Homochirality problem — origin of life obstacle

Amino acids come in two mirror-image forms: left-handed (L) and right-handed (D) isomers. All amino acids used in biological proteins are left-handed. When amino acids are produced in laboratory simulations (e.g., the Miller-Urey experiment of 1953), a racemic (50/50) mixture of L and D isomers is produced. The probability of randomly assembling all left-handed amino acids decreases exponentially with chain length. Furthermore, the proteins needed are not just random chains but specifically crafted sequences that fold into functional three-dimensional molecular machines — making the probabilistic obstacle even more extreme.