Letron_James posted...
The oldest creature is around 700 million years old. Oxygen became available around 2.5 billion years ago, so theoretically there could have been atleast 3 rounds of something similar to our evolutionary history. And I might be incorrect, but I believe that there was a lot more oxygen in the air back then so the creatures could have been massive, or maybe that means theyre small? Idk but either way I think its more plausible than aliens.
You are quite incorrect about the oxygen levels. Oxygen didn't get above a few percent until relatively recently in geologic time.