by alien we mean living organism and life forms coming outside from a planetary habitat in question.
by the definition, the alien is ME and YOU, and everyone else. take note that millions of years ago, earth is a highly sterile place. chemical compounds necessary for life to evolve came from meteorites. yes, the strayed meteorite carry in itself nucleic acid essential for the building blocks of life like RNA and DNA, chemicals including amino acid glycine, carboxylic acid, and some precursors to sugar.
these elements should not have came form earth. our minerals' formation tend tend to be catalyzed by different biomolecule, thus they end up separated and less likely to form life.