Stahl thought that because some parts of plants , like raw cellulose and starch are hard to digest in uncooked form , they would likely not be a part of the hominid diet before fire could be controlled . These parts include stems , mature leaves , enlarged roots , and tubers . Because of the indigestible components of plants such as raw cellulose and starch , certain parts of the plant such as stems , mature leaves , enlarged roots , and tubers would not have been part of the hominid diet prior to the advent of fire .