How quickly do bones biodegrade? It depends on what kind of soil they are buried in, but from 6 years to thousands of years.
The bones we have of dinosaurs are not actually bones but fossils. If a dinosaur dies in the right conditions, minerals can replace the calcium in the bones, creating a fossil that is exactly the same shape as the bone itself, but they are not bones. The oldest real human bones that have ever been found are 233,000 years old. They were found in Ethiopia. There is some argument about this age because the bones were dated by analyzing a layer of volcanic ash that they were found under. That specific volcano erupted 233,000 years ago, so volcanologists say the bones must be older than that.
What are bones made of? There are three parts to our bones. There is the outer layer, which is the hard, outer shell. This is the dense bone tissue that gives the bones their color, strength and protects the softer inner part of the bones. It is very dense and forms about 80% of the weight of our skeletons. The next layer is the spongy bone layer which is made of collagen. This tissue is made of bone cells and is a honeycomb shape because that gives the most support while allowing the bone to be light. This tissue is made of hydroxyapatite, which is a form of calcium. The calcium is constantly broken down and replaced throughout our lives, although this process slows down as we get older. The spongy bone layer allows the bones to be flexible because they would shatter under force otherwise. Then, there is the bone marrow layer. This is semi-solid tissue that makes new blood cells. 5% of our body mass is usually bone marrow and it produces approximately 500 billion blood cells every day.
When someone dies, the body begins to decay straight away. The bacteria that live in our intestines and stomachs to help us digest our food get hungry and go to work on us. They are prevented from doing this when we are alive because as they eat our stomach lining, it regrows fast enough to keep them contained. When we die, the stomach lining stops regrowing, and the bacteria go to work. If someone is buried in moist soil, in a warm climate, with a regular wooden coffin, and isn’t embalmed, then all but their bones will be gone in about one to five years. If someone is embalmed and the coffin is much thicker, they can last for far longer than ten years. However, at some point, only the bones will be left.
So, how do bones decompose? During the decomposition process, bacteria and enzymes get inside the bones and the soft bone marrow is the first part of the bone to go. This only takes a few weeks if the conditions are right. Then the enzymes and bacteria go to work on the spongy bone. The problem they have here is the calcium coats the collagen and makes it very difficult for them to get at the organic matter. Collagen is also a very durable and stable protein, and not all enzymes are able to break it down. However, they can decompose it after enough time. As the enzymes finally break down the collagen, the hard outer shell of the bones is left. This begins to crack and shatter as its support frame is removed. Finally, the outer part of the bones becomes dust, and the minerals are absorbed into the soil.
This is a very approximate explanation, and the process varies widely depending on where a body decomposes. Burying a body in the ground slows down decomposition because it is harder for insects to get at it. Using a heavy duty coffin will also affect the rate. Some coffins can wick away moisture and bacteria don’t work well without water. This can cause mumification. This also happens if somebody dies in a hot, dry environment, like a desert. It is far easier for a skeleton to last a long time in a desert than in wet, warm soil. Bodies that fall into peat bogs are also preserved. The peat rots, releasing an acid that kills the bacteria, basically pickling the body. Still, if given long enough, all bones will eventually decay, and vanish, unless they are fossilized. And this is what I learned today.
Photo by Renato Danyi: https://www.pexels.com/photo/grey-skulls-piled-on-ground-1096925/
Sources
https://www.npr.org/2022/01/13/1072867405/scientists-determine-age-of-some-of-the-oldest-human-bones
https://naturalhistory.si.edu/education/teaching-resources/written-bone/skeleton-keys/inside-look
https://www.businessinsider.com/how-long-it-takes-human-body-decompose-grave-coffin-2019-8
https://pubmed.ncbi.nlm.nih.gov/20646883/
https://en.wikipedia.org/wiki/Bone
https://en.wikipedia.org/wiki/Bone_marrow
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/bones
https://www.osteoporosis.foundation/health-professionals/about-osteoporosis/bone-biology
https://www.livescience.com/how-long-bodies-take-to-decompose