It really is an interesting question, yes! Fires started by frictional heating are pretty uncommon in nature, but early humans could pretty readily see that objects placed near a fire would begin to smolder and burn just from radiant heat.
It really depends on when we were able to take intellectual leap of realizing that all heat is equivalent, and fire is not a prerequisite of making new fire.
It really is an interesting question, yes! Fires started by frictional heating are pretty uncommon in nature, but early humans could pretty readily see that objects placed near a fire would begin to smolder and burn just from radiant heat.
It really depends on when we were able to take intellectual leap of realizing that all heat is equivalent, and fire is not a prerequisite of making new fire.