What happens to our bodies after we die

The breakdown of our bodies after death can be

fascinating – if you dare to delve into the

details. Mo Costandi investigates.

“It might take a little bit of force to break this

up,” says mortician Holly Williams, lifting John’s

arm and gently bending it at the fingers, elbow

and wrist. “Usually, the fresher a body is, the

easier it is for me to work on.”

Williams speaks softly and has a happy-go-lucky

demeanour that belies the nature of her work.

Raised and now employed at a family-run funeral

home in north Texas, she has seen and handled

dead bodies on an almost daily basis since

childhood. Now 28 years old, she estimates that

she has worked on something like 1,000 bodies.

Her work involves collecting recently deceased

bodies from the Dallas–Fort Worth area and

preparing them for their funeral.

“Most of the people we pick up die in nursing

homes,” says Williams, “but sometimes we get

people who died of gunshot wounds or in a car

wreck. We might get a call to pick up someone

who died alone and wasn’t found for days or

weeks, and they’ll already be decomposing, which

makes my work much harder.”

John had been dead about four hours before his

body was brought into the funeral home. He had

been relatively healthy for most of his life. He

had worked his whole life on the Texas oil fields, a

job that kept him physically active and in pretty

good shape. He had stopped smoking decades

earlier and drank alcohol moderately. Then, one

cold January morning, he suffered a massive

heart attack at home (apparently triggered by

other, unknown, complications), fell to the floor,

and died almost immediately. He was just 57.

Now, John lay on Williams’ metal table, his body

wrapped in a white linen sheet, cold and stiff to

the touch, his skin purplish-grey – tell-tale signs

that the early stages of decomposition were well

under way.

Self-digestion

Far from being ‘dead’, a rotting corpse is

teeming with life. A growing number of scientists

view a rotting corpse as the cornerstone of a

vast and complex ecosystem, which emerges soon

after death and flourishes and evolves as

decomposition proceeds.

Decomposition begins several minutes after death

with a process called autolysis, or self-digestion.

Soon after the heart stops beating, cells become

deprived of oxygen, and their acidity increases as

the toxic by-products of chemical reactions begin

to accumulate inside them. Enzymes start to

digest cell membranes and then leak out as the

cells break down. This usually begins in the liver,

which is rich in enzymes, and in the brain, which

has high water content. Eventually, though, all

other tissues and organs begin to break down in

this way. Damaged blood cells begin to spill out of

broken vessels and, aided by gravity, settle in

the capillaries and small veins, discolouring the

skin.

Body temperature also begins to drop, until it has

acclimatised to its surroundings. Then, rigor

mortis – “the stiffness of death” – sets in,

starting in the eyelids, jaw and neck muscles,

before working its way into the trunk and then

the limbs. In life, muscle cells contract and relax

due to the actions of two filamentous proteins

(actin and myosin), which slide along each other.

After death, the cells are depleted of their

energy source and the protein filaments become

locked in place. This causes the muscles to become

rigid and locks the joints.

During these early stages, the cadaveric

ecosystem consists mostly of the bacteria that

live in and on the living human body. Our bodies

host huge numbers of bacteria; every one of the

body’s surfaces and corners provides a habitat

for a specialised microbial community. By far the

largest of these communities resides in the gut,

which is home to trillions of bacteria of hundreds

or perhaps thousands of different species.

The gut microbiome is one of the hottest research

topics in biology; it’s been linked to roles in

human health and a plethora of conditions and

diseases, from autism and depression to irritable

bowel syndrome and obesity. But we still know

little about these microbial passengers while we

are alive. We know even less about what happens

to them when we die.

Immune shutdown

In August 2014, forensic scientist Gulnaz Javan of

Alabama State University in Montgomery and her

colleagues published the very first study of what

they have called the thanatomicrobiome (from

thanatos, the Greek word for ‘death’).

“Many of our samples come from criminal cases,”

says Javan. “Someone dies by suicide, homicide,

drug overdose or traffic accident, and I collect

tissue samples from the body. There are ethical

issues [because] we need consent.”

