You know the anti evolution deniers are breathing Oxygen because cynobacteria evolved photosynthesis and that is WHY WE HAVE AN OXYGEN ATMOSPHERE today and not natural gas one.
The Origin of Oxygen in Earth's Atmosphere
The breathable air we enjoy today originated from tiny organisms, although the details remain lost in geologic time
"It's hard to keep oxygen molecules around, despite the fact that it's the third-most
abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react; it can form compounds with nearly every other element on the periodic table. So
how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff?
The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct
photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them down to this day.
For some untold eons prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes lived the real old-fashioned way: anaerobically. These ancient organisms—and their "
extremophile" descendants today—thrived in the absence of oxygen, relying on sulfate for their energy needs."
The Origin of Oxygen in Earth's Atmosphere - Scientific American