Anaerobic Lifeform (The Green Marauder)

The anaerobic lifeforms were the original intelligent inhabitants of the Earth, about 780 million years before Humans evolved.

Biology
Nothing has been revealed of their appearance, other than that they were small, soft and fragile, and had a very slow metabolism compared to oxygen-breathing lifeforms. They were short-lived but that was compensated by the fact that they inherited their ancestor's knowledge and memories. Free oxygen was lethal for them.

History
About 780 million years before mankind emerged, the anaerobic lifeforms built vast and beautiful cities in the shallow seas along the coastlines of Earth's primitive oceans. In the later stages of development, their civilization had mostly moved on to floating cities, shaped like huge rafts which they lived underneath.

While they never developed space travel, they did send messages to the stars and contacted the alien race known as the Chirpsithra. Their works of philosophy were greatly appreciated by the Chirps and much of their literature is still preserved in the Chirp libraries, long after the anaerobic creatures became extinct.

They died out due to environmental changes when new kinds of photosynthetic life evolved, which released huge quantities of free oxygen in the atmosphere. The Chirps' efforts to relocate the dying population to other planets have been sadly unsuccessful.

Appearances

 * "The Green Marauder", by Larry Niven (1979)