What if....just what if....
What if the Earth was a creature and the plants and everything on it were like the cells and organs that interact to keep it alive? What if humans were just a shift of farmers that came in for a few thousand years to do their work and then, similar to the dinosaurs, our job will be done one day and we will disappear? Or, how about this....What if humanity was really more like a virus on the Earth and eventually, the Earth will warm up (or something) and rid itself of us?
Admittedly, The Happening (movie) was pretty poorly made....but, the idea of nature being able to create chemicals that humanity will respond to, negatively or positively, is already a reality. Also there is a thing called "allopathy" -in which some plants can exude chemicals that kill or repel other plants from growing near them. So...it's really not out of the question that nature could probably come up with chemicals to get rid of us....maybe not instantly like in The Happening...but still...