Stromatolites are thought to be the first living organisms on this planet.  3.8 billion years ago, the Earth was a very different place.  There was no ozone layer, the atmosphere was primarily carbon dioxide, and dry land was constantly sterilized by deadly UV-C rays from the Sun.  Bacteria ruled world, but they made a mistake...  they filled the entire ocean and the entire atmosphere with a corrosive, reactive, poisonous gas and paved the way for higher life-forms to assume control of the Earth.  That gas is oxygen.  It filters harmful solar rays, and its vigorous chemical reactivity allows organisms to maintain a very high, and therefore efficient, metabolic rate.

Cyanobacteria give off O₂ as waste.  It's poisonous to them.  Today, we're filling the atmosphere with poisonous wastes of our own just as they did.  If germs can pollute the entire planet so might we,   Apparently, we're no smarter than bacteria.