According to the theory, human language evolved through opposing forces of destruction (our natural tendency to save effort by shortening and compressing words) & creation (new words).
Deutscher starts with a simple story, told without the structures such as prepositions, tenses, cases, conjunctions etc that give current language its expressiveness
girl fruit pick turn mamoth see
girl run tree reach climb mammoth tree shake
girl yell yell father run spear throw
mammoth roar fall
Justifying the above simple 'Me Tarzan' scaffolding as a legitimate starting point on which his evolutionary forces would have operated, Deutscher presents 4 'natural and transparent principles' (e.g. keep things that are close together in time close together in the story, etc) that, he argues, are sufficient.
The third principle is "Don't be a bore", i.e. those parts of the narrative which are less important, or can be understood from the context, need not be restated. For instance, reworking the story's first part and repeating the actor involved
girl fruit pick girl turn girl mammoth see girl run girl tree reach girl tree climb
Because listeners can work out from the context that it was the girl that turned around, and not the mammoth, there is no need to restate it every time. To do so is wasted effort. The identities need only be made explicit when there are multiple possibilities, e.g. either the girl or the mammoth might have run away. At other times, identities may be safely left implicit (or replaced with time and effort-saving references such as the pronouns 'she' & 'it'.)
As a warning, Deutscher writes
Of course, speakers cannot always assume that the identity of the participants will be obvious to the listener.