Charcoal

300 million years ago, towards the end of the Carboniferous period, Earth saw oxygen levels as high as 35% (vs 21% today). Lightning was far more damaging back then. This period is associated with the formation of large deposits of charcoal, which have been linked to massive wildfires. Not only was the oxygen concentration at a record high, but there were also huge deposits of wood that could serve as fuel. Due to the absence of organisms to break down wood, during this time tens of millions of years’ worth of forests piled up, which became the coal we use today.