Most internal organs are devoid of microbes when

we are alive. Soon after death, however, the

immune system stops working, leaving them to

spread throughout the body freely. This usually

begins in the gut, at the junction between the

small and large intestines. Left unchecked, our

gut bacteria begin to digest the intestines – and

then the surrounding tissues – from the inside

out, using the chemical cocktail that leaks out of

damaged cells as a food source. Then they invade

the capillaries of the digestive system and lymph

nodes, spreading first to the liver and spleen,

then into the heart and brain.

Javan and her team took samples of liver, spleen,

brain, heart and blood from 11 cadavers, at

between 20 and 240 hours after death. They used

two different state-of-the-art DNA sequencing

technologies, combined with bioinformatics, to

analyse and compare the bacterial content of

each sample.

The samples taken from different organs in the

same cadaver were very similar to each other but

very different from those taken from the same

organs in the other bodies. This may be due

partly to differences in the composition of the

microbiome of each cadaver, or it might be

caused by differences in the time elapsed since

death. An earlier study of decomposing mice

revealed that although the microbiome changes

dramatically after death, it does so in a

consistent and measurable way. The researchers

were able to estimate time of death to within

three days of a nearly two-month period.

Bacteria checklist

Javan’s study suggests that this ‘microbial clock’

may be ticking within the decomposing human

body, too. It showed that the bacteria reached

the liver about 20 hours after death and that it

took them at least 58 hours to spread to all the

organs from which samples were taken. Thus,

after we die, our bacteria may spread through

the body in a systematic way, and the timing

with which they infiltrate first one internal organ

and then another may provide a new way of

estimating the amount of time that has elapsed

since death.

“After death the composition of the bacteria

changes,” says Javan. “They move into the heart,

the brain and then the reproductive organs last.”

In 2014, Javan and her colleagues secured a

$200,000 (£131,360) grant from the National

Science Foundation to investigate further. “We

will do next-generation sequencing and

bioinformatics to see which organ is best for

estimating [time of death] – that’s still unclear,”

she says.

One thing that does seem clear, however, is that

a different composition of bacteria is associated

with different stages of decomposition.

But what does this process actually look like?

Scattered among the pine trees in Huntsville,

Texas, lie around half a dozen human cadavers in

various stages of decay. The two most recently

placed bodies are spread-eagled near the centre

of the small enclosure with much of their loose,

grey-blue mottled skin still intact, their ribcages

and pelvic bones visible between slowly putrefying

flesh. A few metres away lies another, fully

skeletonised, with its black, hardened skin

clinging to the bones, as if it were wearing a

shiny latex suit and skullcap. Further still, beyond

other skeletal remains scattered by vultures, lies

a third body within a wood and wire cage. It is

nearing the end of the death cycle, partly

mummified. Several large, brown mushrooms grow

from where an abdomen once was.

Natural decay

For most of us the sight of a rotting corpse is at

best unsettling and at worst repulsive and

frightening, the stuff of nightmares. But this is

everyday for the folks at the Southeast Texas

Applied Forensic Science Facility. Opened in 2009,

the facility is located within a 247-acre area of

national forest owned by Sam Houston State

University (SHSU). Within it, a nine-acre plot of

densely wooded land has been sealed off from the

wider area and further subdivided, by 10-foot-

high green wire fences topped with barbed wire.

In late 2011, SHSU researchers Sibyl Bucheli and

Aaron Lynne and their colleagues placed two

fresh cadavers here, and left them to decay

under natural conditions.

Once self-digestion is under way and bacteria

have started to escape from the gastrointestinal

tract, putrefaction begins. This is molecular

death – the breakdown of soft tissues even

further, into gases, liquids and salts. It is already

under way at the earlier stages of decomposition

but really gets going when anaerobic bacteria get

in on the act.

Putrefaction is associated with a marked shift

from aerobic bacterial species, which require

oxygen to grow, to anaerobic ones, which do not.

These then feed on the body’s tissues,

fermenting the sugars in them to produce

gaseous by-products such as methane, hydrogen

sulphide and ammonia, which accumulate within

the body, inflating (or ‘bloating’) the abdomen

and sometimes other body parts.

