Thanks, guys.
Our original atmosphere was supposed to have a lot of methane in it, which is a very short hydrocarbon. Supposedly lightening working on it caused the formation of other, longer-chained hydrocarbons. There wasn't any oxygen until photosynthesis came with green plants taking water (H2O) and carbon dioxide and making simple sugar hydrocarbons and releasing oxygen as a toxic byproduct.
It's a good thing that plant greenies didn't exist to complain about how they were poisoning the air with the oxygen or oxygen breathing fish and mammals would never have evolved.