The positions of the early republic came directly from the regal period. Other than the Consuls, which was a position to replace the magistracy of the King, the other positions and assemblies already existed while Rome was under a monarchy.

This is one aspect of Rome throughout its period of political independence, the constitution was always contiguous, and never underwent revolution. This is consistent with an unwritten constitution where new conventions and constitutional structures are constantly added. The down-side is, that many of these conventions come in times of emergency; such as the positions of dictatorship, tribune, etc. These can be good, it appears that the unification of the patricians and plebians came from the Gaullish threat (which fits Turchin's argument), but we have examples in the modern world of increasing political equality achieved without a state going into emergency.

Arguably the Roman Constitution was so conservative, as oligarchies are, that the only method for constitutional change was emergency. The civil strife of the late republic where Caesar and Augustus sought to change the fundamental nature of the Roman Constitution could only be done through the imposition of emergency. Since Gracchus, this, rather than statutorial or consular change, became the dominant political form of progression.

I think it is a mistake to look at the Roman Constitution in modern liberal democratic terms. Rome was a martial-state; not a nation-state, and not a modern market-state. It was structured for martial success and security. Its magistrates, assemblies and Senate all had military aspects - such that its democratic nature was militarily based. Consuls and Senators were expected to be generals rather than civil servants. And the plebians that made it into the patrician class, or later into the elected positions, were from the wealthy plebian equestrian class (cavalry).

Since a military structure and militaristic society needs to be able to adapt to constant expansion, warring and infrequent catastrophes such as invasion, it makes sense for emergencies to be the driving force behind organisational, and hence constitutional change.

Another aspect of the Roman Constitution, outside of its deeply oligarchic nature, that would help retard and stop constitutional change, was the veto. This was a central political technology in the Roman Magistracy that could stop any legislation by a single person with imperium. It was intended to halt tyranny, especially from the consuls, which it was feared might seek regal powers, but it became an effective means to halt change. The veto was well established in the early republic as a technology, but I don't know how much it was used. I have not seen or read any discussion on its use during this period.

Part of Scullard's argument for their being two class based states in the early republic was that plebians were not allowed to run for elected positions such as consuls. Plebian law from plebian magistracies and assemblies were also not binding on particians, whereas patrician law was binding on plebians. Not surprisingly, plebians, especially rich ones, agitated for legal equality. Additionally the caste nature of this arrangement was enforced by law that prohibited the inter-marriage of patricians and plebians.

During the early republic many of the constitutional and magistrate positions were duplicated in both the plebian and patriciate states; such as the aediles, and there were four assemblies at this point. There was, however, a written component to the Roman Constitution in the early republic which was the Twelve Tables (I will cover them in detail later). This was the writing down of traditional or customary law into statutory legislation which could be termed constitutional. It appears they were ten commandments like - ie be a good person, where good is defined by the tribe, and don't be an immoral asshole, where immoral is defined by the tribe. Morality always being a moving target in human history.

So we have in the early republic the standard template for Roman Constitutional change and advancement for the martial-state's prosperity. It continued the constitutional positions from the regal period, other the Consuls which were created to replace the King, and Rome went conquesting, such that it became a local power, able to siege Etruscan cities as well as help out its Latium and Hernici allies.

The Roman martial-state was slowly on the path of increasing its size, army, wealth and influence through this period, until it was invaded by Brennus' Gauls and the state collapsed as its military was destroyed. A martial-state is nothing without its military power guiding the state, the society and culture.

This emergency led to the reorganisation of the state and constitutional innovation which was the removal of the partician-plebian caste divide. This became the common constitutional basis for the next stage of the Roman Republic - the middle Republic. Which is probably best defined as the period between the Gaullish invasion of Rome and the Second Punic War.