This causes further discolouration of the body. As

damaged blood cells continue to leak from

disintegrating vessels, anaerobic bacteria convert

haemoglobin molecules, which once carried oxygen

around the body, into sulfhaemoglobin. The

presence of this molecule in settled blood gives

skin the marbled, greenish-black appearance

characteristic of a body undergoing active

decomposition.

Specialised habitat

As the gas pressure continues to build up inside

the body, it causes blisters to appear all over the

skin surface. This is followed by loosening, and

then ‘slippage’, of large sheets of skin, which

remain barely attached to the deteriorating

frame underneath. Eventually, the gases and

liquefied tissues purge from the body, usually

leaking from the anus and other orifices and

frequently also leaking from ripped skin in other

parts of the body. Sometimes, the pressure is so

great that the abdomen bursts open.

Bloating is often used as a marker for the

transition between early and later stages of

decomposition, and another recent study shows

that this transition is characterised by a distinct

shift in the composition of cadaveric bacteria.

Bucheli and Lynne took samples of bacteria from

various parts of the bodies at the beginning and

the end of the bloat stage. They then extracted

bacterial DNA from the samples and sequenced

it.

As an entomologist, Bucheli is mainly interested

in the insects that colonise cadavers. She regards

a cadaver as a specialised habitat for various

necrophagous (or ‘dead-eating’) insect species,

some of which see out their entire life cycle in,

on and around the body.

When a decomposing body starts to purge, it

becomes fully exposed to its surroundings. At this

stage, the cadaveric ecosystem really comes into

its own: a ‘hub’ for microbes, insects and

scavengers.

Maggot cycle

Two species closely linked with decomposition are

blowflies and flesh flies (and their larvae).

Cadavers give off a foul, sickly-sweet odour,

made up of a complex cocktail of volatile

compounds which changes as decomposition

progresses. Blowflies detect the smell using

specialised receptors on their antennae, then land

on the cadaver and lay their eggs in orifices and

open wounds.

Each fly deposits around 250 eggs that hatch

within 24 hours, giving rise to small first-stage

maggots. These feed on the rotting flesh and

then moult into larger maggots, which feed for

several hours before moulting again. After

feeding some more, these yet larger, and now

fattened, maggots wriggle away from the body.

They then pupate and transform into adult flies,

and the cycle repeats until there’s nothing left

for them to feed on.

Under the right conditions, an actively decaying

body will have large numbers of stage-three

maggots feeding on it. This ‘maggot mass’

generates a lot of heat, raising the inside

temperature by more than 10C (18F). Like

penguins huddling in the South Pole, individual

maggots within the mass are constantly on the

move. But whereas penguins huddle to keep warm,

maggots in the mass move around to stay cool.

“It’s a double-edged sword,” Bucheli explains,

surrounded by large toy insects and a collection

of Monster High dolls in her SHSU office. “If

you’re always at the edge, you might get eaten

by a bird, and if you’re always in the centre, you

might get cooked. So they’re constantly moving

from the centre to the edges and back.”

The presence of flies attracts predators such as

skin beetles, mites, ants, wasps and spiders, which

then feed on the flies’ eggs and larvae. Vultures

and other scavengers, as well as other large

meat-eating animals, may also descend upon the

body.

Unique repertoire

In the absence of scavengers, though, the

maggots are responsible for removal of the soft

tissues. As Carl Linnaeus (who devised the system

by which scientists name species) noted in 1767,

“three flies could consume a horse cadaver as

rapidly as a lion”. Third-stage maggots will move

away from a cadaver in large numbers, often

following the same route. Their activity is so

rigorous that their migration paths may be seen

after decomposition is finished, as deep furrows

in the soil emanating from the cadaver.

Every species that visits a cadaver has a unique

repertoire of gut microbes, and different types

of soil are likely to harbour distinct bacterial

communities – the composition of which is

probably determined by factors such as

temperature, moisture, and the soil type and

texture.

All these microbes mingle and mix within the

cadaveric ecosystem. Flies that land on the

cadaver will not only deposit their eggs on it, but

will also take up some of the bacteria they find

there and leave some of their own. And the

liquefied tissues seeping out of the body allow the

exchange of bacteria between the cadaver and

the soil beneath.

