Steps of primary succession:
Initially, there is no life on the rocky ground (A). This could be after a volcanic eruption, or after a glacier melts to expose rock underneath.
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After a while, through wind, rain and animals, seeds for mosses and lichens will enter the area and will start to grow (B).
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As those organisms die and are broken down by decomposers, they will form a thin layer of soil, which can support smaller plants like grasses (C).
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Eventually, there is enough soil (from the previously growing plants) for tree seedlings and shrubs to grow (D).