Dr. Bass is a forensic anthropologist who founded the University of Tennessee Anthropological Research Facility (aka The Body Farm) where Bass and his colleagues monitor the decomposition of human corpses in various environments. He spent much research on studying human decomposition and identifying human remains. He explains in his book how the bones of the pelvis, knee, and jaw all play a part in determining race or gender while the cranial sutures can determine age.
Warning: The following quotes describe the changes through which the human body goes in a fire:
The human body undergoes dramatic changes in an intense fire. The arms and legs are the first to go. Relatively thin and surrounded by oxygen, they’re like kindling, easy to ignite and quick to burn. At temperatures of only a few hundred degrees, the skin quickly blackens, the fat beneath the skin starts to sizzle, and within a matter of minutes the skin splits open and the flesh begins to burn. Then the limbs begin to move–the hands and feet clench, the arms curl up toward the shoulders, and the legs spread slightly apart with the knees flexed. It’s function of biomechanics and muscle strength: The flexors, the muscles that cause our arms and legs to bend, are stronger than the extensors, the ones that cause our limbs to straighten. As a fire cooks and dries out the muscles and tendons of the body, they shrink, just like a steak on the grill, and the flexors overpower the extensors. The resulting position is very much like a boxer’s stance in the ring; for that reason we call it the “pugilistic posture.” If, on the other hand, the arms are ties or pinned behind the back, they won’t be able to curl up, so finding a burned body whose arms are straight can be an important clue that the victim was somehow confined or restrained. (p. 76)
The other truly dramatic change that occurs is to the head. The skull is basically a sealed vessel, filled with fluid and moist brain tissue. It doesn’t take long for that moisture to reach the boiling point and create pressure in the cranium; the hotter the fire, the greater the pressure. If there’s an outlet for that pressure–for example, a bullet hole in the skull–the pressure vents harmlessly. If there isn’t, the skull can literally burst, fracturing the cranium into numerous pieces. (p. 77)
I will spare all you squeamish readers the details Bass and Jefferson share about differential decay, putrefication, adipocere, autolysis, green bone, or any of the other grotesque terms related to human decomposition. For me, I must admit, it is all very fascinating. :-) It reinforces my desire to study forensic anthropology.