When future civilizations dig up the remnants of our civilzation, they will learn that our fall was predicated by a single event. Namely, Brandon Jones' porting of Team Fortress 2 to WebGL (i.e. web browsers). Thankfully, it's merely a demonstration and isn't actually playable – yet. Frankly, if someone manages to make Team Fortress 2 even more accessible, we're not sure humanity will have the mental fortitude to withstand it.
You can learn more about the technical aspects of the demo (or even download the source code) over at Jones' website, TojiCode.