

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.
This is fundamentally a variation on the question of a Temporal Paradox, also known as a Grandfather Paradox (“You go back in time and kill your grandfather. What happens?”). Although no killing happens in this variation, the basic idea is the same: Information is transmitted to the past from the future, but results in a situation where it cannot be transmitted in the first place.
Accordingly, there are several hypotheses to cover this. This isn’t even all of them: