[Irish Wolf]

In one of Niven's Draco Tavern stories, an ancient chirpsithra (a species that operates interstellar passenger and trade liners that travel just under the speed of light, so even the youngest humans have met have been around for centuries) reminisces about the last time she came by Earth, some 780 million years previously, and the advanced civilization of anaerobic life forms dwelling in the young planet's shallow seas. Sadly, there was nothing the chirps could do to save the planet from the threat that came, so slowly and subtly - a green scum floating at the very edge of the land, that sucked in precious carbon dioxide and spewed forth toxic, highly damaging oxygen...