Fungi are much older. Source

Plants are older than thought: 500 million years old. Source

The earliest fungi started to develop 1.5 billion years ago, with other types 635 to 400 million years ago. Source

In fact, both flora and fauna can’t survive without fungi. Source

I thought it was fungi, as they are the ones breaking both plants and us down, are the oldest of all of us, both feed plants and us, etc.

But… Peter, you’re already here, we can’t summon you while you’re here