[general_re]

If X is similar to Y in Z, then it is possible that X and Y came from similar origins. Not definite, but most definitely possible. It is a theory which holds a good amount of credibility, but is by no means certain.

I already pointed out that this is a fallacy in logic (law of the excluded middle).

Certainly not. The excluded middle is only a fallacy in deductive arguments. Since this is explicitly an inductive, probibalistic argument, no fallacy is present.

[general_re]

And that's what I get for doing three things at once. The fallacy of the excluded middle is also known as the fallacy of the false dichotomy, or false dilemma. One typical example is the following construct:

Either scientists can explain strange objects seen in the sky, or these objects are piloted by visitors from outer space. Scientists cannot explain these objects, so they must be visitors from outer space.

Essentially, the false dichotomy is created in restricting the possibilities to two (or sometimes more), and deducing that since one of the possibilities is false, the other most be true. The problem is that there are possibilities that are not listed, bringing about a possibly false conclusion. Reword the argument above, and you get something like this:

Either scientists can definitively explain the origins of life, or God is responsible for the origins of life. Scientists cannot definitively explain the origins of life - therefore, God did it.

Not that you'd ever see such "logic" used, of course....