I keep hearing lately that zombies, unlike vampires (which seem to have a fairly long history in folklore) and werewolves (which date back to the ancient world) are a relatively recent creature feature. In fact, radio experts keep telling me, they were invented by George Romero for his 1968 film Night of the Living Dead. I found this particularly interesting because I'm pretty sure I remember writing about a Classical zombie in the final chapter of my MA dissertation, lo these many years ago.
I think what the radio pundit meant was that the modern popular conception of what a zombie is was invented by Romero, rather than the idea of animated corpses in general. So in order to solve this puzzle, we need to spend time on the Classicist's all-time favourite hobby - defining things. We love defining things, and the longer and more complicated the definition, the better. I once read a book that spent four chapters defining 'myth' (and still didn't come to any definitive conclusions, because defining myth is impossible).
The simplest definition of a 'zombie' is that it's a walking, talking corpse. Unlike ghosts, they are still in their physical body (whereas a ghost is the spirit or soul of a dead person, completely separated from their physical remains). Unlike vampires, they don't have to drink blood, and unlike an immortal (who, depending on the mythology, may 'die' and come back to life or may just be completely impenetrable) the zombie is most definitely dead. Their heart does not beat and they do not heal. If you chop off their arm, it stays chopped off. They are the ugliest form of undead, because they exist in whatever state of decay their body would be in, more or less - they don't go gooey and they last longer than real corpses, but they do slowly decay, or show signs of it anyway.
I suspect creatures that fit this broad definition crop up in the mythology and folklore of lots of different cultures. I don't claim to know about any of them outside of Greco-Roman myth and some elements of European folklore, but I have a vague notion there are lot of zombie-types in some African, Caribbean and Southern U.S. traditions (I'm thinking of voodoo, but my knowledge of the culture and geography of the areas in question is woefully lacking. I only discovered New Orleans is in Louisiana when I started reading the Sookie Stackhouse books).
In Greek and Roman mythology and fiction, zombies are usually raised individually, for a short period, to answer some questions or provide a prophecy. There's an excellent book by Daniel Ogden called Greek and Roman Necromancy (with a fabulous red and black cover in the edition I have, it's a great way to freak out visitors to your home) which details the history of Greek and Roman stories about necromancy, the raising of the dead. Ogden defines 'necromancy' slightly more broadly than I do (I wouldn't count Odysseus' raising of Tiresias in The Odyssey as necromancy, I would tend to label it a katabasis, a journey to the underworld) and some of the necromantic traditions discussed involve ghosts rather than zombies, but there are definitely some zombies in there. The tradition of raising the dead in ancient world is connected to the existence of oracles of the dead, and to the tradition in Greece of elevating heroes to divine or semi-divine status. Zombies are raised when more tasteful forms of oracle just won't do the job.
The scene I talked about in my MA dissertation was from Lucan's historical epic poem about the war between Pompey and Julius Caesar, the aptly titled Civil War. (Lucan had written what we have of the presumably unfinished poem by the age of 25, at which point he was forced to suicide for conspiring against the emperor Nero. This makes me feel very bad about the state of my novel, which so far consists of a bunch of half-started first chapters...). Instead of sending his hero on a journey to the underworld, like Odysseus in The Odyssey or Aeneas in The Aeneid, Lucan takes a minor character (Sextus Pompey, Pompey's son) and has him go to a witch to ask if he will survive the war.
The witch, Erictho, goes to a nearby graveyard and raises a corpse to provide the answer - throughout Greco-Roman literature the dead, whether you call up their ghost, see them in a dream, go and visit them in the underworld or raise them as a zombie, have special knowledge of the future. Lucan describes Erictho herself and her habits in great detail - among other things, she likes to tear apart the bodies of the dead, ripping apart the knots on the ropes that held crucifixion victims up with her mouth and pulling the nails out with her hands. She raises the corpse of a recently killed soldier, who proceeds to prophesy about just about everything except what will happen to Sextus before going back to being dead. (This section is towards the end of Book 7, and well worth a read if you like your literature gory! Follow the link to Perseus' very old-fashioned translation, though Susan Braund's more recent translation for Oxford World's Classics is a much better read).
