There is no evidence of anything remotely similar to that shown in HBO's Rome television show.

The rank rotation within the file, the use of whistle, the grabbing of the baldrics to keep men in the ranks, and the step drag marching in formation, was all the creation of an Italian Roman army reenactment group that acted as the technical advisers from the show, specifically by the leader of the group,who basically made it all up.

- At the time of the Late Republic, when the series takes place, the milite (common soldiers of the legion) didn't wear over-the-shoulder baldrics, iconography and archaeological remains show they wore their swords on waist belts.

- There was no evidence at the time they were even formed into strict files, their formations seem to be rank centric as described by Livy, Polybius, etc.

- There is no proof that they marched in step, the closest is Vegetius writing many centuries later describing the military step, though that seems to more focus on march speed than anything else; certainly their tactics, as described by various sources, were not complex enough to require marching in step.

- They are described as advancing in battle at the run by numerous sources, to include Caesar, so the drag step as shown was a fabrication.

- Though there is some proof that whistles existed in ancient Rome in some manner, there is no proof that they were used in a military setting, the sources describe the use of various types of horns/trumpets.

- The worst fail from the television show was its omission of the key tactical fighting tactic of the Roman infantry, the devastating pilum volley done during the charge right before closing with sword and shield, which was heavily documented for centuries by basically every Roman historian, to include Caesar, who wrote of his various wars.

The historians Livy and Polybius wrote in great detail about the Roman army in the middle Republic during the days of the so-called "Manipular Legion" (a title used by modern historians), and in it they describe the bread and butter Roman formation being to form the infantry into four classes, one class of light infantry who served as skirmishers, the Velite, and three classes of heavy infantry, the Hastati, Principes, and Triarii, who each formed into their own battle line, grouped into maniples of roughly 60-120 soldiers (and those were likely broken down into two equal centuries). The Hastati, the junior soldiers, served in the first battle line, the Principes, the men in their prime, in the second, and as the name implies, the Triarii, the elders of the legion, were the third liners, serving in an operational reserve, to be committed only to reinforce success or to cover a retreat.

Within each maniple, based on the analysis by the historian Michael J. Taylor, the Republican Romans seemed to most often formed their maniplies in a depth of 3-6 ranks. Polybius describes a spacing of 3 feet between each soldier (a roughly shield sized gap), with an additional space of 3 feet between the man in the next rank behind him. Vegetius, writing much later, describes a much closer spacing within the rank, almost no gap between each man, while having 6 feet between ranks. Many have suspected that Polybius describes an open order formation, needed for throwing the pilum and the aggressive sword and shield fighting style the Romans are attested to have used (requiring space), while Vegetius is describing a close order formation, used to deal with cavalry or deal with enemy predominately using missiles. Caesar, in his Commentary on the Gallic War, describes having to personally intervene in a battle to give an order to his soldiers to extend their formation in order to give them room to use their sword and shields properly. And, from experimental archaeology, we know that ranks could not be close to one another, as throwing the pilum would not be possible if the men in the rank behind were within a few feet of the man in front.

There doesn't seem to be any proof that, outside of casualties or exhaustion, there was any sort of personnel rotation within the century or maniple. Contemporary peoples, Greeks, Gauls, etc., are known to place their best soldiers in the front ranks, and its likely considering the aggressiveness of the Roman offensive minded tactics (they were the ones typically conducting the near suicidal charges), it required the bravest and most aggressive men in the front. Likewise, centurions are noted for taking extremely heavy casualties, Caesar describes cohorts with all their centurions dead or wounded, while the rest of the cohort is only slightly hurt; this seems to point that the centurions led from the front. So likely the Romans front loaded their best, place their mediocre or weaker soldiers in the middle and rear ranks, guarded by the centurion's assistant, the optio, who prowled behind the formation giving encouragements and keeping stragglers in the ranks, and the only rotation occurred if a man could no longer fight. When the majority of the front rankers, the best troops, were too exhausted, suffered too many casualties, or were disheartened, it meant the unit as a century/maniple had lost its usefulness and should need to be replaced by those from the lines behind them.

And numerous sources, Polybius, Livy, Frontinus, Caesar, etc., describe those lines relieving each, but they do not describe how it happened, or really provide any sort of consistent explanation when they drop some hints.

Polybius describes at the battle of Zama how P. Cornelius Scipio's (commanding the large army) was responsible for the decision not to relieve the hastati with the principes after the first stage of the battle was over, as he believed the Hastati weren't spent and could continue serving in the fore; that implies that they were replaced all together, possibly army wide, at the discretion of the commanding general.

Later, when the Roman legionary infantry stopped forming by class type in maniples, and started being grouped by maniple of each three class inside a single cohort (this was done over a gradual century plus long process from the 3rd to 1st Cent BC), it seems that they still most often formed in a bread and butter triplex acies/three lines of infantry, with the cohorts in the second and third lines relieving those cohorts formed in line in the first. But its even more confusing how that occurred.

Caesar, in his Commentaries on the Civil War, describes the battle of Pharsalus as having the main battle lines being three lines, that only Caesar himself (who was positioned at the far right of the overall battleline near the first cohort of Legio X) was authorized to commit the third line of reserve; when Caesar finally did commit the third line the infantry of the first and second lines were either fighting together, or the first line had been entirely replaced by the second line at some point (despite the battle supposedly only being of a relatively short duration).

Lastly, none of this had anything to do with Gaius Marius. Nearly every reform most commonly given to him in pop culture, especially on the internet, is not actually relating to him. What the ancient sources stated he did:

- Removed all excess servants and mules from the use by the common soldiers (not officers), requiring the common soldiers (not officers) to carry their own personal equipment, in order to streamline logistics (Marius' Mules)

- Modified the pilum (in a manner that was never replicated by anyone else)

- Recruited the poor one time to augment an existing army (Marius never levied a single legion from scratch).

- Removed four of the five standards of the legions, keeping only the eagle standard

- Gifted HIS veterans with land grants in order to win them as clients