A descriptive statement is a fairly abstract structure, pragmatically speaking, and as such seems likely to have appeared as a type quite late in human language evolution. Obviously it did not spring into existence full-blown.
In other words, there was probably quite a lot of pre-evolution involved, but we don't have nearly enough data about what really happened to do more than speculate about the relative evolutionary advantages of speech act types.
It is instructive, however, that mathematics, logic, and philosophy (and for that matter, science and theology) have seized on the descriptive statement as the communicational vehicle of choice. Descriptive statements have a lot of convenient features.
- They can be independently verified
- They have a recognizable structure
- They are -- in priniciple, at least -- independent of intentions, presuppositions, or emotions
- They have modes of expression for Quantification, Negation, and Modality (the 3 logical Operators)
But the fact is that descriptive statements are by far less common in real language than are isolated words or phrases, fragments of idioms and conversational formulae, and simple sub-morphemic intonation contours (i.e, grunts, including the various versions of "Oh", "Ah", "Eh?).
Mostly people talk in phrases, not clauses. Sentence-level and paragraph-level consistency in structure and focus is the exception, not the rule -- at least for those educated in Anglophone schools, where such consistency is famously uncultivated.
So the evolutionary reasons for preferring descriptive statements in some contexts are probably still the reasons for using them in those contexts.
Descriptive statements also pair up well as responses to Wh-questions (as they are called in Anglocentric syntactic theories), which are pretty obviously elaborations of natural expressions of puzzlement and anxiety -- so natural that there are characteristic facial expressions and intonations for expressing those emotions, and requesting information about their source, that are pan-H.sapiens.
So whatever the force(s) and source(s) involved, the evolution of descriptive statements can't be divorced from the evolution of questions; they're a natural pair.