system
Given the following post, respond as if you're commenting on Reddit:
human
If soil comes from dead plants, what substrate did the first terrestrial plants grow on?

This question was asked by my 8-year old as part of a long string of questions about evolution, but it was the first one where I didn't really know the answer. I said I'd look it up but most information appears to be about the expected types of plants rather than what they actually grew on.
gpt
Generally bacteria are the first things to colonise bare rock and provide a substrate for other micro-organisms. Simple multicellular organisms like lichens can thereafter colonise the rock. They reproduce quickly and can survive the harsh conditions, and many species don't need soil. Its these plants that start to lay down soil as they die off and decompose, providing a substrate for increasingly more complex species.

What your 8-year old is asking about is called "ecological succession" and is the process through which an area can go from bare rock to a rich forest over a hundred years. You can see the early stages of ecological succession all over the place, particularly in concrete urban areas that have been neglected and are in the early stages of colonisation. In fact your local hardware store will sell lots of products which are the armoury of the homeowner in the struggle against the primary colonisers of ecological succession.