This is a rather typical question about the "Fall of Rome". But it has many problems, that you need to be aware of. I now assume that you mean the "fall of the western part of the Roman Empire" and not its eastern counterpart, later by us called Byzantium. Then you would commonly assume you mean the toppling of Romulus Augustulus 476, which is often associated with that and the end of Antiquity. But you have to be aware, that this is an academic notion and not what people thought at the time! The date has been called into question and thus seems rather arbitrary from the perspective of periodization as well as the question at hand here. There have been other interpretations of the from antiquity to the middle ages, as proposed by Henri Pirenne. Connected to this is the very problem you are actually hinting at: No one at the time actually recorded the "fall of Rome" in this sense. The message that the crown was send to the eastern Emperor is rather an offhand comment at the time. But what is actually recorded and what send shock waves was the sack of Rome 410. In particular because of its interpretations in the argument between different religious groups (pagans and christians), the writing of Augustin and Orosius (Books of History Against the Pagans 2.3, 7.39–40) show this, as they now felt the need to justify god, some of their pagan countrymen thought this to be the fault of the new state church that had not protected the empire from such horrors.

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Another important problem is in the idea of "left over Romans" you have to understand that the notion of Romanness is rather complicated, the Eastern Romans are a good example of that. In this context, too, I think that no one actually saw a big change at first. Now coming back to our interpretation of 476 as historians today and in the context of your question of graduality. This is actually ONE of the MAJOR debates in Ancient History and Medieval History and you could ask different professors to get different interpretations of it. There is actually a very good section in the open Yale lecture series on the "Dark Ages" which gives a clue for that. In the last 50 years there was a new idea of the transition process established which is now commonly shared among a certain group of scholars: They believe the change between antiquity and the middle ages was gradual and point to many long lasting institutions that still existed from the end of the emperorship onward in the "barbarian successor-states".

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Entering two very famous books in the 2000s written by Bryan Ward-Perkins and Peter Heather, both of which made the sudden and catastrophic aspects of the change more strong than it was established with the shift I mentioned before. I think today it really depends which aspect of the Western Roman Empire you want to look at, to then make a special diagnosis for this very aspect. I hope I could clear up some of the problems in regards to your question. It is considered a very key question, which also gives us thoughts about our own interpretation of civilization and history, and actually a very hard one to answer shortly, quickly and with a definite answer.

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Brown, Peter, The World of Late Antiquity: AD 150–750, London 1971.

Cameron, Averil, The Mediterranean World in Late Antiquity, AD 395–700, London 1993.

Goffart, Walter, Zosimus, the first Historian of Rome's Fall, in: The American Historical Review 76,2 1971, p. 412-441.

Heather, Peter, The Fall of the Roman Empire, London 2005.

Pirenne, Henri, Mahomet et Charlemagne, Paris 1937.

Ward-Perkins, Bryan, The Fall of Rome and the End of Civilization, Oxford 2005.

Jimros' comment is good, but to add to that: the Roman Empire was so large and so vast in a pre-modern media period that without widescale cultural reproduction of the kind performed in the early Empire it was hard for Romans from different parts of the Empire to really agree on what was 'Roman.' Christianity was a key cultural unifier, and Latin (in the West), but a Romano-Egyptian and a Romano-Briton in 400 AD would have lived very different lives that shared some common aspects.

Just as importantly, by this time, many people living north of the Roman frontier in Europe would also have been living lives that could be said to be variations on 'Roman.' The old Italo-Roman families might not have recognized someone from north of the Danube as 'Roman,' but they often said the same thing about Romans living along the border in Gaul and Briton, or in Syria and Armenia.

One of the things archaeologists and historians note from the late Roman period is that many people outside the Empire's traditional borders were now living lives that archaeologically cannot be distinguished from those of their Roman neighbors. So... who was Roman anymore? Were they all?

