The zombie apocalypse is upon us. Hordes of the undead shamble through the streets, hunting for food. But there’s no need for the living to barricade themselves within homes, offices and shopping malls. It’s just Halloween. The zombie hordes that clog the streets are after candy, not brains. And the chainsaw-wielding zombie fighter and that heroine bearing a sword are just fellow trick-or-treaters. But if there really were zombies coming for our brains, we wouldn’t need chainsaws or swords to save us. Microbes, maggots and time are far better weapons.
After a person dies, explains Kimberlee Moran, their body isn’t going to get up and start shuffling out the door. Moran would know. She’s a forensic archeologist — someone who studies human remains to find out about the past. She works at Rutgers University in Camden, N.J. “The idea that zombies can move at all is just completely wrong,” she says. “They’re not going anywhere.”
A dead body doesn’t have anything to drive it, Moran says. “The brain controls the movement of the body through the electrical impulses from the brain through the nervous system.” When a person dies, those electrical impulses disappear, and any potential shambling of the undead disappears with them.
Even if a mad scientist or evil alien virus managed to get those electrical impulses going again, Moran notes, moving would be tough. “The first thing to hamper the shuffling would be rigor mortis,” she says. When a person dies, their cells don’t immediately die. Many of the processes that take place within them keep going. But since breathing has stopped, these cells are no longer getting any oxygen.
That oxygen is needed to charge a cell’s batteries. Muscle cells (and all the cells in the body) rely on a molecule called adenosine triphosphate, or ATP. They use that ATP when they contract and relax. That energy loss transforms ATP into adenosine diphosphate, or ADP. Then, with the help of oxygen, a chemical process loads up the ADP with energy, transforming it back into ATP that can be used again.
Without oxygen, that recycling process stops. But cells are not left with uncharged ADP batteries. Anaerobic activity — energy production without oxygen — turns on to keep the cells going for a little while
Anaerobic activity has a big downside, though. It produces a product called lactic acid, Moran says. That can be used for a short time to turn ADP to ATP again. “It’s enough to cause muscles to contract but not enough for them to relax again,” she says. “Without the ADP-ATP recycling, the muscles stay contracted.” The body locks up “into a giant cramp.” Rigor mortis begins between three and six hours after death, and it will definitely keep a zombie off the streets.
A broken-down body
Long before rigor mortis sets in, the body has started to break down, Moran notes. Cells contain enzymes that can digest cells. When cells are alive, they carefully store those enzymes in pockets away from the rest of the cell, where they process waste. But after death, those enzymes are released and begin to chew through their former homes, a process called autolysis. “The cell breaks open and goo comes out,” Moran says. “Everything soft and squishy eventually liquifies.” That includes muscles and organs — leaving very little for a zombie to shamble with.
Those enzymes aren’t the only things going rogue. The bacteria in a person’s guts keep on eating. And without food coming down the gullet, they’ll chew through the gut and whatever else is in their way. “You have a relationship with the bacteria that live in your gut where there’s no oxygen,” Moran explains. “But when you die, no food is coming in. The bacteria get restless and start digesting the host.”
That process is called putrefaction, and it produces the strong, stomach-turning odors we associate with death. “No one talks about how bad [a zombie] smells,” Moran says. “You should be able to smell it coming!”
People won’t be the only ones to smell the dead. Help flies in, drawn by the scent. “The blowflies are there first,” explains Nancy Miorelli. “They have sensitive antennae and can smell a body from between 10 and 12 miles [16 and 19 kilometers] away. They show up within minutes.” Miorelli is a science communicator and entomologist — someone who studies insects — in Quito, Ecuador.
Blowflies don’t have a taste for human flesh. But their babies do. Blowflies and other fly species lay eggs on the corpse. Those eggs hatch into maggots, eager to slurp up the liquifying body. “The super goopy stage at the beginning usually has maggots,” Miorelli explains. “It’s all fly [maggots] because flies have a liquid diet.”
The maggots slurp up what the cells and bacteria leave behind. “That leaves your hair, tendons, skin, bones — the tough stuff,” Miorelli says. That tougher material may stick around for longer, but even it eventually gets broken down, too. Beetles arrive to lend a hand — or rather, a mouth. “They have chewing mouth parts, and they can chew and break down the stuff the flies couldn’t do,” she says.
“In perfect conditions,” and without bugs, Moran says, “a body will become skeletonized in one to two years.” But location matters. Warm environments can speed up the process. A lot. “In the Amazon, it’s one to two months.”
Add a few bugs, and, Miorelli says, you can go from months to mere weeks. That doesn’t leave a lot of time for zombie shambling.
The position of the corpse also matters. Maggots are legless, worm-shaped larvae, so they tend to fall off surfaces that aren’t flat. If a body is lying down, decay it would be faster because the maggots wouldn’t fall off. If the zombie were walking, though, “they would be leaving a trail of maggots,” Miorelli says.
A cool environment can slow the process. Microbes and bugs alike work best in warm weather. Cold weather could help a zombie hold itself together for much longer — months or even years. “Antarctica, parts of Canada, that’s where zombies would be scary,” Miorelli says.
But Moran is reassuring. Even when dead bodies stay fresh, they won’t ever be coming for your brains. “If a person is truly dead … there’s no shuffling after somebody,” Moran says. “If they’re shuffling, they’re not technically dead.”
So movies, books and games about the undead shouldn’t keep you up at night, she says. Biology and bugs have zombies well in hand. “You shouldn’t lose sleep over zombies.”
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