The diet of each type of animal must include a specific groups of essential nutrients essential amino acids essential fatty acids , vitamins and minerals. To provide through  material needed to synthesize the variety  of organic molecules the animal must have to sustain life. The essential nutrients are materials the animal must have but cannot synthesize in its cells. The nutrients that are essential differ for each type of animal because the ability to synthesize particular molecules is a genetic trait acquired through natural selection and because each type of animal has a unique evolutionary background .

In general the more simple and primitive animals are able to synthesize more of the materials they need  than can the more complex and advanced animals. The ability to construct many materials was lost during the evolution of more complex animals. Because these materials became so richly  available in the diet that the enzymes required to synthesize them were no longer necessary and were lost for the genetic repertoire.