Do you grasp the difference between identifying the DNA of individual human beings and between virii and human beings, squash, and bacteria? Using PCR in paternity tests is not specifying that the DNA is human or not. It is based on the assumption the DNA is human.

It cannot be used to ascertain if the DNA is from a human being or not. It can only amplify extant material, not specify the source of that material. Means of specifying the source of material have been developed, and Koch's Postulate is how that determination is made.