The expression of the gene dpp in the bilateral form in a variety of animals is evidence that

The presence of the same gene in all the animals suggests that all the organism, which have the same gene are somehow derived from a common ancestor.

The expression of the same gene in all the animals suggests that at some stage of their evolution, a single event was occurred, which made them to evolve the same gene.

Later, the dpp gene was known to perform different function in different animals. So, it can be said that bilateral form in all the animals is homologous.