Part 1 - Protein Functional Redundancy...
Part 2 - DNA Functional Redundancy...
Imagine you are teacher with suspicions that some of your pupils have been copying from each other. Comparing the correct answers in all of their assignments might not provide conclusive evidence. They could simply claim they had all carefully revised the same textbooks so it shouldn't be surprising that they all gave the same answers. The way to prove there has been cheating going on is to examine their mistakes. There might be only one way to answer some questions correctly but there are almost limitless ways to get it wrong.
By examining a number of identical mistakes you could even build up a family tree of plagiarism and show who wrote the original essay and who copied from whom.
In a similar way geneticists have been able to confirm the evolutionary history of humans and other species by examining errors in our DNA.
Occasionally a virus will infect an animal and get its DNA implanted in its host genome, HIV is an example of such an infection. Most infections are of soma (body) cells and end with the death of the hapless victim. Very occasionally a virus will infect a germ (sex) cell of its host and will then become part of the genome of all of its victims descendants.
These kind of infections, known as "endogenous retroviruses", are very rare and very random. Your DNA consists of 3 billion base pairs and is split into 24 pairs of chromosomes. If the same remnant of a virus was found in exactly the same place in the genome of two different people it would be irrefutable evidence that they both inherited it from a common ancestor.
If many such examples were found that exactly matched in two people we know for an absolute certainty that they have shared the same family tree.
Many such examples have been found in the human genome, each one in exactly the same place proving the common ancestry of every human on earth. However scientists next looked at the genome of the primates and found the very same remnants of ancient virus DNA in exactly the same places.
Not only that but they found a "family tree" of endogenous retroviruses that confirmed exactly the relationships they previously believed to be the case. In other words those species that were known for other reasons to be our closest relatives share more endogenous retroviruses than those more distantly related. We can therefore show exactly when each of these viruses infected our common ancestors.
Here is a diagram of the common ERVs that have been found in Homo sapiens and primates showing how the genetic evidence confirms the tree of life...