The invention of the bow is the first landmark military event in human history. Its power is far beyond that of earlier throwing weapons, and so is its range. Thus, archers grant an important advantage to an early civilization over barbarian tribes, not to mention a nasty surprise to rival neighbors which lust for their lands. Note that in order to activate the Eureka moment for this tech, you need to achieve a kill through an attack! If your Slinger kills an enemy during self-defense, the Eureka won't activate.