We have phylogenetic techniques to interpolate when certain genes might’ve appeared in evolutionary history. Not surprisingly, the ability to breakdown plastics is quite new.
Not only that, but the very few microorganisms that can degrade some plastics only express those enzymes under extreme pressure, when no other sources of carbon are available. Literally every sugar is a better alternative than plastic, as the process of degrading it is massively inefficient.
Making a usable polymer out of the absolute insane mixture that is crude oil is also way beyond what any human civilization could ever achieve without industrialization.
I get your point of “but we did amazing things in the past! look at the complexity of steel!” but artificial plastic polymers is in another league.