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Essential amino acids are those that the human body cannot synthesize on its own and must be obtained through the diet. There are 9 essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Nonessential amino acids, on the other hand, can be synthesized by the human body and do not need to be obtained through the diet. Even though the body can synthesize them, they still play crucial roles in various bodily functions. There are 11 nonessential amino acids: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.

In evolutionary terms, the requirement of essential amino acids through diet is a result of genetic mutations and alterations in the enzymatic pathways responsible for amino acid synthesis. Over time, as species evolved and faced environmental pressures, the ability to synthesize certain amino acids might have been lost due to superfluous or redundant processes that would have required unnecessary energy expenditure. As a result, these amino acids became essential in the diet as a way to satisfy the body's needs and maintain an efficient metabolic process. On the other hand, nonessential amino acids are synthesized by the body as they are needed. This ability to synthesize nonessential amino acids has been conserved through evolution because it ensures the availability of amino acids for efficient metabolism, even if the diet does not provide sufficient quantities. In conclusion, the functional definition of essential and nonessential amino acids can be explained in evolutionary terms as the result of genetic mutations, enzymatic pathway alterations, and natural selection, which led to the development of efficient metabolic processes ensuring the availability of necessary amino acids for survival, growth, and reproduction.