If our ancestors had not learned how to master fire the history of the world up to modern times would not have happened.
Wow. This is a cool thread and very hard to choose.
We can pretty much rule out religion, music and art (sadly ), animal domestication and trade/money. All are important, but not at the level of the others. We can also rule out fire, as it wasn't an invention.
I'm going to have to go with the wheel. It makes all the other ones easier.
Beer, duh - 9,000 BC