The first amino acid, discovered in 1806, had a disarmingly ordinary source: asparagus juice. Hence the name asparagine. The next amino acids to be discovered had sources that were, if anything, even more humble. By 1820, cystine had been isolated from urinary stones, glycine from gelatin, and leucine from muscle and wool. All of the common amino acids were discovered by 1935, when threonine emerged from studies of animal nutrition involving growth stimulation assays.

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[genengnews, 2023.9.1, https://www.genengnews.com/topics/omics/noncanonical-amino-acids-inspire-the-development-of-novel-drugs/]