Controlled fire

Controlled fire means a fire you can start, keep, and put where you want it, not a wildfire you flee. Good evidence of habitual fire in human ancestral sites is about a million years old, with stronger hearths by a few hundred thousand years ago. The exact first controlled fire is not known.

Heat is the contribution. Cooked food yields more energy and is safer. Fire hardens wood, deters predators, and extends the day. Much later, the same heat fires clay and melts metal. Bronze and iron are fire plus rock. A steam engine is fire plus water plus iron. There is no metal age without this.

Fire also changes the body over time. Smaller guts and larger brains in the human line are often linked to cooking, because cooked food is easier to digest. That link is a strong hypothesis, not a single fossil that proves it.

A campfire is not a technology you can scale by itself. The next steps, farming and metal, need people living in one place long enough to keep the fire and the knowledge.