My knowledge of earth science is basically null so please bear with me. My first question relates to the soundness of assuming a primitive atmosphere rich with H2O CH4 NH3 and H2 gases (Also not too much presence of O2, as the Urey-Miller atmosphere has to be a "reducing atmosphere"). The wikipedia article mentions this has been criticised but I was left wondering what exactly can you criticise about that? What was the composition of the primitive atmosphere? Where did O2 come from then? What's so special about a "reducing atmosphere"? Also, versions of these experiments use 100,000 Volt sparks to initiate chemical reactions. I was searching for the typical potential difference in a thunderbolt and couldn't find anything. What is it? Does this question make sense? On the other hand, thunderbolts assume clouds, I think. What evidence is there for the formation of clouds in the primitive atmosphere? And my final question is: There exist 500+/- different amino acids. I don't know if they are all found in nature. The ones that are common to life are only 20. Does that mean that all over earth one can only find 20 different amino acids? Or can one find those actual 500? Why would life prefer 20 only out of all those? Thanks!