For 50 million years, dead trees didn't decompose because the microbes capable of digesting wood hadn't evolved yet. The dead trees just piled up miles deep, eventually being crushed into the massive coal reserves we use today. This is why it’s so impressive that microbes capable of digesting plastic exist, it’s only been maybe 200 years since plastic was invented and already things are evolving to eat it. Plastics are extremely similar to natural hydrocarbons like cutin, the waxy stuff on some plant leaves. Bacteria existed that ate and broke down cutin, and eventually found some modern plastics that had biodegradable polymers in them. See, those biodegradable polymers, that were meant to break the plastics down into... Let's skip over that... Were designed by copying Cutin hydrocarbon chains. So it wasn't a big jump for the cutin eating bacteria to start eating cutin adjecent polymers. Kinda their version of a Twinkie. We just kinda accidentally domesticated a bacteria. Like how we domesticed wolves and cats by just attracting garbage and pests around us.