The Second Discovery

Before the invention of the harness, animals could potentially be used for transportation, but not much else. The reason is simple; when you tie a rope around a horse's neck, the harder he pulls the less he breathes. With the invention of the harness, which distributes the weight on the shoulders, mankind's ability to do work was no longer limited by his own muscles. The invention of the stirrup made riding easier (and armed combat possible) and the invention of the yoke meant you could use several animals together, but the invention of the harness was one of the fundamental discoveries for humanity.