Archaeologists often dig up bones from hundreds, thousands or even millions of years ago. These
remains tell us a great deal about how ancient humans and prehistoric animals,
such as dinosaurs, lived on Earth. The question is, how do bones survive the
decomposition process? Why don’t they decay? In fact, bones do decay, albeit very slowly.
Our soft tissue, like muscles, fat and connective tissue, are decomposed through
cellular death and microbial decay, but bones RESIST such decay due to their structure.
Bones are made of two components—cells that make up approximately 5% of the bone’s volume and the
extracellular matrix, which accounts for the rest. This extracellular matrix is composed of
organic matter, and is primarily a matrix of the protein COLLAGEN and minerals, chiefly CALCIUM,
which account for 50 to 70% of the bone. The secret of the bone’s decay resistance lies in
the last two components—collagen and calcium. Collagen is a very stable protein designed
to give SUPPORT and STRUCTURE to tissues like bone. Collagen and other structural proteins,
such as keratin, the protein our hair is made of, resist decay due to their linear
helical structure and the amino acids—the building blocks of protein—that form them.
Within the bone, collagen forms a matrix and, along with calcium, it makes our bone extremely
sturdy and resistant to decay. Within bones, calcium is found primarily as hydroxyapatite
and is bound to the collagen within bones. This association between the protein and
minerals gives bones added strength. When the decomposition process begins,
mere minutes after death, cells of the body die through cellular death, called AUTOLYSIS. This
is the primary form of cellular degradation. At that point, microbes, especially those in our gut,
no longer kept in check by the immune system, will begin to break down the remaining organic matter.
However, this process doesn’t take place in extreme cold conditions, since microorganisms
can’t grow in these freezing climes. This is why the bodies of climbers who have died
on Mount Everest are perfectly preserved, even serving as ominous landmarks for newer climbers.
For most bacteria and fungi, eating bones is a challenge. First of all,
not every microbe can break down collagen. Specific enzymes, called COLLAGENASES,
can digest collagen, but only certain microbes produce it. Second, even if a microbe produces
collagenase and other proteases, enzymes that break down proteins, they CAN’T reach the collagen
protein due to its interaction with calcium. Despite this, most bones WILL decompose,
though it might take years. There isn’t a standard estimate for how long bone decomposition will
take. Different bones, depending on their calcium content and structure,
will decompose at different rates. Teeth, for example, tend to survive decomposition very well,
since their outer enamel coating is very hard and contains very little protein, if any.
The external conditions also affect bone decomposition. Very dry and hot conditions tend
to preserve bone incredibly well. Dry and arid conditions usually slow bone decomposition since
many microbes can’t survive the dry conditions. Alkaline soil is also known to preserve bone well,
since it prevents mineral leaching. Bones preserved in many archaeological sites remain
intact for a combination of these reasons. Acidic soil, on the other hand, increases the
rate of decay. This type of soil helps dissolve the minerals within the bone,
exposing the organic matter. No matter what, over time,
minerals will leech out of the bones, and physical factors and microbes will break down organic
matter. Certain fungi can tunnel their way through the bone and break down collagen. Eventually,
the bone will break down to nothing but minerals, which will get integrated into the soil.
So what about the dinosaur bones that we see in museums? They’re millions of years old and have
managed to survive the test of time. Now, while those structures may look like bones,
the remains aren’t true preserved bones; they are actually stones in the shape
of bone. Once all the organic matter has been degraded, the mineral shell remains,
making the bone into a kind of stone—a fossil! This process of fossilization starts when water
carries minerals like calcium and iron into 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. So, all those T-rex skeletons aren’t made
of real T-rex bones… they’re just fossils! With that being said, in extremely rare
circumstances, some organic material manages to survive over the course of millions of years,
like the preserved cells that were discovered in a dinosaur skull in 2020.
The resilience of bones have helped countless forensic scientists discover evidence about victim
identity or how they might have died. It also helps archaeologists and historians understand the
past—of both the natural world and our own human existence. The clues left behind in these bones
and fossils can reveal to us how a creature, human or otherwise, may have lived and died.
In life, these bones support us and give structure to our otherwise soft bodies. Without them,
we would be amorphous slugs. Even in death, they support our knowledge of the world around us,
as well as its rich and varied past. Finally, even if bones don’t survive long enough to tell
us their history, they help to replenish the Earth and the bountiful life forms that call it home.