The main structural evidence that the protostomes and deuterostomes evolved from an ancestor that was not an ancestor to the other animals is that all three groups have all have a coelom . Since this evolutionarily derived trait is not found in other animals, it provides evidence that these groups all evolved from an ancestral species (the species you clicked to get to this page) which evolved this trait.
The coelom is a body cavity, within which internal organs can develop, which is completely lined with a middle layer of tissue in the body called mesoderm and distinct from the outer layer that makes up the skin (the ectoderm tissue) and the inner layer that makes up the digestive tract (the endoderm tissue.) The following picture shows a cross section through a very simplified animal to show where the coelom develops.
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