What is primary succession?

Primary succession is a type of ecological succession that occurs in an environment that is initially devoid of life and soil. This means it starts in a barren environment where no community has existed before. Pioneer species, such as lichens and mosses, are the first to colonize these areas, gradually creating soil and paving the way for more complex plant and animal communities.

Let's evaluate the given options:

• A. It occurs when organisms start to live in a barren environment for the first time. This statement accurately describes the beginning of primary succession. A barren environment implies no pre-existing soil or life, which is the defining characteristic of primary succession.
• B. It occurs when plants begin to grow over lichens and attract other animals to the area to feed. This describes a stage within primary succession, after pioneer species like lichens have already established and started to form soil. It's not the initial condition for primary succession itself.
• C. It takes place in areas where there is soil and living things have already made that place their home. This describes secondary succession, which occurs in areas where a community has been disturbed or removed (e.g., by fire or logging), but the soil remains intact.
• D. It occurs when a shrub community is replaced by small trees which may eventually develop into a forest. This describes a later stage of ecological succession, where one plant community is replaced by another. This could happen in both primary and secondary succession, but it doesn't define the start of primary succession.

Therefore, the most accurate answer describing when primary succession occurs is when organisms begin to colonize a barren environment for the first time.

The final answer is $\boxed{\text{A}}$