We know that insects played a huge role in the decomposition of the body, because the body was placed only in a 55 gallon barrel. If insects can be excluded, a body could decompose very slowly because the most damage is done by flesh eating organisms like maggots. In the worst possible conditions a body could decompose to just bone in 10 days but if a body is buried at least 1 meter underground most of the tissue can last up to a year. 

The main enemies of preservation are:

 Blowflies: can easily locate and burrow down to bodies buried at 0.3 meters.             

Flesh Flies: Flesh fly maggots that are left to develop turn into adult flies in 5-7 days. These are large, sluggish, black or shiny green or blue flies. These flies are harmless. The flies will not deposit eggs or further develop inside the house. By the time the flies emerge, the original carcass is too old and dry for reinfestation.

Adult coffin flies can burrow 0.5 m into the ground in four days. These three organisms from the kingdom Animalia and the Order Diptera.

The skeleton itself is not permanent; acids in soils can reduce it to unrecognizable components as well, but in this situation we know that the body was burned. Bodies exposed to cool; damp soil may develop a waxy substance called adipocere, caused by the action of soil chemicals on the body's proteins and fats. The formation of adipocere slows decomposition by preventing the bacteria that cause rotting.

Research was done by : Forensic pathology, entomology and anthropology.                                                              Forensic pathology studies the clues to the cause of death found in the corpse.

Forensic entomology studies the insects and other vermin found in corpses; the sequence in which they appear, the kinds of insects, and where they are found in their life cycle are clues that can shed light on the time of death, the length of a corpse's exposure, and whether the corpse was moved.

Forensic anthropology is the branch of physical anthropology that studies skeletons and human remains, usually to seek clues as to the identity, race, and sex of their former owner