When they take samples from cadavers, Bucheli

and Lynne detect bacteria originating from the

skin on the body and from the flies and

scavengers that visit it, as well as from soil.

“When a body purges, the gut bacteria start to

come out, and we see a greater proportion of

them outside the body,” says Lynne.

Thus, every dead body is likely to have a unique

microbiological signature, and this signature may

change with time according to the exact

conditions of the death scene. A better

understanding of the composition of these

bacterial communities, the relationships between

them and how they influence each other as

decomposition proceeds could one day help

forensics teams learn more about where, when

and how a person died.

Pieces of the puzzle

For instance, detecting DNA sequences known to

be unique to a particular organism or soil type in

a cadaver could help crime scene investigators

link the body of a murder victim to a particular

geographical location or narrow down their search

for clues even further, perhaps to a specific

field within a given area.

“There have been several court cases where

forensic entomology has really stood up and

provided important pieces of the puzzle,” says

Bucheli, adding that she hopes bacteria might

provide additional information and could become

another tool to refine time-of-death estimates.

“I hope that in about five years we can start

using bacterial data in trials,” she says.

To this end, researchers are busy cataloguing the

bacterial species in and on the human body, and

studying how bacterial populations differ between

individuals. “I would love to have a dataset from

life to death,” says Bucheli. “I would love to

meet a donor who’d let me take bacterial samples

while they’re alive, through their death process

and while they decompose.”

“We’re looking at the purging fluid that comes

out of decomposing bodies,” says Daniel Wescott,

director of the Forensic Anthropology Center at

Texas State University in San Marcos.

Wescott, an anthropologist specialising in skull

structure, is using a micro-CT scanner to analyse

the microscopic structure of the bones brought

back from the body farm. He also collaborates

with entomologists and microbiologists – including

Javan, who has been busy analysing samples of

cadaver soil collected from the San Marcos

facility – as well as computer engineers and a

pilot, who operate a drone that takes aerial

photographs of the facility.

“I was reading an article about drones flying over

crop fields, looking at which ones would be best to

plant in,” he says. “They were looking at near-

infrared, and organically rich soils were a darker

colour than the others. I thought if they can do

that, then maybe we can pick up these little

circles.”

Rich soil

Those “little circles” are cadaver decomposition

islands. A decomposing body significantly alters

the chemistry of the soil beneath it, causing

changes that may persist for years. Purging –

the seeping of broken-down materials out of

what’s left of the body – releases nutrients into

the underlying soil, and maggot migration

transfers much of the energy in a body to the

wider environment.

Eventually, the whole process creates a ‘cadaver

decomposition island’, a highly concentrated area

of organically rich soil. As well as releasing

nutrients into the wider ecosystem, this attracts

other organic materials, such as dead insects and

faecal matter from larger animals.

According to one estimate, an average human

body consists of 50–75% water, and every

kilogram of dry body mass eventually releases 32g

of nitrogen, 10g of phosphorous, 4g of potassium

and 1g of magnesium into the soil. Initially, it

kills off some of the underlying and surrounding

vegetation, possibly because of nitrogen toxicity

or because of antibiotics found in the body, which

are secreted by insect larvae as they feed on the

flesh. Ultimately, though, decomposition is

beneficial for the surrounding ecosystem.

The microbial biomass within the cadaver

decomposition island is greater than in other

nearby areas. Nematode worms, associated with

decay and drawn to the seeping nutrients,

become more abundant, and plant life becomes

more diverse. Further research into how

decomposing bodies alter the ecology of their

surroundings may provide a new way of finding

murder victims whose bodies have been buried in

shallow graves.

Grave soil analysis may also provide another

possible way of estimating time of death. A 2008

study of the biochemical changes that take place

in a cadaver decomposition island showed that the

soil concentration of lipid-phosphorous leaking

from a cadaver peaks at around 40 days after

death, whereas those of nitrogen and extractable

phosphorous peak at 72 and 100 days,

respectively. With a more detailed understanding

of these processes, analyses of grave soil

biochemistry could one day help forensic

researchers to estimate how long ago a body was

placed in a hidden grave.