JOESTALIN may be full of shit here but there are bio-pathways to methamphetamine and amphetamine.
How do I know this? Well, methamphetamine and amphetamine both occur naturally in Acacic rigudula. Since plants create all their goodies via enzymes I conclude that there is some enzyme that makes meth. From what I don't know, but phenyalanine is a good guess. PERHAPS, the meth is a result of a bacterial symbosis, but still there is an enzyme.
The most fucked up path I can imagine is that the meth is a result of accidential reduction or oxidation of another molecule being synthesized imperfectly. For example the synthesis of Molecule A is done via an enzyme B that results in some free engery that acts on molecule C which turns into meth. (Not very scientific am I)
If you accept that the presence of meth and amphetamine reported in the paper on my website is indeed correct the possiblity of some enzyme exsisting becomes more plausible.
THE PAPER:
http://chemguy.homestead.com/AcaciaRigudula.html (http://chemguy.homestead.com/AcaciaRigudula.html)