Zombies - Examining the Modern Undead in a Scientific Light
Humans are fragile. They can be taken down by diseases, electricity, fire, lack of oxygen, food, or water, blood loss, or a well-placed strike by anything from a baseball bat to a trained fist. What about zombies? Popular conception (brought forth by George Romero's Night of the Living Dead) says that zombies can only be killed (in the words of a news anchor in Sean of the Dead) by removing the head or destroying the brain.
So they can only be killed by severe trauma to the brain, fine. They're also dead and don't require most of the vital functions that a normal human needs to live. But, taking into account these exceptions, their bodies are still governed by basic laws that allow a normal human to function. I'm going to document commonly held beliefs about zombies and whether or not (if Romero zombies are real or ever come to be real) those beliefs are scientifically sound.
How likely do you think a zombie apocalypse is?
Brain Function and Sense Retention of a Zombie
Zombies are dead humans and are commonly believed to retain many human senses and functions, but many of these functions rely on several factors that work in perfect unison with one-another and when one is damaged, the function is lost. In this section I'll talk about the physiological functions that a zombie can plausibly retain and the ones that would most likely be lost.
BRAIN FUNCTIONS - After brain-death, the brain starts decaying, and due to the nature of the brain being a fleshy and very delicate piece of organic machinery, the zombie disease would need to somehow preserve key areas in order to retain certain functions. The occipital lobe for seeing, the cerebellum for coordination and movement, parts of the parietal lobe for allowing the zombie to navigate through its environment effectively, and parts of the temporal lobe for basic memories. They can't work through their environment if they don't know how doors work and can't use sensory information to navigate in an efficient way if they are unable to store memories. Simply by the fact that a zombie would NEED at least part of its memory as a human suggests that the Romero concept of the zombies being able to learn and remembering bits of their past life seems more plausible than a creature devoid of any complex reasoning or humanity.
Spinal Cord - Even in the undead, the spinal cord would be required to send signals to any part of the body a zombie would be able to control. Breaking a zombie's spine would still disable it, so stabbing a zombie in the neck might be enough to completely incapacitate it (besides its snapping jaws).
As for the brain stem, I'll cover that later when I go into other required bodily functions.
ONSET OF DECAY - Most Romero-style zombies tend to place the time between death and undeath at several minutes up to 8-12 hours. It takes 3-6 hours on average for rigor mortis to set in, with it greatly speeding up the hotter and more humid it gets and slowing down the colder it gets. In a tropical rainforest, it could take as little as 10 minutes, in below freezing, dry weather it could take several days. Again, depending on temperature, rigor mortis progresses faster in heat and humidity and slower in cold and dry, can last for days or weeks depending on conditions, and reaches a maximum stiffness in 6-24 hours, with 12 hours being average. As well, it affects the face and jaw first, moving downward, affecting core muscles, arms, and then legs. All of these factors suggest that most of the recent additions to the zombie horde, as long as they hadn't come back in minutes, would be incredibly stiff, and for several hours to a day after they return they would be unable or very hard-pressed to attack you with arms or teeth, possibly even being unable to move.
Besides rigor mortis, in the first stages of decay, enzymes are released that weaken cell walls and allow true decay to start. It's usually implied that animals don't eat the flesh of the undead, so I'll rule out flies and vermin that aid in decomposition. I'll also give some leniency when it comes to microorganisms, as the disease that turned them into a zombie in the first place might act as a sort of new immune system, killing microbes that would seek to attack the brain or other functions vital to the zombie. As well, assuming that the disease didn't kill those microbes and preserve certain functions, the transition from death to undeath would be completely impossible because the nerves necessary to the zombie would be too damaged by onset decay to function.
Sight - The sense of sight requires the use of the eyeball, the retina, the occipital lobe of the brain, and the nerve connections between those parts. Assuming the disease stops microbial agents from decaying these parts, the eyeballs would still be vulnerable to environmental factors. In any dry conditions, the eyes would dry up within hours to days, and in cold conditions, they would freeze and become too damaged to function. This means the likelihood of a zombie retaining its sight for more than a few days maximum is extremely low.
Smell - Never-mind the parts of the brain that interpret smells, the nose is notorious for being damaged in battles. It'd likely be one of the first things pulled off by the zombie hordes. Zombies are usually shown as having a keen sense of smell, but the lack of a nose would probably inhibit that, as well as being composed of a fleshy material very prone to decay. Most of the facial appendages are the first things to rot off of a corpse, and even if the disease slowed the decay, the nose and the ears would still quickly rot off, if they weren't torn off before death.
