let's be clear about this.

when the romans annexed (through a mix of conquest and other means) what was at the time the remnants of the conquests of alexander (a collection of loosely affiliated greek kingdoms), they set up two provinces around the dead sea, the first of which was named iudea after what they thought were the indigenous peoples of the region - a claim that we think today is probably false given that there is no archaeological evidence to support it, even if the ultimate origin of the israeli people remains somewhat unclear. i tend to lean towards a west asian hypothesis, which is actually their own origin myth in that they claim that abraham came from persia. they seem to essentially be a zoroastrian iranian nation that was absorbed by the local (west) semitic culture upon migration, but that refused to give up it's monotheism. judaism seems to be a syncretic mix of zoroastrianism and local canaanite lore. there does not even appear to be a kernel of truth in the exodus story - it appears to be entirely fabricated from whole cloth, a work of complete fiction, probably invented out of nothing at all, during the captivity. the romans also set up on arab province around the city of petra, on the red sea, but there would not have been arabs living on the coast of the mediterranean during this period, even if the province of arabia did briefly touch the sea west of gaza in modern day egypt. the arabs of this period were illiterate desert nomads that were frequently used as soldiers by the empires of the region, but lacked any centralized state of their own; this area would have been primarily greek-speaking and greek in culture, like the rest of the eastern part of the empire (including egypt and what we now call iraq). the province was really what is called a march, a concept that has created existing states like austria (literally translated to the eastern borderlands) and ukraine (also literally translated as border area). the romans had similar marches on the border areas with the germanic tribes. so, what the naming of the province really meant was "this is a buffer state between the empire of the civilized world and the desert barbarians outside of it".

so, during the roman period, the jews were inside the boundaries of the empire and the arabs were not; the jews had a province, and the arabs had a march.

as is widely known, the jews were a restive people that sought a nation state defined by their own laws and refused to submit to the roman emperor. their religion prophesied a warrior-king they called a messiah that would lead them to autonomy, so, with every potential rebel that appeared, the people saw some kind of divine revelation. so, the province of iudea was frequently in revolt. the romans found the jews to be a great annoyance, as they had little taxable wealth but were sitting on top of one of the most strategically important regions of land on the planet. as they could not grant autonomy in this region due to it's strategic importance, and the jews would not submit on account of their religious beliefs, it was eventually decided that they should just be destroyed altogether. in truth, the attempted genocide of the jews was one of many attempted genocides by the romans (the most famous being the carthaginian genocide), but also probably the least successful. the jews were said to be scattered (although it is understood today that many of them stayed) and the area was renamed from iudea to palestinia, after the historical tribe of philistines (who had long disappeared from history).

the philistines are mentioned in hebrew scripture as a race of giants, and actually explicitly referred to as originating from greece. while the bible is usually a questionable source, at best, we can actually corroborate this fairly well with the destruction horizon around 1200 bce. during this period, the entire eastern mediterranean, from greece all the way around to egypt, was invaded by a shady group of characters that we call sea peoples because they appear to have come from the sea. this invasion led to a widespread collapse of civilization and a lengthy dark age outside of egypt (which was transformed, but survived). when the dust settles, we have these people the egyptians called pelesets settled just outside of their borders, on the coast. it is widely accepted that these pelesets were a greek tribe that tried to invade egypt but were repelled, and that they are in fact the same people as the philistines, which is the etymological origin of the palestinians.

so, palestine is not a term that is historically connected to an indigenous group of arabs in the levant, which in truth never existed, but rather a term that is historically connected to a group of europeans that settled in the region roughly 3200 years ago. about 1800 years ago, the romans renamed the region after the philistines in an explicit attempt to destroy the semitic history of the region and europeanize it in a colonial roman image. so, your palestinians are both european in historical origin and resurrected from long historical death by different europeans in order to colonize the area. some indigenous group.

but, if the indigenous people of the region were neither jews nor arabs, who are they? it's maybe impossible to be specific beyond pointing to a concept of (north)west semitic as a cultural grouping, a group that is referred to in the very ancient world as canaanite and after the destruction horizon of 1200 as phoenician. these were important people, in world history. but, they were neither arabs nor jews, even if the jews were eventually absorbed by them after they invaded the region.

while there would no doubt have been some arab migration into the region in the upcoming centuries, the empire's borders held fairly firm for quite some time, well past the conversion of the region to christianity. palestine, jewish or not, remained inside the now christian empire, while the arabs remained outside of it. conflict in the region mostly took the form of civil wars around theological aspects of christianity, and the continuing thousand year war with the persians. an arab in the levant in this period would have probably been a mercenary in a roman legion, and not a roman citizen or inhabitant inside the empire.

it was the thousand year war with the persians that led to the decline of both empires by around the year 600. neither of them saw an arab uprising coming, and it's unclear if either could have stopped it.

as it is, while the dates around the arab conquests are blurry, and the invasion of the levant was not a singular event, it was certainly the case that the roman provinces of palestine were under the control of arab forces by the year 700. it was only after this point that you have a process called arabization take place, where the peoples that were conquered by the arabs became arabs in culture and language. there was romanization, hellenization and even iranization before this, so there is actually some historical process at play. the region would later undergo a turkicization, as well. arabization was successful throughout northern africa (including egypt) and parts of the middle east, but it was rejected in iran and asia minor, who insisted on maintaining their own culture. turkicization eventually succeeded in asia minor, where arabization failed.

while it was initially thought by modern scholars that arabization was a migration process - that is, that large amounts of arabs moved into the region and displaced the local inhabitants - this hypothesis failed to live up to much scrutiny. the historical records might have led somebody to believe that, but it just didn't add up when faced with any kind of scrutiny. how can a small amount of arab nomads repopulate not one but two empires? that's absurd on it's face. however, it wasn't until the advent of dna testing that the issue became an empirical question. who were the palestinians? were they arabs that invaded after the 7th century? were they greeks from antiquity? phoenicians?

and, we now have the science to state it.

the palestinians are the descendants of the jews that lived in the region in antiquity.