustion is essentially an oxidation process that gives off heat and light. And third, there must be a heat or ignition source that allows the fire to begin. We would not expect fire on a barren Earth; there must be plant life on land that can provide a fuel source. And vegetation fires can’t occur until the oxygen level in the atmosphere has reached around 15%. (It is 21% today.) This is why we smother a fire with a blanket or sand, pump carbon dioxide on it, or even flood it with water to extinguish it — to cut off the oxygen. The main sources of ignition before humans appeared were lightning strikes.
Our evidence of fire in the fossil record (in deep time, as we often refer to the long geological stretch of time before humans) is based mainly on the occurrence of charcoal. This is the partially combusted plant material left after a fire has been extinguished. The oldest fire recorded on Earth has been identified from charcoal in rocks formed during the late Silurian Period, around 420 million years ago. Though plants had spread on land at that point, fluctuating levels of atmospheric oxygen meant that the first extensive wildfires recorded came somewhat later, dating from around 345 million years ago, the early Carboniferous Period.