Hang on, I forgot it's just after Xmas.
Somebody has been listening to their old Chris De Burgh albums again.
Rational means just that. Whilst nobody can give evidence that a spaceman didn't come a travelling, it doesn't mean it is a logical explanation for life on earth. After all, the chemical stew on the comets seem to be the best bet till we get a better picture.
Explanations without basis, whether it be religious or just dreamt up today, is not a good description of "rational". Mental leaps based on evidence such as the chemical compositions of comets is the start of a rational investigation. Not necessarily the truth, but uses a few facts to start the hypothesis.
Noahs ark however is not a rational basis for explaining reality. It is a fairy story based on, perhaps, the opening of the black sea into the med, but who knows?
It certainly ain't two by two etc. For that you would need a God and thats where it falls down.