I can't say there is one KEY invention that made mankind what it is. But here are some favorites:
The printing press--the invention that made possible the mass disemmination (sp?) of information and knowledge. Before it existed, hand-written books were very expensive and thus only accessible to the elite. After its invention, literacy could increase and Mr. Everyman could contribute more and more to solving various problems man faced: he could give more knowledgeable input, or he could dedicate himself to the study of various sciences that become more and more collaborative with their complexity.
Agriculture--this made possible man's advance from a precarious position not always at the top of the food chain, to masters of the entire planet. Continued innovations in this field ensured more efficient food production that made cities and civilizations possible.
Use of fire--the first advance in chemistry! Man controlling natural phenomena, rather than being controlled by them all the time--this was the first step.
Rule of law--advanced man from an animal into a more responsible, sentient being with a sense of higher purpose. "Might makes right" still rears its ugly head from time to time when we forget this concept, but the concept would evolve into a sense of man's natural rights, and equality of all in a moral framework.
Space flight--our key to expansion beyond our crowded planet. I would say interstellar flight will be a more significant invention, but we haven't invented that yet....