Purple Earth hypothesis. The first (AFAWK) photosynthetic critters were cyanobacteria. They produced enough oxygen to kill off everything which couldn't withsta

Purple Earth hypothesis. The first (AFAWK) photosynthetic critters were cyanobacteria. They produced enough oxygen to kill off everything which couldn't withstand its reductive effects. Oxygen levels have been much higher than they are today, presumably this is what made e.g. 6 foot centipedes a possibility.