and: "The central idea of biological evolution is that all life on Earth shares a common ancestor"
Right. Common ancestry is a reasonable inference form the way we know evolution works.
If it is common ancestry you are trying to disprove then you have three possible approaches:
1. Show that it is not a reasonable inference.
2. Show that mechanism of the evolutionary process doesn't work.
3. Present evidence of special creation.
so evolution cant be true if special creation is true.
Not quite. We know that the mechanism of evolution works. So even if special creation was true, there is no reason to suppose that the process of evolution would not cause the specially created creatures to evolve. Many creationists believe this. They contend that the many species alive today evolved from the relatively few "kinds" Noah took with him in the Ark.
Thus, we know--and creationists agree--that evolution has taken place from some starting point. The creationists believe this starting point to be the creatures of special creation. Evolutionsists theorize that the starting point was the first self-replicating life form.
Consequently, IF there had been no special creation, then common ancestry would remain a reasonable inference (unless you can show otherwise) from the evolutionary mechanism which we know works.
You might say, "Wait! I can show that the evolutionary mechanism
doesn't work."
But that won't help you. If you show that the evolutionary mechanism doesn't work, then all you have is an evolutionary mechanism which doesn't work. But that does nothing to prove special creation, because that is not the only possible alternative to the evolutionary mechanism as we understand it.
What this boils down to is that at some point you are going to have to present evidence of special creation. Just showing that evolution won't work isn't going to do it.