Evolution-denying creationists actually believe that God created every single species as entirely distinct from every other species, so that there is no evolutionary connection at all. That's why they say misinformed things like they believe in microevolution but not macroevolution (note: this bogus distinction as an argument against evolution is not one that anyone who understood science would ever make). If God had wanted to create all species as entirely distinct, not giving the appearance of having any evolutionary connection, then the genetic relationships between species is not something you'd expect to see in a single act of creation. All of the world's organisms (with the exception of a few bacteria and archaea strands) use the exact same set of 20 amino acids to code for their proteins. And with that set of 20 amino acids, they use the same specific 3-bit DNA sequences to code for their specific amino acids.
Why would
God do that if we are not genetically related to all other species, and they to each other? It's especially strange when you look at the genetic sequence for
these same-function genes in a bacterium and then at the genetic sequence for
the same protein in a human - they have a remarkable similarity. In fact, some
of the genes are almost identical, despite being hundreds of bases long. In a
special act of creation, like the one to which the Genesis 1 literalists
subscribe, would God really have wanted to create a system in which we share
the blueprinting for the same proteins with the bacteria in our mouth?
But it gets even deeper. If each of the world's creatures were all created in a single act of creation, this 3-bit DNA sequence could have easily been assigned to code differently for the 20 common amino acids between groups; and furthermore the DNA sequences themselves could have been radically different between groups, or radically unique to species. God could have shuffled it around and made every species different for these attributes; with a 3-bit system of 4 varying nucleotides, and with a total of 64 different codons being issued across 20 different amino acids independently, there are literally enough combinations of codons and amino acids to assign each living species its own unique genetic code. In other words, there is absolutely no reason for God to give each organism the same genetic coding blueprint, and exhibit the computations of the relational distance of every species to each other, unless He wanted to make it look like all evolved in the same family tree of life over billion of years. Creationists are encouraged to go figure that one out!