Hearing - Just like the nose, the ears would quickly be pulled off by hungry zeds. That wouldn't prevent the zombie from hearing, but it would prevent them from being able to focus the sounds they hear and pinpoint their locations. In addition to that, the eardrum is paper-thin and would likely rot away quickly. I don't think it's likely a zombie would retain its sense of hearing for more than a couple of days.
Touch - Most zombies are shown to have no sense of touch, but the sense of touch is the easiest to preserve in the event that someone dies. It has no required fleshy bits to function (like the nose, ears, or eyes), and could easily become a primitive form of location based on changes in air pressure and vibrations, assuming it was enhanced greatly and the brain didn't have to focus on other bodily senses. It would be evolutionarily advantageous for the disease to either greatly increase the host's sense of touch or be able to infect other hosts before the primary location senses became useless (I'd give that a couple days to a week).
Taste - The tongue is protected in the mouth from hungry zombies and from environmental factors, but the sheer amount of microbes in the mouth of a flesh-eating corpse would quickly rot it away. Unless the disease greatly modified human physiology, taste would not be preserved and could not be used in any way for location.
Humans can only move their limbs thanks to a complex set of contracting and extending muscles and tendons, pulling and pushing bones in the directions your brain signals. The normal physiological response to injury is a pain response, release of adrenaline and endorphins, and recoiling from the injury or involuntary reflexive contraction around the injury in order to prevent further injury. Zombies are portrayed to have none of these responses and have no need for them thanks to a headshot being the only way to kill them.
Like I said, the human body works off of contracting and extending muscles and tendons. Even in a zombie, the severing of a key muscle or tendon can cause it to be completely unable to move certain parts of its body, possibly rendering it unable to move or attack. Cutting the pectoralis major of a human, living or dead, will make them completely unable to use their shoulder, meaning their arm is disabled. Cutting their achilles tendon or hamstring will make them unable to use their leg. The human body, regardless of if it's undead or living requires these things to move, not just by biology, but by gravity and physics. They're a lever and pully system and one part damaged can disable the device.
In reality, a zombie can be incapacitated by many, MANY means. Learn the major muscle groups, learn what muscles control what parts of the body, and if you're ever in a position where a zombie is on top of you and you can't reach its head with a knife, you might still be able to disable it.
Other Physiological Functions
Blood - The human brain requires an incredible amount of oxygen to function properly and though the brain is only on average 2% of your body weight, it receives 15% of the blood pumped through your body, 20% of your oxygen intake, and 25% of the energy you get from food. It cannot function without oxygen and sugar, and all mammals transport this to their brain through the use of their blood. Even with reduced function it would require living blood cells or something that can perform the same function, a pump (the heart) to get them through the brain's capillaries, and a way of getting energy into the system, which leads us to...
Digestive System - Romero zombies are portrayed as having no actual reason for eating the flesh of the living. They don't require the food and the only reason they might be driven to bite people is to spread the disease and increase the size of the horde. This makes sense if all of their functions have stopped and they don't require energy, but as I've said earlier in this article, it would be impossible for ALL of their functions to stop, and all of them that are still functioning require ATP (bio-energy) to function. In humans, blood turns sugar and fat into ATP and transports it through the body. But the only way to get the fat and sugar required for synthesis is to eat, which requires your entire digestive tract be undamaged enough to process the food.
Lungs - The brain also needs a significant amount of oxygen to function, and so do all of the other processes that I've established are required for zombies to even function. So the lungs would also need to function, though they might not need to breathe as much humans due to many of the systems being inactive, so they could probably do with only having one.
Brain Stem - The brain stem is the portion of the brain that controls all involuntary muscles and the functions of everything required for life in our body. It needs to be intact for the heart, lungs, digestive system, and any other possible required involuntary physiological functions.
Romero Zombie Plausibility
Put all of the factors I've discussed together and there is only one logical outcome; Romero zombies are impossible without added unknown factors. It's probably for the best that Romero hasn't ever tried to scientifically explain his zombies, instead merely having the characters speculate on the cause. This hasn't stopped most other influential zombie movies, games, and other mediums from explaining it away as a viral infection, but taking into account all of the evidence, they don't have a leg to stand on.
That isn't to say Romero zombies are impossible, just highly highly improbable taking into account all currently known factors. It seems far more likely that a meteor bringing an unknown factor into the mix would cause it than a mundane or even manufactured virus.
