Perhaps plants and animals are not so far removed from each other as you think. At the cellular level they are far more similar to each other than to any bacte

Perhaps plants and animals are not so far removed from each other as you think. At the cellular level they are far more similar to each other than to any bacterium. And as to why they manufacture similar molecules, i would suspect reason #1 to be that they derive similar utility. Plants and animals both perform chemical signalling between cells. Both use ion channels. Similarities go on and on. And when we're talking about molecules with only a few dozen constituent atoms, there really is a very finite set of possible combinations which are not toxic and structurally suited to perform some task in the biochemical mechanistic computational environment of the cell. Also, this is not a refutation of your point, just interesting, but complete opioid synthesis has been engineered into yeast by at least two different teams at his point: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4924617/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4924617/