While there are ways to experimentally test the process of evolution, known as natural selection, by mimicking it artificially, ways to test the historical issue of evolution, known as common descent, must rely on making observations and making predictions of the outcome of those observations.Inside the cells of all Eukaryota (plants, animals, fungi, and protists) are organelles called mitochondria. Likewise, inside the cells of plants are organelles called chloroplasts. Both mitochondria and chloroplasts have their own DNA and are thought to be descended from bacteria that took up residence inside the larger cells. In any case, the DNA in those organelles are completely different from the DNA in the nuclei of those same cells.

We can take the DNA of cell nuclei and check them to see if the differences between those of various organisms can enable us for build a family tree of those organisms. The more similar their nuclear DNA is, the more closely related they are. But since mitochondria and chloroplasts also have DNA, we could also take them and check to see if we could build up a family tree that is the same as the one we would build up with the nuclear DNA.

Indeed, there is no reason, if all life was produced by a single recent act of creation, for the DNA of the mitochondria of all animals to be significantly different from each other. So if I was a Creationist, especially of the young Earth kind, I would predict that it would be impossible to make a family tree from mitochondria DNA, or if I did, it would be completely different from the nuclear DNA. But if I was testing the theory of common decent, I would predict that the readings in animals of both their nuclear and mitochondrial DNA would produce the SAME FAMILY TREE in all cases! This would make perfect sense if the mitochondria and the rest of the cells have been evolving together ever since they first came together over a billion years ago.

Mitochondrial DNA is already used in forensics to determine who the mother of a child is, while nuclear DNA must be used to determine the father of that same child. This would only be an extension of that function, since the parents of the child must be of the same species, or at least very closely related, to even produce offspring at all.