When this concentration rose above 13%, it permitted the possibility of wildfire.[2] Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago, by fossils of charcoalified plants.[3][4] Apart from a controversial gap in the Late Devonian, charcoal is present ever since.[4] The level of atmospheric oxygen is closely related to the prevalence of charcoal: clearly oxygen is the key factor in the abundance of wildfire.[5] Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7 million years ago;[6] this kindling provided tinder which allowed for the more rapid spread of fire.[5] These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.[5]