There really are only two possible explanations for the existence of complex life. Just natural unguided processes, or there's this ancient book claiming some great intelligent being designed life. So it all boils down to a simple debate over the evidence and which theory does the evidence best fit? Here's the problem... whenever you have two opposing theories in which the same type of evidence is predicted to be observed, then in that particular debate that type of evidence becomes polysemic and useless. It must be thrown out of the debate. I'll give you an example of what I mean.
In the famous Robert Blake murder trial where he was accused of shooting his wife to death, the prosecution claimed that gun shot residue on his hands and cloths was evidence of his guilt. The problem is that his defense argued that any gun owner who goes target practicing often, as Blake does, would be expected to have GSR on his hands and cloths. Both sides predicted GSR and therefore that evidence became polysemic and useless in the trial. Better and stronger evidence was required in order to get a conviction.
I pointed this out because both naturalism and design theories predict similarity between the organisms. Naturalism says if all life has a universal common ancestor then we predict to observe many similarities in the features of those various organisms. However design says that if all life had a common designer then we predict he would have used many similar design features in the various organisms. Thus similarity argument cannot be used by either side of the issue. These arguments in this debate are polysemic and useless to the debate. Both sides are required to bring better and stronger evidence to the table. That means to prove all life is the result of natural unguided processes then evolutionists must present evidence that the process either did or at least could occur. This evidence would have to be in the form of at least one finely (without giant leaps) graduated chain of fossils between any two major forms. Or at least one example of observed new and beneficial information being added (gene increasing) to the genome of a multi-celled organism. (Note: I stress the words "observed, beneficial, and multi-celled" here.) Likewise Design has to step up their game. They must be able to point to scientifically accepted clues for design and then demonstrate how those clues are in fact observed in all living organisms.
The mainstream world today parades the fact that evolutionary evidence is seen in every fossil and biology lab throughout the world. Indeed I agree they have literally so much evidence supporting evolution that you could probably fill every football stadium in the USA with it all. The kicker is that every last bit of it...relies on similarity arguments. All of it. And since similarity arguments are not evidence in this particular debate then they must be thrown out. This leaves Naturalism with nothing. Zero. Not a single shred of evidence they can bring to the table in a debate between naturalism or design.
So what has Design got to bring to the table? Well the scientifically accepted clues that suggest design are anything which we can observe was formed with a particular intent or purpose. The term for this is specificity. Examples of how specificity is used by scientists today can be seen in the way Archaeologists look for specific design features in objects they dig up. A feature that he/she recognizes from a previous experience that tells them this object was designed rather than naturally formed. Another example would be in the way marine biologists studying dolphins looking for specific sound patterns emitted by the creature that correlate with certain behaviors. This type of specificity tells them they actually have some form of intelligent language. Another example would be the way astronomers with SETI institute search for specific compressed light or radio signals coming from deep space. Such signals are very specified and would suggest an intelligent source. So if specificity suggests strong evidence for design in these cases then why would it not suggest design when we observe it in things like DNA code? This code is so specified that it warps our most intricate and sophisticated computer software codes by comparison. Again with no evidence that the code could have formed by natural processes, the most logical conclusion is design.