Explain why complementarity would have been necessary for the development of self-replicating molecules.

It allows the existing molecules to direct the process of assembly of a new molecule from the smaller and simpler substances.  

Explanation:

Complementarity is required for self-replicating molecules as it allows the existing molecules to serve as a template for the formation of a new molecule of its own type. For example, DNA is a self-replicating molecule as it can direct the synthesis of new DNA strands by serving as a template. This is possible as two DNA strands are complementary and the complementary bases of two DNA pairs with each other to make a double helix.

A DNA template with a sequence "" would direct the synthesis of new DNA with a sequence "" since Adenine is complementary to the thymine and makes two hydrogen bonds with it.