A short story about an ancient anaerobic civilisation on Earth

Question

I read this story many decades ago. Probably in a collection since I very rarely read magazines, but I don't remember at all what it might have looked like.

A very, very old alien tells her memories to Earthlings. She was born billions of years ago. Her physiological age is much less, since she travelled a lot at near light-speed velocities, but she is still very old.

She has visited Earth, billions of years ago, at a time where there was a brilliant civilisation of anaerobic creatures. IIRC, she came on several occasions, hundreds of millions years apart. On her last visit, they knew they were doomed. Our ancestors, i.e., cyanobacteria, were starting to perform photosynthesis and fill the atmosphere with oxygen that was poisonous to them...

Answer 1

It's a classic! The Green Marauder by Larry Niven. I read it in the anthology Tales from the Spaceport Bar.

The alien is Chorrikst, and as the name of the anthology suggests it's a story related while drinking at the bar. Chorrikst relates:

“They had evolved for as long as you have,” Chorrikst said with composure. “Life began on Earth one and a half billion years ago. There were organic chemicals in abundance, from passage of lightning, through the reducing atmosphere. Intelligence evolved, and presently built an impressive civilization. They lived slowly, of course. Their biochemistry was less energetic. Communication was difficult. They were not stupid, only slow. I visited Earth three times, and each time they had made more progress.”

Almost against his will, Noyes asked, “What did they look like?”

“Small and soft and fragile, much more so than yourselves. I cannot say they were pretty, But I grew to like them. I would toast them according to your customs,” she said. “They wrought beauty in their cities and beauty in their philosophies, and their works are in our libraries still. They will not be forgotten.

Then:

“They sensed worldwide disaster coming,” Chorrikst said, “and they prepared; but they thought it would be quakes. They built cities to float on the ocean surface, and lived in the undersides. They never noticed the green scum growing in certain tidal pools. By the time they knew the danger, the green scum was everywhere. It used photosynthesis to turn carbon dioxide into oxygen, and the raw oxygen killed whatever it touched, leaving fertilizer to feed the green scum.”