Good afternoon :)
Is this the one by Daniel Quinn? That's what I'm assuming, anyway :)
How about starting with something broad, such as the main thesis of Quinn's book. Something like how it is difficult for any species or tribe (in this case the human race) to be aware of their myth because it is so deeply rooted in every facet of civilization, and that it is difficult for anyone to effectively "point it out" because they too are part of the myth.
Then, what about saying something about the theory that humans are on the final step of evolution, and that humans are destined to rule, using that as the foundation for taking the lives of anything that stands in the way of that "evolution," including the environment itself (through global warming, etc.)
The key to the thesis is that it should be almost all-encompassing and broad in the beginning, and gradually dwindle down in scope until you come to the actual statement, which should be a sentence or two. Perhaps you should fill in your outline more before writing the thesis so that you know what you will be including; perhaps that will help you identify an undercurrent theme.
Best of luck!