Of course it has always existed.
I agree, but not for the reason you've offered. As long as men have believed in Gods that they needed to worship, there's always been someone else standing to the side and saying, "you guys are doing it wrong! That's not how we did it last week".
People have been fighting because their religion is the right one or because their king or priest tells them their god wants them too. Is that not a fundamentalist?
No, it is not.
Killing people because you believe you're right and they're wrong (eg religious crusades) is a behavior that STEMS from believing the fallacy of fundamentalism, but it's not a synonym. (And if we're going to use certain terms, we have to abide by their meanings to avoid confusion).
The problem with the concept of fundamentalism is that it implies that some ancient standard of "pure" or "fundamental" form of worship actually existed in the distant past, and if we can ONLY know and attain that, then we can worship in a "more pure" fashion.
However, such ideals have often been shown to be just myths stacked on top of more myths, actually being an idealization of a past that never actually existed.
Such fantasties are part-and-parcel of religious thinking, and have been encouraged by relying on the same flawed idealizations of prior generations of history, Eg in the case of Catholicism, early Church historians offer a picture of Early Christianity that is highly idealized and uniform, since surprise! They had a dog in the race, and offered an idealized version of early Christianity which never was, but the result of wishful thinking. We KNOW with some certainty (confirmed from independent sources) that someone who called themselves a Christian in Egypt worshiped very differently than someone in Rome, even using different religious texts to guide their beliefs. So in the case of Christianity, the idea of "pure worship" is an ideal that never existed, since there ALWAYS been different concepts that existed about how to conceive of Christ; the discovery of the Nag Hammadi texts shows the wide divergence in what later got lumped together into the bin of "Gnostic" beliefs, a sub-set of Christianity which were targets of later suppression when the Romans (who had officially accepted and defined "orthodox" Christianity) tried to stamp out these heretical beliefs like brush-fires. Heck, THAT'S the dirty secret of Christianity: there's NEVER been a consensus, hence no fundamental beliefs to return to.
Similarly, Judaism wasn't some monolithic religion, either, which was unified in Jesus' day; aside from well-known Pharisees vs Sadduccees battles recorded in the NT, there were HUNDREDS of Jewish sects which coexisted that are mentioned in various Mishnas, and they likely developed side-by-side, where any common source is lost in the fog of time.
So the same human cycle likely occurs, whether we're talking about Christianity, Judaism, Islam, Buddhism, etc. It occurs today, where Mormonism and Scientology are relatively young religions, the former being an offshoot (sequel) splintering from same seed-bed of the OT/NT.
Sure, some have suggested that the scientific advances during the Age of Enlightenment have triggered a strong reaction towards fundamentalist beliefs in society, but that's likely a reflection of an intrinsic psychological human trait, where people feel the need to counter challenges to their belief system.
The JWs are a PERFECT example of how some groups respond after being confronted by overwhelming scientific evidence which challenges their World view, and they'd rather fill their heads with fantasies of some perfect past that never actually existed, thinking that if only they can assemble enough like-minded individuals who agree to believe, it'll become true. It's the stuff of Peter Pan, where if you only believe strongly enough, fairies can be willed into existence, and "paradise can be regained".
So JWs spend their lives trying to model their society based on theocratic ideals, as if they fit into the storyline and THEIR actions will help usher in the New System. They use the conception of an ancient Israelite society which archaeology indicates didn't even exist as they imagine.
These personality types have likely ALWAYS existed (although they likely didn't always ACT on their beliefs, and weren't always in a position to influence the actions of others), just as there's always been those willing to question the main group, being willing to explore and even die in the name of truth, not group fantasy (Giordano Bruno comes to mind).
That speaks to a wide diversity and spectrum in personality types found in most human populations, and to claim fundamentalism wouldn't exist at some time in large populations is a bit hard to swallow; that type of hyperbolic claim is often a vestige of absolutist thinking, relying on the fallacy of mutual exclusivity ('either/or' dilemmas, as if the existence of fundamentalist beliefs is a BINARY situation of ALWAYS existing vs moving on a variable slider that changes with time).
How on earth can you be a fundamentalist "non-religious" or a fundamentalist atheist?
I actually COULD make a few arguments to defend the concept of a fundamentalist atheist (based on looking at the idealized beliefs of the first-recorded atheists), but won't, since I suspect the person who made that statement needs to think a bit more about the definition of fundamentalism and reflect on the foolishness of the tired ol' canard, "atheism is just as much of a religion as that of believers, and demands MORE faith than religion".