I'll try to answer this question by talking about why the reforms were needed. After that I'll dive into Caius Marius's contribution and if there were any signs these reforms werealready taking place.

This is a question that, from the research I've done, is debated continually even now. The importance of the "Marian Reforms" (whether it was Marius himself or more gradual processes) results from the fact that the Republican army could no longer function on old traditions. The old Roman legion was based on the idea of farmers and workers leaving their fields for a few months to fight, then return home. The Punic Wars, among other events, changed this dynamic. The old system slowly eroded over the decades preceding Caius's appointment as consul in 107 BCE.

The original idea of the Republican army as that based on the idea of a soldier providing their own equipment, making their own way, and eventually being disbanded again until they needed to be called up again by a different Consul. This "militia army" built a strong and flourishing Republic. But it had its flaws. This militia system relied on the idea that these soldiers would eventually be able to return home to their farms. With additional territory acquired after the Punic Wars and the annexation of Greece, the need for standing armies that never left conquered territory meant soldiers could spend up to ten years abroad. Hardly a "citizen soldier." By the time they returned home farmland had been stolen, their wives taken, and their family left in destitution. This was no way to repay the service of a soldier. The other problem, of course, is that of manpower. With holdings further and further abroad, more soldiers were needed to protect conquered territory.

These problems were already cropping up by the time Caius Marius instituted his reforms. So, the question is, how much of this revolution happened at the hands of Caius Marius and how much change in the army happened as a result of gradual pressure placed on the Legions themselves? That's really the heart of your question. And this question is really the confrontation of two different theories of history. The Great Man theory says that, by and large, any major change that takes place within a society or culture can be attributed to a powerful and influential few. People like Hammurabi, Caius Marius, or Albert Einstein buck traditional trends, innovate and change the world. I think that's true sometimes. Maybe. The other idea is a newer trend in historical research which attributes cultural/societal progress to long-term trends taken up by society as a whole. The so-called "Great Men" are just cashing in on a trend many millions had been working toward. I think both theories have merit.

Let's take for a second the story behind Caius Marius's need to "reform" the army. Why was it him and not someone else? Caius Marius was elected consul in 107 BCE and sent to replace the commander of the Numidian War. The Roman Senate was furious with his appointment and as punishment denied Marius the right to raise new legions to strengthen his army in Africa. He could only take volunteers. Marius, who'd already been known to shatter convention as a novus homo (new man), was popular amongst the people, as a speech from his 107 BCE consular campaign can attest to. While this speech is the invention of the historian Sallust, it shows Marius's bellicose attitude:

"Now, compare me, fellow citizens, a new man, with those arrogant nobles. What they know only from hearsay or reading, I have seen with my own eyes or done with my own hands – what they have learned from books I have actually done during my military service. Work it out for yourselves whether words or deeds are worth more. They hold my humble origins in contempt – I scorn their worthlessness... … If they (the nobiles) justly look down on me, then let them also despise their own ancestors, whose nobility began with courage, as did my own." (In the Name of Rome 134)

Marius, therefore, had no problem in breaking with tradition. In response to this embargo by the Senate, the consul took to the streets and asked for volunteers from the poorest citizens. Those who lacked proper possessions to qualify for military service, the capite censi (headcount), responded enthusiastically to Marius's overtures. The rest is history.

That's the traditional story and it is most likely true. But how much was Marius playing on trends already established within the army? There is evidence that since the Second Punic War there had been periodic reductions or suspensions of the property requirements. One soldier who served in the 170's BCE, Spurius Ligustinus, did not have the required land to serve in the legions but still volunteered many times over the course of his 22-year career. Ligustinus was a semi-professional soldier within the legions, serving as a centurion even though he did not meet the requirement. There is other evidence of poorer soldiers serving within the legion before the Marian reforms. It may be the case that Caius Marius was making open a practice that had already existed in principle. I tend to think that this is probably true. Caius Marius may have finally institutionalized a practice already going on within the legions. I tend to think that the Marian reforms are important, but not the ground-breaking, earth-shattering change to the legions older historians have made it out to be. Caius codified a system that would, however, allow the Roman legions to transition from a militia army designed for local warfare into a fully professional army intent on acquiring and keeping foreign territory.

Sorry I can't be more thorough hopefully this helps!

Goldsworthy, Adrian. In The Name of Rome. London: Orion Books, 2003.

Goldsworthy, Adrian. The Complete Roman Army. New York: Thames & Hudson, 2003.

Beard, Mary. SPQR. New York: Liveright, 2015.