Taming fire was surely one of the most significant developments in human history. Probably the discovery happened numerous times and numerous places, but the results were profound. Fire provided warmth, hot food, the ability to venture out into darkness, fire pottery and bricks.
It could also be a tool for long distance communication. There's a marvelous scene in The Return of the King, the third Lord of the Rings movie, where signal fires are lit to summon the troops to battle. It's breathtaking to watch the fires catch on distant peaks.