The Purple Scum Belts were bands of worlds dominated by "purple scum", located West of the Datum.
These Earths had neither multicellular life - such as plants and animals - nor cellular life with internal nuculi. Only the very simplest of bacterium existed, in mats and banks as well as in ponds. These worlds didn't vary much from one to another.
The closest belt of these worlds to the Datum spanned from before Earth West 30,000,000 to around Earth West 35,000,000. This first band of worlds contained structures like stromatolites, mounds of bacteria built up layer by layer in the sunlight. These forms were capable of movement.
There was a second, larger band of purple scum worlds from approximately Earth West 50,000,000 to Earth West 100,000,000. The climate conditions here were similar enough to those of Datum Earth that humans were able survive without significant protective clothing. This belt also contained green scum worlds.
On one of these worlds, a stromatolite-like structure crept on an unwary crewman who was taking samples with her back turned.