The question of social transition and transformation of Late Ancient lower classes is particularly interesting, but is still an important academic debate and thus difficult, if possible at all, to answer.

With the Late Empire, while the distinction between citizens and non-citizens remained relevant, a new distinction between *humiliores* (humbles) and honestiores (honourables) became prevalent, the latter being defined positively by its access to public functions or state network. The former status comprised every other citizens, including tenants farmers.This distinction was later exacerbated by the fall of the Imperial state as elites previously defined by their integration trough the political, social and economic network that was the Empire re-centered themselves to more regional, or even local, foci in the Barbarian kingdoms, in a world where sur-production to met fiscal and mercantile need wasn't this important anymore; but association to the state and the court remained socially vital.

Already in the IIIrd and IVth centuries, intermediary status between free and servile peasantry appeared with the colonate. Originally contractual tenants from big landowners (that still took a great care directly managing a significant part of their estates) coloni saw their status undergoing an important transformation at the end of the IIIrd century. The increased fiscal needs of the Empire (especially for military and bureaucratic expenses) were addressed by Diocletian fiscal reforms which were nevertheless based on a pre-calculated and necessarily stable fiscal income; and thus fiscal subdivisions had an expected income to provide in money, goods and personal service (corvée, military, etc.) that was ensured by legally biding tenants to their farm or to their patron, making what was a private agreement a public and inheritable matter.

This look, at first glance, close enough to serfdom and was indeed a "semi-servitude" of sort, but the state of dependency to patrons wasn't particular to coloni, with other forms of clients, waged or hosted, being integrated within local social-economic network headed by an aristocratic land-owning elite. While the status of colonate quite possibly reinforced the social confusion among humiliores by providing an intermediary status; this confusion made it harder to distinguish people by their status themselves, coloni being able to flee (temporarily or not, although necessarily illegally) the farms either as a protest against treatments or situations, or more or less definitely to either go to other estates or other patrons; as a form of negotiated treatment, if you will and eventually, the colonate status seems to have been negotiated on a local basis rather than being systematized (unfree status being plausibly a means of social promotion for a free peasant in some circumstances, would it be trough protection from taxes, raiders, or obtention of a landed farm that the patron couldn't chase you off from); other forms of rebellions could include "passing to Barbarians" (as Italian destitute populations did in the early Vth century with joining Alaric's army/people) or simply turning to banditry or open revolt as it happened with the Bagaudae. In the eastern part of the Empire, colonate seems to have evolved towards a free, de facto land-owning, peasantry under fiscal patronage during Justinian and Anastasius' reigns; which would underscore the historical malleability of the status in the Late Empire.Colonate status (statutes, rather) lived on after the fall of the Roman Empire in the West, but there's no real element pinpointing to a systematized status : most probably the variance of situations continued, possibly exacerbated by the economical regionalisation.

Slavery itself clearly didn't disappeared in Late Antiquity or Early Middle-Ages (presence and importance of ancient chattel slavery north of the Alps should be significantly relativized) being a regular occurrence in law codes (the Gothic Law in particular took great care on the matter of fugitive slaves), saints' hagiographies, and other accounts. But being a slave might not have the same meaning in the VIIth century as it did in the IVth century. While the Roman law underscored the distinction between free and non-free (Slaves were attributed a "tag" in the Late Empire for their identification), their everyday function tended to be similar to semi-free peasantry, some slaves being likened to colons, other to waged laborers, etc. although the social-legal distinctions remained important as legal statements were expected to be applied on a complex situation nevertheless.

Effectively, slavery coexisted with semi-free peasantry and free peasantry in Late Antiquity; something that continued after the fall of the Imperial state in the Vth century. The social and economical simplification that followed, with the disappearance of the urban middle-class (itself slave-owning) led to a stronger distinction between upper and lower classes, at the expense of the free/non-free distinction, by sheer contrast. With lower productive expectations and requirement, the need for a servile workforce diminished, and slave trade tended to focus to Mediterranean markets (the Eastern Roman Empire, and later Arabo-Islamic states), with local demand which is hard to estimate. Eventually, slavery really disappeared in western Europe only in the Late Middle-Ages, more or less in the same time serfdom in the strict sense did; which does tends to defuse the idea that the latter proceeded directly from the former (slavery, widespread outside western Europe doesn't seem to have led to serfdom or a comparable situation in Byzantium, Scandinavia, Ireland, etc. either)

The problem is that we lack detailed sources on the situation of lower classes in former western Romania, slaves, colons, clients and peasants being treated "in passing" or as narrative tropes. Use of vocabulary and maintain of Late Roman status and legal vocabulary makes it hard to really pinpoint the transformation of slavery, coloni and "unfree" peasantry as a whole.

The current tendencies tend to counter traditional explanations that a set ensemble of these status (slaves and/or colons; colons/free; etc.) gave birth to serfdom, rather putting the transformation as a matter of regional evolution in the long time, serfdom being less a status than a whole set of statutes for medieval Europe (not even touching the topic of Prussian, Polish or Russian serves).

Somewhere in the early Middle-Ages, between the VIIIth and the Xth centuries, the distinction between free/non-free was updated depending on the circumstances (especially in a troubled late Carolingian and post-Carolingian world, with a mix of opportunities and increased social violence) with some communities managing to hold or to gain a communal freedoms (e.g. allods or salvetats), some kept or put in servitude defined by their lack of freedom as servi, in a status that was hard to translate as slaves or serfs, and probably something that was both (taking from statues that were basically mixed up on a case by case situation, keeping in mind inspiration from Carolingian laws, customs and inspiration from what happened regionally were influential); some other in a semi-free tenant peasantry or other statues (e.g. bordarii, cotarii).

Eventually, what made the distinction was the capacity to renegotiate one status (serfdom disappearing in Xth century Normandy and Brittany, quite possibly out of social revolt) for some being an opportunity of betterment or promotion (even if it meant being at one's service), for other a degradation.Servility itself tended to be peripherised in the Late Carolingian world (the remaining slaves in Francia proper being essentially serves) and tied to slave trade from Slavic marches to the Mediterranean markets, thus the change from servi to sclavi.

- De servus a sclavus, la fin de l'esclavage antique; Didier Bondue; 2011

- Les sociétés occidentales du milieu du VIe à la fin du IXe siècle; Philippe Depreux; 2002.

- Sauvages et ensauvagés : Révoltes Bagaudes et ensauvagement, ordre sauvage et paléomarchand: 1982 ; Pierre Dockes, Jean-Michel Servet

- Slavery in the Late Roman World, AD 275-475; Kyle Harper; 2011