Cheese by John Spottiswood.

Our early ancestors probably discovered cheese when they first used animal stomachs to carry milk. An enzyme in the stomachs called rennet would have caused the milk to curdle and separate into cheese and a watery liquid called whey. People have been tinkering with that basic recipe ever since then, and there are now hundreds of different kinds of cheeses. Cheese-makers impart different flavors and textures into their cheeses by using different milks, adding various bacteria and molds, aging for different lengths of time, and so forth.