A dead body can tell us a lot. By monitoring how corpses decompose, we can increase our understanding of the subtleties of the process and improve the accuracy with which we can locate and identify dead people, and determine their time of death.

After death, the body breaks down into simpler organic matter through biological and chemical processes. This process can take anywhere from a few weeks to a few years, depending on a number of factors.

Chemical reactions involved in the decay process will speed up as the temperature increases. Likewise, a body can remain 'fresh' for longer in the cooler winter months, or in colder climates. And there are other factors that delay decomposition, including burial in soil or sealing in a casket, while the presence of insecticides in the vicinity of the body will delay the arrival of insects (and therefore slow decay), although it won't hold them off indefinitely.

A study carried out by researchers at Australia's first 'body farm' also found that corpses can move during the decay process. And it's more than just a twitch. They found that movement occurred in all limbs after death, including in the advanced decomposition stages. In particular, they noted that the arms moved from being positioned down beside the body, to stretching out on either side.

“We think the movements relate to the process of decomposition, as the body mummifies and the ligaments dry out,” said taphonomy researcher Alyson Wilson.

Stage 1: Rigor sets in

The 'fresh' stage can last a few days to a week. One of the first visible changes to occur is the onset of pallor mortis. This is when the body begins to turn pale as a result of the lack of circulation and blood stops moving through the capillaries. Pallor Mortis usually sets in within around 15 minutes after death.

Rigor mortis sets in around two to six hours after death. During this phase, the cells break down as the lack of oxygen and nutrients prevents them from replenishing themselves. After this, proteolysis (the breaking down of proteins) causes dissolution of the stiffness, and the body becomes limp again. In general, rigor mortis disappears 36 hours after death, and the next phase is known as 'secondary flaccidity'.

If a body is left out, insects will arrive quickly after death, usually after 10 minutes, or so.

Stage 2: Bloating

When bacteria in the gut can no longer be kept in check, they start to reproduce and feed on the body. A study carried out by The Human Microbiome Project, found that it takes bacteria around 58 hours to spread around the body to the liver, spleen, heart and brain. This produces gas that causes the abdomen to bloat. In temperate climates, bloating will occur over a period of two to three days.

Stage 3: Skin sloughs off

Gas building up increases pressure within the body, pushing fluids in between the layers of skin and causing the outer layers to slough off.

Stage 4: Marbling

With no oxygen to bind to, haemoglobin in the blood binds to sulphur instead, filling the arteries and veins with a greenish-black substance. This gives the flesh an appearance known as ‘marbling’.

Stage 5: Expulsion of liquified organs

Increasing pressure forces the body’s fluids and liquefied organs out of any available orifice. Eyeballs can be dislodged and bodies have even been known to explode.

Stage 6: Insects multiply

Chemicals released by the body attract flies, which lay eggs in and around the orifices. Soon after, maggots hatch and begin feeding on the body’s flesh and organs.

Stage 7: Scavengers arrive

Other insects, such as beetles, are attracted to the body, as well as small birds looking to feed on them. Local scavenging animals will also appear to pick the flesh off the bones.

Stage 8: Skeletonisation

The final stage is skeletonisation, when the soft tissue is fully lost. Bleaching and exfoliation of the bones begin at around nine months of exposure. Wind, rain, erosion and abrasion take over and the bones are disarticulated over the following months and years.

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Holly SpannerStaff Writer, BBC Science Focus

Holly is the staff writer at BBC Science Focus, and specialises in astronomy. Before joining the team she was a geoenvironmental consultant and holds an MSc in Geoscience (distinction) from UCL.