Other Kinds of Zombies
Living viral zombies - These are FAR more likely than undead ones. There are already many diseases that destroy or heavily damage portions of the brain, rabies being one of the most similar to a zombie virus. In popular media, the rage zombies presented in 28 Days Later seem to me the absolute most scientifically plausible. As well, they have a method of transferring the virus that doesn't damage required functions (blood vomit ftw), and can be taken down with a knife to the gut, blood loss, or simply dehydration, fever, or starvation.
Parasitic zombies - These are also far more likely than the undead, though they would probably be far less mindless than any other likely counterparts. There are several parasites that control the mind of their host already in this world, however the likelihood of a parasite that controls human functions developing naturally is incredibly low, especially when you take into account how developed our brains are compared to the things that mind control parasites work on. All of the mind control parasites prey on one specific creature (mice, bees, ants) and can't work with creatures not of that type. Evolution takes a long, LONG time, so the only way this is truly plausible is if it's developed in a lab specifically to prey on humans, or an unknown being introduced (alien in origin possibly). That said, a parasitic zombie would likely be able to use weapons and would have a transference method other than biting. Mind control parasites work with the framework they're given, so they would require all required bodily functions be intact.
Other Possible Types
Bio-engineering - When we know more about how genes work and can effectively manipulate them, there is the possibility of simply growing zombies in labs. These would be living, but could certainly act like zombies and have the general properties of zombies. Extra capacity for survival could be added too, like incredibly thick skin, tougher bones, or other enhancements.
Nanomachines - This, in my opinion, is the only plausible way for Romero-style zombies to exist in their full undead glory. Nanomachines programmed well could replicate all of the functions required for a human to function, but the body would still be dead. The machines could be built with the basic materials that human flesh is composed of and programmed to reproduce. It's possible the zombies could become inactive if they don't get enough energy for the nanomachines to function, so this might be mitigated through partially shutting them down unless they are actively hunting. These kinds of zombies might not be as active as other kinds, but it's likely any human that gets even the slightest bit of bodily fluid in (or maybe even on) them, would likely die very quickly and come back when the machines could reproduce enough to make the body work.
Slime mold - Slime molds are a type of single-celled organism that attempts to congregate and form a mass. When they get to a certain density, they split into groups within the mass and perform different functions (absorb sunlight, produce spores, transport materials, etc). In theory, it's possible a slime mold could be a parasitic organism and could be fine-tuned to perform functions required to control a human body. One coming about as a result of evolution is practically nonexistent, but it could be created in a lab. Slime molds reproduce by spraying out spores, so it's likely a simple touch would be enough to infect someone, maybe even just being near them. It's also very likely it would be spread through air and zombies that expired would become an incredible biohazard to anyone wandering near them. If this was produced in a lab and released on humanity, the only way to beat it would be to wait until it all died out. Live in an underground bunker, on a ship or island in the middle of the ocean, or some other way that isolates you completely from it. And then wait. Possibly only weeks, possibly years. Depending on its resilience and ability to survive without a host, it might never go away. If this ever happened, it's possible humanity would go extinct or be completely unable to recover.
Plasmid or retrovirus - Plasmids and retroviruses alter the genetic code of the host. Plasmids do so in ways that slightly change the environment and make it more friendly to their survival, but one created in a lab could change as much as the geneticist was able to code into it. Retroviruses infect cells and replicate by the way of infecting DNA and changing the code to turn it into the virus' genetic code. Both of these could be developed in a lab and could change a human into a mindless, infectious killing machine, or even into a genetic monstrosity (like the lickers from Resident Evil). By the same vein though, plasmids and retroviruses might be able to turn people into superhumans (think spider-man), so if we get to the point where we can create genetic monsters out of normal humans, there might be others who have the power to kill them with a glance. At the time we can do this, if these things aren't strictly controlled, we will have a lot more to worry about than zombies.
Don't expect Romero zombies, expect the much more dangerous rage zombies, because they are FAR more likely. Gear your zombie plan toward this so you aren't surprised and get torn to pieces by blood vomiting lunatics when you expect them to be slow and stupid.
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Acknowledgements and Attributions
I don't own any of the pictures on this hub or any of the video games, movies, or books referenced in it. Please don't sue me, I'm a dirty hobo and I need my money to buy soap and video games to make life less miserable and smelly.
If you have the source for anything that doesn't acknowledge its source, tell me and I'll put them in place.
Thanks for reading this, if you managed to get through all of it. If some of my scientific crap is horribly off, please tell me why and refer me to some sources explaining why I'm wrong. I want it as scientifically sound as possible and I am not a omni-disciplinary scientific badass, just a layman.
Good luck when the zombie apocalypse comes - I hope you don't get torn limb from limb by mutant, blood-vomiting cyborg zombies.