There are lots of well known candidates (fire, the wheel, gunpowder) but I'm going to break with the pack and suggest the Fence (and it's closely related cousin, the Wall). While it seems everyday and obvious now, it is worth recognizing how different the world became once humans recognized the protective value of enclosing land with a fence or wall. Other animals just don't do this (as far as I am aware). The invention of fences spurred agriculture as our ancestors could protect crops from animals, and livestock from predators. Fences took the hassle out of protecting the food supply, freeing up people for other tasks. As fences came to denote boundaries and ownership they have continued to play an important role in our relationships with each other. Traditional uses of fences have endured (a ring of thorny bushes to protect cattle in Africa) and new ones constantly emerge (like 'protecting' the southern US border from immigrants).
The Wall is often used much like the fence, but can also be paired with the Roof to make a shelter. Its a big step from using natural shelters, like trees and caves, to building walls to make a home. While other animals build shelfters such as nests and dens once humans had developed the concept of the wall they were able to adapt the idea to whatever local materials were at hand (rocks, mud, wood, snow, etc ). People were no longer tied to natural shelters or one kind of environment. Wall (and roof) buliding let humans spread across the globe.