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Molecules like amino acids are '''chiral''', meaning they exist as optical [[isomers]] of each other. A "chiral" molecule (from the {{Greek Name|χειρ|cheir}}<ref name=bio>{{cite book|author=Voet, Donald; Voet, Judith V|title=Biochemistry|edition=4th|publisher=John Wiley & Sons|page=74|location=River Street, Hoboken, NJ|year=2011|isbn=978-0470-57095-1}}</ref>, {{Greek Name2|χειρός|cheiros}}, a hand) is one that can not be superimposed on its mirror image. Just as left and right hands are mirror images and not the same, chiral molecules have the same things attached in the same order, but opposite of each other.
The two isomeric forms ([[enantiomer|enantiomers]]) of [[amino acids]] are known as the D and L forms. Enantiomeric molecules are physically and chemically indistinguishable by most of the techniques available and only when probed asymmetrically, for instance, by plane-polarized light can they be distinguished.<ref name=bio /> Although most amino acids (except for [[glycine]], which is non-chiral) can exist in both L and D forms, [[life]] on Earth is made of only L-form amino acids. <ref>{{citar livro|autor=Sarfati, Jonathan|autorlink=Jonathan Sarfati|título=[[By Design]]|editora=Creation Book Publishers|ano=2008|local=Australia|página=175|isbn=978-0-949906-72-4}}</ref> The L form is found in proteins. The D form is found in only some [[proteins]] that are formed by exotic sea dwelling organisms. No one knows why this is the case, but it offers strong evidence that life was designed rather than the result of random [[Abiogenesis|chemical evolution]].
