Prior to the reforms of Marius in the late 2nd and early first century BC, the Republican Roman Legion had a completely different organization than that which is commonly illustrated for the Imperial period. The earliest Roman army was based originally on the Greek Phalanx system, and the legion continued to evolve from that origin, and from elements of Latin tribes in Italy.
By the time the Romans began to resist the yoke of Etruscan rule, the unique legionary system was firmly rooted. The Romans abandoned the use of the hoplite spear as its primary weapon of war and instead moved onto the large shield (scutum) and short sword (gladius) as their weapons, and corresponding tactics of choice.
The most significant difference between the Republican and Imperial legions dealt with its makeup of social and financial qualifications. Except for extenuating circumstances where the very survival of Rome depended on using anyone available for defense, the Republican legion maintained a strict social hierarchy. Only landowning citizens were allowed to serve under optimal circumstances, and the status of one's total wealth along with military experience determined their place in the infantry. As each citizen prior to Marius provided weapons and equipment from their own estates, gear could vary, but there was a basic uniform code to be followed within reason. This system was devised out of necessity, but perhaps incorporating the concept of loyalty to the city and morale of the men. Since the army was made up of citizens who could afford their own armor, they were the obvious choice for service. However, in the earliest days, the common concept that only the land owners had a real stake in the outcome of the battle likely played a key role in the development of the early Roman Legion.
Each rung of the ladder on the status of a soldier was based on his ability to equip himself. There is some debate, however on the social mobility of the legionary ranks. Beyond the lowest orders, its quite likely that status of wealth may have allowed a citizen to move into a different rank class within the army. In favor of general cohesiveness, however, upward mobility once an initial rank was established was probably limited until a legion was disbanded. It's far more likely that experience was the more determining factor in deciding a soldier's promotional within a legion after the initial status was determined. Once a veteran was 're-enlisted' his class within the army was based again upon his wealth, but also age and experience.
A soldier within the Republican Legion served much the same as those within the Imperial one. They were eligible for service for a 20 year period and were retired or exempted from further service after that point. Unlike the Imperial army, however, the earlier Republic didn't maintain a standing army so to speak, and soldiers may or may not be entirely active during this time period. In some cases, such as during the Punic Wars, a single soldier may have seen nearly continual service throughout his 20 year eligibility, but at others, he may have had give only periodic service based on the need for the army. Though the Romans rarely had a time of complete peace throughout their history, there were times when it was conceivable that these citizen soldiers could spend a great deal of time working their own lands.