meaning of "giddy-up"

Since giddy-up commands a horse to get moving, it can be used to mean from the start. A similar phrase would be from the get-go, which M/W has as "from the very beginning". Primordial then is an intensifier, "from the really very beginning, way back when."

A little more context would help but I understand this dense and effective prose to refer to the goddamn (=disgusting and deplorable) primordial (=original) brutal reality of black slavery, to the reduction of human relationship to the coarse master-slave ordering to giddy-up (=hurry-up with the work, with progress; giddy-up being a demeaning order usually given to horses). Such barbarous inhumanity both characterised and defined the status of black people in those days.

The phrase “goddamn primordial giddy-up” is thus an adjectival noun phrase, modifying “homo sapiens”, which is finally equated to “nigger” by the separating comma, which puts the modified “homo sapiens” and “nigger” into apposition.

This interpretation fits with the narrator’s additional modification of “homo sapiens” towards the final appositional “nigger” by his mentions of “pre-black” (=already defined as black), to rural life in the fictional Dickens agrarian suburb, and to the OG (=authentic) membership of the Crips gangs (black-influenced Los Angeles gangs).