“Historical science” is really not science at all, plants and animals have a symbiotic relationship and require each other to survive and thrive. No, never was a poisonous atmosphere and never had millions nor billions of years of plants only.
Plus DNA requires an uber-intelligent author to write and also to adapt it’s code via natural selection. Mutations accumulate with each and every generation leading to devolution and mass extinctions, never been observed to create anything truly useful rather deleterious [removal, loss of some genetic code and functions].
Going to disagree with you there: Before photosynthesis (solar energy breaking up the CO2 in the air by green plants) there was no excess free oxygen for life as we know it.
Thus -> a poisonous atmosphere. Until plants formed/were created/evolved/were designed.