A mystery may have been solved as to why Animals didn’t flourish earlier in earth’s history even though oxygen levels should have permitted it. Scientists claim it’s because oxygen levels at that time were a fraction of what we thought they were.
Big multi-cellular life really proliferated on the earth in the period of geologic history called the Proterozoic about 800 million years ago. But the preceding billion years or so seemed to have enough oxygen for the simple organisms then existing to take advantage of. The bottom line question then has been: What the hell took so long?