There is an inherent bias in all such surveys, because everyone strives to be original and surprising and so shuns the obvious but probably more correct answers — such as steel, or moveable type, or antibiotics, to name but three obvious things that have utterly transformed not only how people live but the way theyexperience life.
The only way I can think of being surprising is to violate John's terms and go back 6,000 years. But if I will be permitted to do so, I would argue that the single invention that has changed human life more than any other is the horse — by which I mean the domestication of the horse as a mount. The horse was well on its way to extinction when it was domesticated on the steppes of Ukraine 6,000 years ago, but from the moment it entered the company of man the horse repopulated Europe with a swiftness that announced the arrival of a new tempo of life and cultural change. Trade over thousands of miles suddenly sprang up, communication with a rapidity never before experienced became routine, exploration of once forbidding zones became possible, and warfare achieved a violence and degree of surprise that spurred the establishment and growth of fortified permanent settlements, the seeds of the great cities of Europe and Asia. For want of the horse, civilization would have been lost.