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Michael: Not long after your heart stops beating the body starts to break down. And as you probably know, it almost always leads to the same thing unless the body is preserved or cremated - dust. But between death and dust, the human body goes through a lot of changes. What happens in the first few days after death is a lot different from a few weeks or months later. The process and rate of decay also depends on different factors like whether the body is exposed to the air, and the temperature, and the humidity of that air. And we're still learning exactly how it all works which is why scientists who study body decomposition, known as forensic anthropologists, monitor the different phases of decay at special facilities called body farms. Their findings are especially important for crime scene investigators who can use the information to learn more about a crime and hopefully nap the perpetrator. Decomposition is a fascinating scientific process, and in this episode we're going to talk a lot about death and decay.

So if that sort of thing makes you squeamish and you [chuckles] wanna watch something else, here's an adorable video about koalas. The first body farms were created because scientists really wanted to understand what was going on when bodies decomposed. But for that, they needed to see the whole timeline, and human bodies generally aren't found decaying unless there's some kind of foul play involved or the death isn't noticed for a while. Either way those bodies don't make good specimens so for a long time our understanding of how bodies decompose mostly came from animal studies and the occasional exhumed corpse. Until the 1980's that is, when a forensic anthropologist named William Bass established the first body farm at the University of Tennessee in Knoxville.

At body farms, researchers observe human corpses decomposing under all kinds of different conditions. Some might be posed inside cars, while others are underwater, covered with concrete, laid out in the sun, or buried in shallow graves. Since the researchers are studying actual human decomposition instead of just animal corpses the information they get is much more accurate. These days we know a lot more about the different phases of decay and exactly how they happen in human bodies exposed to different conditions. So first, let's talk about what happens in a fairly neutral environment when a cadaver is left out in the open air in moderate temperatures.

The first stage begins right away. Without a heart circulating blood or lungs working to provide more oxygen, the body cells begin autolysis - they start digesting themselves. Carbon dioxide begins to build up, increasing cell acidity, which starts breaking down the cell membranes. Cells contain digestive enzymes which are normally kept safe in a pocket of membrane called a lysosome, this way they can't digest the cell itself. But once these membranes disintegrate the enzymes start to chew up the rest of the cell which eventually dies. Meanwhile the body is experiencing changes on a larger scale, too. Without the chemical processes that kept the body warm while it was alive, it starts to cool down until it gets to the ambient temperature in a process known as Algor mortis, Latin for "the coldness of death."

And since the heart isn't keeping blood circulating anymore, the heavy red blood cells are pulled to the bottom of the body by gravity, causing a blueish-purple discoloration known as livor mortis, which is Latin for "blue color of death." Those ancient Romans didn't really beat around the bush. The blue color shows up about 20 minutes after death and peaks after around 12 hours, eventually fading away when the blood cells decompose. It's just one example of how the science of body decomposition can be helpful for forensic investigators. They can use livor mortis patterns to figure out how the body was positioned soon after death and whether it was moved.

Then there's the famous rigor mortis, or "the stiffness of death" which starts around 2 hours in. The stiffening has to do with the fact that human muscles actually use energy to relax but the cadaver isn't providing those energy rich molecules anymore so the muscles stay contracted. Rigor mortis sets in between two and six hours after death, and lasts for around two days until the muscles themselves finally break down enough to relax, permanently. Those first few days are the cleaner part of the decomposition process - but soon, things start to get a little messier.

There are microbes that live all around and inside us. And in life our bodies mostly keep them in check. But as more and more cells die anaerobic bacteria, which don't need oxygen to survive, start to take over, kick starting the decay process known as putrefaction. These microbes feast on the carbohydrates, fats, and proteins left in the body, producing gasses like carbon dioxide, natural gas, and hydrogen sulfide, the same stuff you'll find in swamps and sewers. The gasses build up inside the body, causing what's known as bloat - the first sign of putrefaction that you can really see.

