Continuing from the previous part, we look at why biomineralization arose independently in several lineages, and why calcium of all things was the one chosen.
The Independent Origins of Biomineralization
In the Cambrian, animals with shells and skeletons appeared in an explosive burst. Trilobites, brachiopods, mollusks, echinoderms, and the ancestors of vertebrates all came to possess hard structures in this period. And here one fundamental question arises. Were these shells inherited from a common ancestor, or did they evolve independently in each animal lineage? If a common ancestor had possessed a shell, then the genes and molecular machinery for making a shell would have been passed to all its descendants, and the structure of the shells would be similar to some degree. If, on the other hand, they evolved independently, then each lineage would have made its shell in a different way, and the structures of the shells would be considerably different.
The current consensus in science is the latter. Biomineralization evolved independently in several animal phyla, yet at almost the same time. The evidence for this is manifold. First, there is phylogenetic analysis. When you draw the evolutionary family tree of the animals that have shells, animals without shells are wedged in among the animals with shells. For example, among the mollusks, snails and clams have hard shells, but slugs and some sea slugs have no shell. If the shell had been inherited from the common ancestor of the mollusks, then the shell-bearing species should be gathered on a single branch. In reality, however, the species that have lost their shells are scattered here and there across the family tree, which suggests that the shell evolved independently several times and was also lost independently several times. It is the same when we broaden the scope to the entire animal kingdom. Between the animal phyla that have shells, there are animal phyla without shells wedged in, which tells us that the shell did not evolve only once.
Second, there is the diversity of the minerals used. Already in the early Cambrian, shells made of several kinds of minerals appear. Even calcium carbonate has two forms: aragonite and calcite. Their chemical formula is the same, but their crystal structure is different. Some animals use aragonite and some animals use calcite. There is also calcium phosphate. The bones and teeth of vertebrates are made of a calcium phosphate mineral called hydroxyapatite. Some brachiopods, too, have shells of calcium phosphate. There are also animals that build their skeletons out of silica, that is, silicon dioxide. The skeleton of the glass sponge is such a case. If biomineralization had evolved only once, then at first only a single mineral would have been used and later diversified. But the fact that several minerals were already in use at the same time from the early Cambrian suggests that several lineages began biomineralization independently.
Third, there is the diversity of microstructure. Even in the case of the same calcium carbonate shell, its microstructure differs greatly from one animal phylum to another. When you look at a cross-section of a clam shell under an electron microscope, many layers are stacked up, and each layer has a different arrangement of crystals. The structure called the nacreous layer, or mother-of-pearl, has aragonite crystals stacked like bricks, which provides both strength and luster. But when you look at the skeleton of a sea urchin, it is a completely different structure. Each spine and shell plate of a sea urchin is made of a single enormous calcite crystal, within which countless tiny holes are bored, so that it is like a sponge. The skeleton of coral is different again. If all animals had inherited the method of making a shell from the same ancestor, the microstructures could not be this different. It is because each phylum independently evolved its own shell-forming mechanism that this kind of diversity appears.
So why did animals so distantly related to one another independently evolve shells at almost the same time? One compelling explanation is the biomineralization toolkit hypothesis. According to this hypothesis, the genetic and molecular tools needed to make a shell already existed in the common ancestor of the animals. Only, these tools were originally used for other purposes. Pumps that expel calcium out of the cell, proteins that bind calcium, carbonic anhydrase that regulates carbonate ions, systems that secrete a protein matrix outside the cell. These molecular tools had already evolved in order to maintain the cell's calcium homeostasis. As we saw in Chapter 1, this is because every cell can survive only if it manages calcium rigorously. These tools did not evolve for biomineralization but for cell survival, and were later repurposed for shell formation.
In the early Cambrian, when the marine environment changed, calcium concentrations rose, and predation pressure increased, several animal lineages began at the same time to put these existing tools to a new purpose: shell formation. Each lineage had the same basic tools, but the way it combined and made use of those tools was different for each. That is why the mineral composition and microstructure of shells evolved differently from lineage to lineage. This is called convergent evolution. It is the phenomenon in which different lineages, facing similar environmental challenges, independently evolve similar solutions. Just as the eye evolved independently in several animal phyla, the shell too evolved independently in several animal phyla. Now the evolutionary context becomes clearer. Over 3.8 billion years, life had evolved the ability to keep the cytoplasmic calcium concentration at about 100 nanomolar. This was an essential condition for survival. By the Cambrian, some animals extended this calcium-control ability to a new dimension. Beyond managing the calcium inside the cell, they began to form calcium-based structures outside the cell. The shell and the skeleton are the outward expression of the ability to control calcium within the cell. To be able to make a shell is proof that the animal can control calcium with precision. Where and how much calcium to deposit, what crystal structure to form, when to begin and when to stop growth. All of this must be finely regulated. The Cambrian explosion was an event in which a new chapter opened in the evolution of the ability to control calcium.
