Cats have been prowling the earth for over 30 million years. During this time they have become efficient carnivores. Cats are considered obligate carnivores, which means that they really don't need any protein in their diet.
Unlike ominivores, who have the ability to make the nutrients their body needs from a variety of dietary sources, cats are unique in that they are not able to make many of the nutrients their body needs. The story of the amino acid, taurine, is an unfortunate example of this lack of adaptivity from cats. Food manufacturers did not realize the importance of taurine for cats, and therefore did not provide it in commercial diets. Ominivores (dogs and humans for example), are able to make their own taurine if it is not provided in their food. Cats are not able to do this. If they are not fed taurine (animal protein is a source of taurine), they will become taurine deficient. Taurine deficiency in cats manifests as blindness and a heart disease called dilated cardiomyopathy. (Read more about Taurine deficiency Here). After taurine deficiency was identified in the late 1980's, it began to be supplemented in feline diets.
For more information on the unique nutritional needs of cats, Click Here.
How much protein is needed