In nature, animals exhibit two common types of skeletons: one with a hard external skeleton, as seen in many mollusks, primarily composed of calcium carbonate, and the other, like humans and other vertebrates, whose main skeletal component is calcium phosphate, more precisely hydroxyapatite.
Regardless of the skeletal form animals choose for survival, the primary material they utilize is calcium.
So, the question arises: why did animals opt for calcium instead of other metals during evolution?
As we know, calcium is not the most abundant metallic element in the Earth’s crust; aluminum and iron are more plentiful. Moreover, calcium is not the only metallic material utilized by the human body—our bodies are filled with various metals.
However, all animal species have chosen calcium as the primary material for defensive structures and body support.
Our bodies are filled with various metals.
1. Calcium Strengthens Bones but Isn’t the Best Choice
Many might think that our bones are fragile, prompting hopes for advanced science to create a steel skeleton for enhanced durability.
In reality, calcium compounds such as bones and hydroxyapatite have a tensile strength of 150 MPa (the unit of pressure in the International System of Units, where 1 Pa = 1 N/m²), a deformation to failure of 2%, and a fracture strength of 4 MPa(m).
About 35% of human bone composition is organic matter, while the remaining 65% is calcium phosphate along with a small amount of calcium carbonate. These substances determine the strength of the bones.
If hydroxyapatite were as dense as steel, it would be significantly stronger than steel; calculations suggest that human bone strength is five times that of steel.
The reason bones break lies in their not being completely solid; they consist of tubular and hollow structures.
Bones consist of tubular and hollow structures.
This structure withstands pressure better while remaining lightweight, and the fact that our bones account for only about 1/6 of body weight makes running more energy-efficient compared to bones with a solid structure.
Studies have shown that the activation energy for the oxidation-reduction reaction of iron is similar to the energy ATP requires for the body to produce hydroxyapatite. However, if the body were to construct a skeleton of steel and exist in a form akin to hydroxyapatite, it could weigh 3 to 5 times more than the current skeleton, resulting in various disadvantages for mobility.
Overall, it is challenging to find excellent building materials for bones like calcium compounds in nature. However, according to our current technology, among the industrially produced materials, bones are certainly not the best choice for the body.
Among them, titanium alloys can achieve a weight and volume similar to bones but will have better parameters. For example, the stress of titanium alloys is 1.3 times higher than that of bones of equal weight, and their strength is five times that of bones.
Most importantly, the tensile strength of titanium alloys reaches 500 MPa. If we had a titanium alloy skeletal frame, we could support a heavier and larger body, essentially eliminating the need for cells to repair bones.
Additionally, carbon fiber, currently widely used in prosthetics and other fields to help reshape the human body, has also proven to yield better results for humans compared to existing bones.
2. Why Do Most Organisms Choose Calcium?
Approximately 635 to 485 million years ago, a series of chemical changes occurred in the oceans, including a shift in the composition of ocean rocks from dolomite to limestone, coinciding with the emergence of animal bones along with the change in rock composition.
In addition to the fact that the increase in limestone correlates closely with the emergence of animal bones over time, researchers have also discovered that the aragonite and calcite crystals found in limestone form more quickly and require less energy than dolomite.
In other words, if an animal were to utilize dolomite to create calcium-containing bones, it would expend more energy.
Dolomite forms when oxygen concentration in the ocean is low, and researchers believe that the sudden increase in limestone is due to the oxidation of rocks as Earth’s oxygen concentration rose during that period.
Dolomite forms when oxygen concentration in the ocean is low. (Illustrative image).
A similar chemical process occurred on land—perhaps even more intensely than in the oceans—and later, under the influence of rain, a significant amount of limestone was dissolved and flowed into the ocean, making calcium ions one of the most abundant ions in the ocean (ranking behind sodium and magnesium among metal ions), thus promoting the evolution of bones.
Biological evolution tends to utilize available materials; organisms will spend a long time finding the most suitable materials for survival and then use and optimize these materials.
Generally, there are two important considerations when selecting biological materials: one is how much energy is wasted in using this material, and the second is the abundance of the material and the convenience of obtaining it. Therefore, the abundant calcium in the ocean has become a suitable material for animal evolution.
The abundant calcium in the ocean has become a suitable material for animal evolution.
Under normal circumstances, titanium alloys are very durable and lightweight, making them excellent materials for bone construction. Furthermore, titanium is also abundant in the Earth’s crust, but it primarily exists in the form of water-insoluble titanium dioxide. Thus, it is a challenging material to utilize.
Another energy index is that many oxidation reactions of calcium compounds are energy-releasing processes, and animals can also obtain energy when extracting calcium compounds.
For this reason, some researchers even believe that the initial reason animals acquired calcium-containing bones could be due to calcium-rich waste produced during metabolism that was eventually utilized as bone.
While organisms on Earth efficiently use carbon, it is very challenging to obtain carbon fibers through biological processes. Moreover, carbon fibers require a massive amount of energy and extremely high temperatures.
There is another crucial reason why animals do not build their bodies from titanium alloys and carbon fibers: calcium phosphate and calcium carbonate are both poorly soluble in water.
The importance of water for life lies not only in its participation in specific chemical reactions but also in the fact that water serves as a solvent carrier for living substances; all chemicals involved in life activities must be dissolved in water to complete their biological processes.
When the human body is suspended in the womb, the developing body starts to form, and the seeds that will form future bones—cartilage—begin to appear. Cartilage is a type of tissue that is not as rigid as bone but is more elastic and, in some respects, serves more functions.
Later in the development of the fetus in the womb, a large amount of cartilage begins to transform into bone—a process known as ossification. During ossification, cartilage begins to calcify.
The main component of vertebrate bone is calcium phosphate, not calcium carbonate, because the solubility of calcium phosphate in water is 1.5 times that of calcium carbonate.
The solubility of calcium carbonate in pure water is 14 mg/L, while that of calcium phosphate is 20 mg/L. This also means that bones made from calcium phosphate will develop 1.5 times more effectively than those made from calcium carbonate.
It is precisely because materials like aluminum, iron, titanium, and carbon fibers cannot dissolve in water that cells cannot utilize them as building materials for the internal skeletons of vertebrates.
