So the assumption of common descent is built into the thing you're using as evidence.
No. There is no assumption built in. I am not constructing a circular argument.
I'm fairly comfortable that your idea of what constitutes the evidence is a useful starting place, but not under those conditions. What if what you call a "universal" genetic code could be shown to have distinct an non-transferable classes?
What are 'distinct an non-transferable classes'?
If it is your assertions that DNA is common to all living entities and thus evidence for common descent, then that would be an acceptable starting point.
Good, so the demonstrated fact (not my assertion) that all living species use the same system to store and transmit their genetic code, and the molecular machinery of any one species can read the DNA of any other species, is the first piece of evidence for common ancestry.
Here is a second important piece of evidence: The phylogenetic tree of life made from comparisons of the physiology of fossil species and modern species matches very closely the phylogenetic tree of life made from comparing the DNA sequence or amino acid sequences for the same proteins in different species.
Here is an example found in the Holy Wikipedia. The phylogenetic tree of life made by comparing physiology in humans, chimpanzees, mice, rats and cows shows that chimpanzees and humans are much more closely related to one another than either is to mice, rats or cows. The conclusion is that chimpanzees and humans shared a common ancestor much more recently than the last common ancestor of all these animals. To make an independent phylogenetic tree, scientists determine the sequences of amino acids in proteins doing the same job in the different animals. Here is an aligned sequence for the H1 histone protein. Histone proteins are the packaging mechanism for DNA in the cell nucleus (you can find the letter code
here):
The sections of the amino acid sequences that are not conserved are shown as lighter grey. These changes are due to mutations in the relevant section of DNA (the gene for this protein). The differences marked 'conservative' are cases where one amino acid has been replaced by another that has a similar biochemical effect; non-conservative changes are where the choice of amino acid in that position is less important to the functioning of the protein. The sequences for chimpanzees and humans are identical because we have a much more recent common ancestor and so less time has passed for mutations to accumulate in the code for this protein.
To restate it, this evidence for common ancestry is in the correlation between the two independently determined trees made from fossil/physiological evidence and from comparing sequences in either DNA or in proteins.
Stuart