This is one of core issues that led to the decline of Imperial unity, and is a part of why there's not really any point in history before the Lombard invasion when the West suddenly realizes that they are no longer part of the 'Roman' world. Northern Europeans with very Romanicized ethnic identities had been living and fighting and serving inside the Empire for more than a century before the deposition of Romulus Augustulus. Many of them had merged into Roman civilization. Some had become high-ranking officials. One even became an Emperor, briefly. But instead of accepting this gradual fusing together of their culture with that of Northern Europeans, a lot of elites in the central and Eastern Empires became extremely xenophobic about who could and who could not call themselves Roman, and redefined 'Roman' in such an exclusionary way that many Gallo-Romans and Hispano-Romans and Romano-Britans and Romano-Illyrians - and virtually the entire Western Army - found themselves lumped together with 'the Barbarians' of Northern Europe as being 'insufficiently Roman.' This despite the fact that no one had questioned the Romanness of the North Western provinces before.

And in case you are wondering, no, the inhabitants of the Eastern and Central Empires were not somehow More Roman than the Romans of the Northwestern Provinces. Christianity had brought many changes to Roman society, and elite Romans of the Central and Eastern Empires had just changed in ways that made them very different from their Northwestern comrades.

This polarization of Roman identities suddenly made North-Western Romans start to realize that they now had a lot more in common with the 'Barbarians' along the border than they did with their fellow Romans in the Central and Western Empires. When Roman Imperial authority began to retract in the West, the Franco-Roman, Romano-Burgundian, and Romano-Gothic overlords that replaced it looked and acted Roman, in the way that people from that part of the Empire understood Romanness, and they recognized the overlordship of the Eastern Roman empire in a way that they recognized as 'Roman-enough.' A bunch of elites and intellectuals may have seen the writing on the wall, but the average Roman would not have noticed the difference for quite some time.

The Gothic Wars were probably a big wakeup call for Romans in the West. Of all the sub-Imperial Kingdoms, the Ostrogoths had been the most consciously Roman of them all, so much so that the Italo-Roman senatorial elites had treated Theodoric as a sort of returned Western Emperor (they called him Augustus in one inscription), and two successive Eastern Emperors had recognized his rule as legitimate. He was even sent Imperial robes from Constantinople, and allowed to issue coins in his image. When Justinian set about conquering and reconquering Italy, I think a lot of Romano-Gauls and Hispano-Romans may have suddenly realized they were no longer a part of Imperial Roman civilization anymore. Some of the Hispano-Romans even asked Justinian to invade.

So from the perspective of Romans in Rome there were gradual changes, and then some less gradual changes. The later empire regularly featured Germanic generals with significant power, and that was a feature of the period of the end of the western empire as well.

The traditional end date of the Western Empire is in 476 when the Germanic general Odoacer deposed the last emperor, Romulus Augustulus. Odoacer didn't declare the Western Empire ended, he basically just told the Eastern Emperor that he felt that there was no longer a need for two emperors, and that he would rule Italy on behalf of the Eastern Empire. Odoacer ruled Italy for 17 years, until he was displaced by Theodoric and his Ostrogoths, who were acting on behalf of the Eastern Empire as well, which had decided they wanted Odoacer replaced.

The immediate post-empire period from 476 AD to 535 AD was very similar to the late Empire, it was characterized by powerful Germanic military leaders who recognized the Roman Empire, as well as Roman civil and religious authorities. The only difference was there was no Western Emperor, and the Emperor recognized by the Germanic military leaders was the Eastern Emperor.

The real point of discontinuity came, counter-intuitively, when the Roman Empire attempted to re-establish direct control of Italy during the Gothic War of 535-554. The brutal and lengthy ward between the Romans and the Ostrogoths devastated Italy.

The final point of discontinuity came in 568 AD when the Lombards invaded Italy. The Lombards dispossessed most of the remaining Roman landholders. With the exception of the Church, by 575 AD, there was no significant continuity between the Roman era and the situation at that time. The 40 year period between 535 AD - 575 AD represented the end of the immediately post Western Empire period of time.