All currently known living beings from this planet share the same basic gene expression mechanism, which supposedly originated from spontaneous amino acid and nucleotide formation, as hypothesized by Miller and Urey with their seminal 1952-53 experiments. What I have trouble believing, though, is that no parallel life forms ever originated other than what we know. I'm no molecular geneticist, but you mean to tell me that the molecules of DNA, RNA and 20 particular amino acids are the only possible molecular combination that can give rise to self-replication? Why only those 20 amino acids? Why only the L-isomers? There are many other macromolecules that can be generated with the existing quantities of C, N, O and H. In those 1-2 billion years before DNA/RNA/Protein based life-forms reached a critical mass, no other spontaneous generation events took place? Or they did, but they all went extinct? Leaving no trace at all? Even the most isolated, extremophile prokaryotes that have been discovered replicate with the same DNA/RNA/20 Amino acids mechanism.