1932-1967: Classic beginnings
In the beginning, zombies in film reflected traditional Haitian mythology: they were the reanimated dead, toiling under the control of a Voodoo master. They were not hungry for flesh or brains — in fact, they had no motivation outside of following their master's orders.
Even early on, however, cinema decided to tweak things a little. The first feature-length zombie film, 1932's White Zombie, introduced the idea of Voodoo possession of a living person: the result was a mindless being, but one who could ostensibly be cured (to varying degrees of success). The 1943 film I Walked With a Zombie picks up on this. Ultimately, its zombie is ambiguous: is Jessica actually a zombie, or merely afflicted with tropical fever?
The modern zombie has little association with these early films: the Haitian mythology got dropped entirely in favor of the more popular virus explanation. But it's worth noting that the classic zombie movies of the '30s and '40s did introduce the idea of the zombie as a living person. This put them more in line with humans under a vampire's thrall (see 1931's Dracula) rather than simply dead people crawling out of the earth.
1968-1984: George A. Romero makes his mark
The modern zombie film owes an incalculable debt to George A. Romero, who reinvented the genre with Night of the Living Dead in 1968. The look and feel of the zombie is based on Romero's model — not to mention that persistent hunger for flesh. These were zombies as antagonists.
To be fair, there were predecessors. Romero based his zombies in part on the creatures in the novel I Am Legend, though those were closer to vampires in nature. (The adaptations of the book — The Last Man on Earth, The Omega Man, and I Am Legend — have offered various interpretations.) Romero also never used the word "zombie" in the script, instead employing the term "ghoul." In later interviews, he began referring to his creatures as "zombies."
Outside of the zombies themselves, one of the most important innovations introduced by Night of the Living Dead was the idea of zombie as social commentary. The hero of the film is Ben (Duane Johnson), a Black man described in the script as "comparatively calm and resourceful Negro." Despite staving off the zombie horde, Ben is gunned down by fellow humans who mistake him for a zombie, a reflection on contemporary race relations. Romero's 1978 sequel Dawn of the Dead was a satire on consumer culture.
1985: Romero's zombies wise up
While controversial among zombie fans, Day of the Dead — Romero's 1985 sequel to Dawn of the Dead — has also had significant influence on the genre. In particular, the character of Bub (Sherman Howard) introduced the idea of a domesticated zombie. Bub was calm, curious, and remembered aspects of his past life as a human.
Moreover, Day of the Dead gave us scientists experimenting on zombies, something that has become a popular trope, most recently on AMC's zombie series The Walking Dead. The post-zombie apocalypse world of Day of the Dead, perhaps again inspired by I Am Legend, is less about escaping zombies and more about understanding them.
But Bub is the essential point here: he is a Romero zombie who shows emotion, displays human behavior, and even speaks at one point. This went against everything we'd come to understand about cinematic zombies up until that point, which is why Bub has been rejected by some zombie purists. But where would the zombie protagonist of Warm Bodies be without him?
1985-2001: Zombie humor
When did zombies get funny? There's humor in the Romero movies, to be sure, but that was taken to a whole new level with 1985's Return of the Living Dead. The film was based on a novel by John Russo, who had co-written Night of the Living Dead with Romero. Fun fact: when the team went their separate ways, Romero was allowed to make his own sequels, but Russo had the rights to the "Living Dead" title.
Return of the Living Dead embraced slapstick, which kind of feels like a natural fit for the genre. Zombies really got hilarious in the '90s with films like Peter Jackson's Braindead (or Dead Alive) and My Boyfriend's Back. The former featured over-the-top gore, itself a source of humor, but also slapstick. My Boyfriend's Back wasn't gory so much as silly. More recent zombie comedies like Fido have drawn from this tradition.
While unrelated to humor, it's worth noting that Return of the Living Dead is also notable for introducing zombies who hungered for brain instead of for flesh. This idea has somewhat fallen out of favor, but it's still well known in popular culture and resulted in a great gag on The Simpsons.
2002-2012: Run, zombie, run
Something important happened in the early 2000s: zombies started to run. Suddenly the idea of a shuffling, flesh-hungry creature wasn't scary enough — movies like 28 Days Later (2002) and the Dawn of the Dead remake (2004) gave us zombies who were really, really fast. Not only that: they were agile, crafty, and otherwise smarter than their predecessors.
In both 28 Days Later and Dawn of the Dead, which were hugely influential on the present-day zombie movie, the zombies weren't dead but rather people infected with a virus. This is a major change from Romero's vision, and it has come to be the more common story, perhaps owing to our fears of pandemics and biological warfare. (By contrast, The Walking Dead has traditional zombies as a deliberate homage to Romero.)
Unlike Romero, who lingered on the living dead, Dawn of the Dead director Zack Snyder eschewed close-ups of his zombies because he was worried that they appeared too human. Either way, zombies were becoming more like us: they weren't exactly mindless, and — more importantly — they weren't dead. In 28 Days Later, the transformation from ordinary human to bloodthirsty monster is near-instant.
2013-: Zombie love story
Which brings us to Warm Bodies and R, its zombie protagonist. The film is based on a book by Isaac Marion, which in turn was based on his short story "I am a Zombie filled with love." Welcome to the new genre: zombie romance.
Warm Bodies isn't the only one. S.G. Browne's Breathers: A Love Story is currently in development at Fox Searchlight. Of course, paranormal romance is nothing new, but with the success of Twilight (and the expected impending success of Beautiful Creatures), it makes sense to let zombies in on the action. The problem is, zombies aren't glamorous. That's why Warm Bodies is a dark comedy: how seriously can you take this love story?
It's hard to draw a direct line from White Zombie to Warm Bodies — there have been so many diversions along the way. But with some context, the progression makes some sense. This is all the natural development of a species that began as a sort of blank slate. Warm Bodies and all that follow are the next step in the line, drawing from the humor and the humanization of their predecessors, and adding the love.