In $1953,$ Stanley Miller and Harold Urey synthesized amino acids by passing an electric discharge in a mixture of methane $\left( CH _{4}\right)$, ammonia $\left( NH _{3}\right)$, hydrogen $\left( H _{2}\right)$ and water vapours $\left( H _{2} O \right).$

Carbon dioxide $\left( CO _{2}\right)$ was not present in the Urey-Miller experiment mixture.