Since the skin cells are breaking down, all the extra pressure can rupture the skin, letting the gasses escape, and they don't smell too great. Two of them, called cadaverine and putrescine were actually named after the things they're most associated with: dead bodies and decaying flesh. But you can also have smaller amounts of both of those gasses in your mouth, they're partly what makes bad breath smell so bad. Meanwhile, that hydrogen sulfide we talked about earlier starts to react with hemoglobin proteins in the blood, forming a compound called sulfhemoglobin. Sulfheomglobin is green and it's what causes the greenish tint associated with dead bodies. Since the discoloration often follows the patterns of blood vessels under the skin, it's also sometimes called marbling.

But those are just the changes that come from microbes, if the corpse is exposed to the air, it's going to attract insects, too. Mainly flies which lay their eggs in the flesh and then those eggs hatch into maggots which start to eat the remains. That marks the transition into the active decay phase which begins a week or two after death. As the maggots keep eating, the body starts to lose more mass. Fluids build up and leak out which creates more holes in the skin, oxygen comes in through those holes, speeding up the decomposition process. Generally the maggots consume most of the available flesh within a week or two, and then they leave the body to grow up into adult flies, signaling the start of the advanced decay phase otherwise known as dry decay.

This stage can last anywhere from a few months to several years depending on what the surrounding conditions are like. By now most of the easily digested parts of the body have been broken down and it's just bones and cartilage and maybe some dried skin left. So the microbes and insects become less active and there's not much more decomposition happening at this point, at least, not inside the body. All those fluids that were produced during the active decay phase create sort of a barren island around the corpse where the changes in acidity and chemicals in the soil mean that most plants can't really grow for a while. Sun, wind, and rain will continue to dry, bleach, and eventually break down these remains as the bones chemically interact with the surrounding soil and mechanical pressures grind them into dust.

So that's what happens if a cadaver is left out in the open air in a more average climate on earth. But what about the more extreme conditions or even if the body is just buried underground? It'll probably go through the same processes, just at a different rate. According to a rule known as Casper's Law, a body will take twice the time to decompose in water than it would in air, and eight times longer if it's underground. This is because water and soil make it hard for the usual scavengers and insects to reach the corpse. Plus, they're usually cooler than the exposed air. And extreme cold and dryness can even slow down or even stop decomposition. The cooler temperatures and lack of moisture can make it harder for microbes and enzymes to break things down. So instead of decaying the body mummifies, preserving the bones, skin, and organs.

In 1991 for example when two hikers in the Swiss Alps discovered Otzi the Iceman. His remains were so well preserved that they first thought he was a lost climber, until researchers took a closer look at his clothes, tools, and tissue, and discovered he'd lived and died more than 3,000 years ago. On the other hand, a body left in an extremely warm and humid environment, like a tropical forest, will decompose much faster than normal. More insects are found in tropical environments so they remove mass from the body much more quickly, speeding up decomposition, and instead of cooling down, the body would actually heat up to match the ambient temperature, if the surroundings are warmer than a living human body.

Bodies buried in certain kinds of soil or found in warm water go through a totally separate process known as saponification where the bacteria in the soil interact with the oils and fats in the corpse. It's similar to the reaction used to make soap and it forms a waxy grey material known as adipocere or grave wax which starts to cover the body and if there's enough of it, the body will stop decaying and become a so-called "soap mummy." Peach bogs, or swampy stretches of mud filled with decaying plant matter, can also mummify corpses, a process we've talked about before. As the peat grows and decomposes, the water water becomes cold, highly acidic, and oxygen-poor. A combination that has some unusual effects on any bodies in the bog. The acidity of the water acts like vinegar, pickling any submerged flesh. The corpse ends up with well preserved skin. It's basically tanned by the peat bog and it turns into a natural mummy.

So after a few decades worth of body farm research, we know a fair amount about how the processes of decomposition affect human tissue. But there are still a few knowledge gaps. The first non-American body farm is being built in Australia right now so that researchers can study how the Australian climate influences decay. But for the most part, forensic anthropologists can look at a decomposing body and know exactly where it is in the stages between death and dust.

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