Perhaps aliens eat right handed life, but left handed life is poison to them. Seriously. It would be a pretty good selector, and said “alien” need be no more t

Perhaps aliens eat right handed life, but left handed life is poison to them. Seriously. It would be a pretty good selector, and said “alien” need be no more than a snippet of RNA - and it would be entirely gone from earth now, eliminated by us sinister life forms. The only evidence would be the ubiquitous absence of R-entantiomers in life. I think I might be lifting from Asimov - The Left Hand of the Electron.