“Discovered” seems like the wrong word to use.
“Understood” doesn’t apply, because it was being implemented before being understood.
“Made into a tool”… “Mastered into the domain of human culture”
…
Do you think it was completely by accident, and that lightning struck a dead tree and started fire, and then the fire was kept sacred, and tended to and fed (worshiped) by early humans around it?
Maybe later, when the “great fire” ended, a devout shaman spent days rubbing sticks and stones together as a ritual offering?
Eventually somebody figured out that the repeated behavior brought about the desired results – even if all points of thinking in between were flawed. (Like pigeons being fed food through a hole when they press a lever.)
How was the power of fire first harnessed by humans?


I’m guessing that people, who knew the benefits of fires (having seen them started by lightening, and then preserving them for future use), discovered how to set fires when they were making flint arrows. Sparks landed on their bare legs.
Perhaps someone rubbing sticks together to give them a soft finish with a nice patina (again, for the purpose of attaching an arrow), noticed that the sticks were getting hot. some curious mind must have wondered just how hot those sticks could get, and used his bow (with a longer string) to create char or smoke.
Isn’t the brain a wonderful organ?
The accidental lightning fire is a likely start. Since light kept the predators away and the fire kept cold away, early humans would have wanted to find a way to hang onto this. In early cultures, they would have kept a permanent fire going, protected from the rains, that they could use to light other fires.
How they eventually discovered the use of sparking rocks, rubbing sticks together, etc., is a bit of a mystery to me.
Accident.
It was stumbled upon by two rocks beings scraped against each other.
It was lit under a large stone box to heat up food, the world first microwave