Primary succession occurs on completely barren land where no soil exists. This means that no parts of the ecosystem need to be established beforehand.

Here's why:

* Primary succession is a brand new start. It's like starting with a blank slate. Think of a newly formed volcanic island or a glacier retreating and exposing bare rock.

* No soil: There's no existing soil or organic matter to support life. This is what differentiates it from secondary succession, which happens on land that has been disturbed but still retains soil.

* Pioneer species: The first organisms to colonize this barren land are called pioneer species. They're hardy and can survive in harsh conditions, breaking down rock and creating the first thin layer of soil.

So, instead of pre-existing parts, primary succession is about the creation of an ecosystem from scratch.