Human action is one major source, but an ancient rival is lightning, that electrostatic product made of water, ice and turbulent winds. Like humans, lightning prefers land to sea. We see many relics from the deeper past of fire, especially across the wide African savanna. Often woodlands lie on flats that fringe forested mountain slopes. Those retain forest only where they are sheltered from fire-bearing strong winds.
Locations more exposed to wildfire soon surrender trees to grass.
One big bird, the bald ibis, is found only where lightning often sets fire, and it disappears where fire is suppressed. That bird feeds well on fire's leavings, perhaps some half-cooked turtle too slow to flee. Fire making has been practiced by only one or two species. First used by our forebears Homo erectus and now by ourselves, fire is a tool as sovereign as language. When our ancestors foraged, they cooked to gain new sources of food, lighted the evening darkness against big cats, set wide plains afire in season to lure the herds to new grass and opened easy paths through dense underbrush.