I'm going to try to stay distinct with inventions rather than discoveries.
Start with whatever ranged weapon happened along. Some places atl-atl, some maybe just a throwing spear. Whatever some bright fellow came up with to avoid getting too close to horns or hooves or whatnot when batting things over the head.
Horse driven plow. First indication people didn't really have to do the hard part for themselves, ultimately leading to computer programmers.
Smelter. Without some sort of heat producing device rocks would still just be rocks.
War machines, be it land tanks or air tanks or sea tanks. When 'defense spending' meant production of equipment (putting people to work) rather than putting men under arms (forming units based on manpower and taking people away from work) it allowed Keynsian economics to revolutionize capitalism.
Fleet ballistic missile submarines. By providing a threat of certain and ultimate vengeance the FBM sub puts a limit on open warfare for countries beyond a certain technology level, happily coinciding with the technology level where open warfare has a very good chance to produce extinction.