The classic 1952 [[Miller–Urey experiment]] and similar research demonstrated that most amino acids, the chemical constituents of the [[protein]]s used in all living organisms, can be synthesized from [[inorganic compound]]s under conditions intended to replicate those of the [[History of Earth|early Earth]]. Scientists have proposed various external sources of energy that may have triggered these reactions, including [[lightning]] and [[radiation]]. Other approaches ("metabolism-first" hypotheses) focus on understanding how [[catalysis]] in chemical systems on the early Earth might have provided the [[Precursor (chemistry)|precursor molecules]] necessary for self-replication.<ref name="Ralser 2014">{{cite journal |last1= Keller |first1= Markus A. |last2= Turchyn |first2= Alexandra V. |last3= Ralser |first3= Markus |date= 25 March 2014 |title= Non‐enzymatic glycolysis and pentose phosphate pathway‐like reactions in a plausible Archean ocean |journal= [[Molecular Systems Biology]] |volume= 10 |issue= 725 |page= 725 |doi= 10.1002/msb.20145228 |pmc= 4023395 |pmid= 24771084}}</ref> | The classic 1952 [[Miller–Urey experiment]] and similar research demonstrated that most amino acids, the chemical constituents of the [[protein]]s used in all living organisms, can be synthesized from [[inorganic compound]]s under conditions intended to replicate those of the [[History of Earth|early Earth]]. Scientists have proposed various external sources of energy that may have triggered these reactions, including [[lightning]] and [[radiation]]. Other approaches ("metabolism-first" hypotheses) focus on understanding how [[catalysis]] in chemical systems on the early Earth might have provided the [[Precursor (chemistry)|precursor molecules]] necessary for self-replication.<ref name="Ralser 2014">{{cite journal |last1= Keller |first1= Markus A. |last2= Turchyn |first2= Alexandra V. |last3= Ralser |first3= Markus |date= 25 March 2014 |title= Non‐enzymatic glycolysis and pentose phosphate pathway‐like reactions in a plausible Archean ocean |journal= [[Molecular Systems Biology]] |volume= 10 |issue= 725 |page= 725 |doi= 10.1002/msb.20145228 |pmc= 4023395 |pmid= 24771084}}</ref> |