The need for a polymerase that wouldn't denature at high temperature. Obviously, organisms that live in hot springs and the like will have adapted enzymes that can survive at these temperatures.

The native form of a protein is the three-dimensional shape (conformation) assumed by the protein that is required for normal functioning. If this structure is disrupted (typically by heat or acid), the protein is said to be denatured. Changes in the shapes of proteins also alter their taste and texture.