In humid conditions, bones might break down in a matter of a decade or so, but in a dry climate, it could take thousands of years! Bones do decay, just at a slower rate than other types of organic material and tissue.
When someone passes away, one of the most common phrases heard at the memorial or funeral is “Ashes to ashes, dust to dust”. In that context, the phrase is meant to suggest that our bodies come from the earth, and will eventually return to it. It’s a sentiment that explains the circle of life, and helps people cope with the pain of loss and death.
However, that phrase isn’t entirely true…”Dust to dust” suggests that our bodies completely disappear, but that isn’t always the case. Sometimes, bones are found in the earth that have been buried there for thousands of years!
So, although flesh and tissue tend to break down rather quickly, bones have a much more impressive ability to stick around. As it turns out, this subject is a bit more complicated than it appears at first glance, and is actually quite fascinating once you “dig” a bit deeper.
As with most mysteries in the realm of science, there isn’t a single answer to this particular question. As it turns out, bones decay at varying rates, and some don’t decay at all! To properly understand the variability of bones in human and animal bodies, a bit of background on the process of decomposition can be quite helpful.
How does decomposition happens?
Decomposition happens to all organic matter, and while every organism breaks down in a different way, the basic concept is the same. In order to recycle organic matter, chemical processes break down organisms into simpler forms that can be absorbed and reused within the biome. This affects everything that is considered “living”, from trees and badgers to kings and paupers.
Bodies can be broken down in two main ways: by chemical/physical processes, or by other living organisms breaking down the living tissue. The rate of decomposition depends on many factors, including temperature, humidity, insect presence, exposure to air, acidity of the soil, and dozens of other variables.
A human body could lose all of its flesh and tissue in as little as a week, or it could remain in place for thousands of years! It all depends on the conditions the body is in, and the same thing is true for bones.
Collagen and calcium in bones delay decomposition
Although many people believe that bones never break down, when you think about it logically, that would be impossible. After the hundreds of millions of years of life on this planet (in which humans have only been around for a minute fraction), if bones never decayed, we would find them everywhere!
Fortunately, bones aren’t that different from our flesh and blood. We think of bones as solid and firm parts of our skeleton, but also elements that can snap like a piece of chalk when we’re badly injured. The truth is that bones are living tissues, just like our other organ systems, containing blood vessels and nerves.
Bones are largely composed of collagen, which creates a strong porous matrix, rather than a solid structure. Therefore, the same chemical, physical, and micro-organic processes that break down tissues will also cause bones to decompose!
Compared to other tissues, bones can escape decomposition for two reasons – collagen and its association with calcium. Collagen is a very durable and stable protein due to its structure and chemical composition. Only certain enzymes can break down collagen.
Another protein, keratin, makes hair difficult to break down as well. Both keratin and collagen are part of the linear structural protein camp and their linear and tightly coiled helical structure make them particularly strong.
Furthermore, collagen associates with calcium and other minerals within the bone. This gives the bone its strength during life, and allows it to resist decay in death. The minerals “coat” the collagen, making it difficult for microbes to access the organic matter and digest it.
Bones last longer in dry and arid conditions, since microbes cannot survive in high temperatures with no water. This is why the iconic image of a skeleton in a desert is morbidly accurate.
If a body is exposed to water, insects, open air, or highly acidic soil, then bacteria and fungi will be able to invade that porous network, and seek out the proteins of the collagen within the bones, which causes those bones to break down and eventually crumble to dust!
Bones can turn into fossils
Some bones do manage to achieve true immortality, and you’ve probably seen dozens of them over the course of your life—most likely in museums!
Fossils are bones that were so rapidly encased in sediment that air was completely shut out, making it impossible for any decomposition to occur. This is the case in the aftermath of volcanic eruptions, and other catastrophic events that displace large amounts of sediment in the earth.
Only an extremely small fraction of living organisms manage to be fossilized, and even then, we still call them “bones”, but that isn’t actually the case. Fossils were bones that never decomposed, but over time, minerals in the surrounding materials began leeching from the living tissue, replacing living cells with rock! So, when we talk about dinosaur bones being dug up after millions of years, we’re actually just digging up ancient rocks that are shaped precisely as the original bones once were.
This process of fossilization starts when water carries minerals like calcium and iron into the cavities of the decayed bone. The minerals settle there, fortifying the pre-existing minerals in the bone. Over time, all that remains is stone that has taken on the shape of bone. The oldest fossils humans have discovered are roughly 2.8 million years old (source), and were found in Ethiopia.
The most famous examples of ancient bones remaining intact comes from Egypt, where the mysterious practice of mummification prevented bones from decomposing in certain cases.
When powerful drying salts, like natron, were used to clear the body of any liquids, it prevented bacteria and fungus from starting the decomposition process. Furthermore, once the mummy was sealed in linen and a sarcophagus, the lack of oxygen and moisture almost entirely prevented the breakdown of tissue and bone.
Bones do decay, just at a slower rate than other types of organic material and tissue. Based on a wide range of extrinsic and intrinsic factors, bone can last for a few months to a few geologic eras, but the truth is that nothing lasts forever.
Even fossils and mummies will eventually be pulverized or broken down over the course of millions (or billions) of years. As they say, elbows to ashes, bones to dust!