What was the most important discovery of early man?

Depends exactly on the definition of "Ancient Man", are we talking 5,000 years or 50,000 years?

Also there is a difference between discovery and invention. Discovery is something which was already existing (normally something natural), whereas invention is something totally new.

Invention: Anything from the knife blade, spears to the bow and arrow in "caveman" times, to the wheel a few thousand years ago.

Discovery: Probably first place would go to fire.