A Classical zombie, then, has usually been raised by a living figure for a short period of time (in Latin literature, usually a witch using magic) to act as a particularly gruesome oracle (there is also a story about a Greek who has sex with a corpse, then foolishly raises her to ask for an oracle, and she points out she knows what he's been up to). The oracle part seems to be the part that hasn't survived so much in later literature. Animated dead bodies certainly crop up every now and again - it's been a long time since I last read The Lord of the Rings (hmm, I should re-read The Lord of the Rings) but Tolkien includes both wights (which are very mysterious, but presumably both dead and corporeal, i.e. not ghosts) and an entire undead army (portrayed as ghosts in the films, but if they're insubstantial, how do they kill people?) as well as the unseen 'Necromancer' of The Hobbit. Edited to add: See comments below on the army of the dead, who are in fact ghosts - it is clearly a lot longer than I realised since I last read The Lord of the Rings!
Possibly what the nice lady whose name I can't remember on the radio meant was that a very particular type of popular zombie was invented by Romero. The brain-eating aspect of the zombies seen in many modern films is apparently even more recent, but it was Night of the Living Dead that gave us an inexplicable plague of very slow-moving dead people attacking the living. It may also have been a Romero movie that introduced biting as a way of becoming a zombie - or possibly that was imported from werewolf mythology. More often, in stories featuring this type of zombie, everyone who dies becomes a zombie automatically - which is very different from the individually raised special cases of older literature.
These zombies are different to most Classical zombies in several ways. One important difference in modern zombies is the idea of the plague of zombies. In Classical literature, if you want to attack people with a plague - you use a plague. In a world where such things were more common, you don't really need zombies for the survival/zombie hybrid type of story, like The Walking Dead. Another is the very slow movement - Romero made it terrifying on film, but in a book (or poem) it would be harder to explain how awful it was for the hero as the monster walked very, very slowly towards him, and the ancient theatre didn't usually show any action scenes, those all happened off stage.
Ancient zombies are also never inexplicable. They are raised by necromancers for a very specific purpose. Modern zombies' frequent mysteriousness is partly because of their plague-like functions, partly (perhaps) a little bit of writerly laziness, but it also relates to their use as metaphors for modern life. Zombies are often a metaphor for being trapped in humdrum jobs or stuck in a rut, or for having given up on life, especially in lighter stories like Shaun of the Dead or Warm Bodies. In the ancient world, the people stuck in humdrum jobs weren't usually the ones writing the literature (and everyone was much more likely to die of plague, war or conspiring against the emperor at any minute anyway).
MINOR GAME OF THRONES SPOILER WARNING. One of the interesting things about George RR Martin's use of zombies in A Song of Ice and Fire is that he includes both the older model and the Romero model. His Westerlands zombies aren't quite Classical - they aren't oracles, and they can last quite a long time. However, they are individual dead people, raised by an individual necromancer for a reasonably specific purpose (they're certainly quite single-minded) and it's implied their 'life'span is not limitless. (They don't seem to decay, once revived - but neither do they heal).
On the other hand, the zombie army approaching from North of the Wall is made up of Romero-zombies, a plague of dead people, and the living who are killed might become them (they may also have necromancers ultimately behind them. I can't even remember exactly which ones are wights/White Walkers/Others/etc etc, I've got so confused between the book terminology and the TV series - but their sheer numbers, their relatively slow movement and the fact the dead become them is very Romero-style-zombie). It'll be fascinating to see what, if any, connection there is between the two, if Martin ever finishes the books.
END SPOILER WARNING. So basically, several of the various quirks of modern zombies can be traced back to Romero and especially to Night of the Living Dead. The most notable is the plague-like aspect. The random attack by shuffling zombies presumably starts with Romero, and the metaphorically juicy plague-like aspect of zombie attacks, plus the potential for stories about hardened groups of survivors, goes back to that film. But if we're defining 'zombie' as a walking, talking (sometimes prophesying) corpse, they're much, much older than a 1960s horror film.