on how humans are destroying the earth.

http://leo.stcloudstate.edu/acadwrite/thesistatement.html

This is one of the very best places I've seen online to help students write good thesis statements. It shows you sentences that aren't thesis statements and how to turn each one into real thesis statements.

Just remember: your thesis statement must include factual information plus your position/opinion/stance. Without your position on the topic, it isn't a true thesis statement. So think of this sentence as the angle you want to take on the topic and what you intend to prove by the end of your paper. (If your statement is simply factual, then there's nothing to prove!)

So... do you believe humans are destroying the earth? ...do you believe that overall the earth can renew itself no matter what humans do? ...or what?