Back in the 1960's there was an ASTM flamability test that was run by suspending a sample of the material being tested in a stream of gas. The gas was basicly dry air either enriched with oxygen or diluted with nitrogen. The sample was ignited with a small controlled flame. The "oxygen index" was the oxygen percentage in which the sample would just burn.

Like most R & D folks would have, we played around a bit using our own samples. A small piece of wood burns like a Roman candle at 25 or 26% oxygen as I remember. I'm sure that green leaves and grass would burn like a blowtorch at 30% oxygen.

So if the atmosphere ever was 30% oxygen after plants can out of the water, there would be a worldwide marker - a layer of ash.

Best wishes

Bill