Why Calcium of All Things
There is a clear reason why the Cambrian animals chose calcium as the material for their shells. There are many other elements, so why calcium of all things? First, abundance. Calcium exists abundantly in seawater. The calcium concentration of modern seawater is about 400 to 420 milligrams per liter. In the Cambrian it may well have been higher than this. That the raw material is abundant means that the energy cost of making a shell is low. It does not take much energy to extract calcium from seawater. Metals such as iron or copper, by contrast, are dissolved in seawater in far smaller amounts, so to make a shell out of them would require enormous energy to concentrate them. Second, chemical stability. Calcium carbonate is quite stable at the normal pH and temperature of seawater. Once it has formed, it does not dissolve easily. Of course, if the pH drops, that is, if the seawater becomes acidified, calcium carbonate can dissolve, but in a normal marine environment it faithfully performs the role of a hard protective barrier. Third, ease of formation. Calcium carbonate can be precipitated with relatively low energy. If the cell locally adjusts the pH or the carbonate ion concentration just a little, it can deposit calcium at the desired location. No complicated chemical reactions or extreme conditions are required.
Beyond their defensive function, shells and skeletons harbored a decisive function that would later become the key to the advance onto land. That is the role of a calcium battery. In Chapter 1 we learned that calcium is a signaling molecule involved in almost every function of the cell. For a muscle to contract, a calcium signal is needed; for a nerve to transmit a signal, calcium is needed; for a hormone to be secreted, calcium is needed; for a cell to divide, calcium is needed. In almost every moment of the activity of life, calcium ions are consumed. And yet, in a harsh natural environment, it is not always possible to receive a steady supply of calcium through food. There may be seasons when food is scarce, it may be hard to find food while on the move, and the calcium concentration of the environment may change. An animal that lives in the sea can absorb calcium from the seawater, but an animal that lives in fresh water or an animal that lives on land cannot do so.
Shells and skeletons are the solution to this problem. When calcium is plentiful, it is stored in the shell and the bones, and when calcium is scarce, it is dissolved out and used for the activity of life. Just as a battery is charged and then discharged, calcium can be stored and then taken out and used when needed. The bones of vertebrates play exactly this role. Bone is not merely a supporting structure that holds up the body. Bone is the largest calcium reservoir in the human body. About 99 percent of the calcium in the human body is stored in the bones and teeth. When the calcium concentration in the blood falls, parathyroid hormone is secreted, which dissolves calcium out of the bones and releases it into the blood; when the calcium concentration in the blood rises, calcitonin is secreted, which stores calcium in the bones. Bone is a giant calcium battery.
The secret by which vertebrates were later able to leave the sea and climb onto the calcium-scarce land lies precisely here. For a life form born in the sea to climb onto land was an enormous challenge. It had to breathe out of water, it had to withstand gravity, it had to overcome dryness. And there was the problem of calcium. The sea is rich in calcium, but the land is not. The calcium concentration of fresh water is only a few tens of the value in seawater, and extracting calcium from the soil and rocks of the land is far harder than absorbing it from seawater. Had there been no bone, life on land would have been impossible. If it could not take in enough calcium from food, its muscles could not contract, its nerves could not send signals, and its heart could not beat, and it would have died. Because there was bone, calcium could be stored in advance and taken out and used when needed, and so it could survive even in an environment where the supply of calcium was unstable. The vertebrates did not leave the sea. They packed a portable, solid sea, in the form of bone, inside their bodies and climbed onto land. This story will be dealt with in more detail in Chapter 3.
Animals with shells enjoyed several other advantages besides this. The defensive function of protecting the body from predators is the most obvious advantage. A hard shell fends off teeth and claws. From the predator's point of view, it is far more efficient to catch and eat soft prey, so an animal with a shell is more likely to be excluded from the list of targets of attack. The supporting function is also important. If there is a hard skeleton, an animal can have a larger body. Mollusks rely on the buoyancy of the water to maintain their shape, but with a skeleton an animal can maintain its shape on its own, so a greater variety of postures and movements becomes possible. Since a firm point for muscles to attach to is created, it can also produce more powerful force. Some animals use their shells for buoyancy control. The chambered nautilus has gas-filled chambers inside its shell, so it can control its buoyancy and move up and down. Some animals have even endowed their shells with a sensory function. The chiton, a kind of mollusk, has hundreds of small eyes made of aragonite on the surface of its shell. These eyes contain calcium structures that act as lenses, so they can detect light. In this way, the shell, which at first began as a simple detoxification mechanism or defensive structure, came to acquire a variety of functions in the course of evolution. This is a characteristic of evolution. A trait, once it has appeared, gains new functions in new contexts.
Conclusion: The Meaning of the Calcium Revolution
The Cambrian explosion is one of the most dramatic events in the history of life. In a short span of less than 1 percent of the 3.8-billion-year history of life, almost all the animal phyla appeared. The basic blueprints of the animals we can see on Earth today were all created at this time. And the most conspicuous feature of these animals was the shell and the skeleton. Creatures that had had only soft bodies suddenly began to don hard armor. Why did the shell appear in this period of all times? A number of factors probably worked in combination: the surge in the calcium concentration of the ocean, the rise in atmospheric oxygen concentration, and the increased need for defense following the appearance of predators. But at the center of it all was calcium. The sea overflowed with calcium, that calcium had to be dealt with, and in the course of dealing with it the shell was made.
Life had, over 3.8 billion years, evolved the ability to manage the calcium inside the cell. The ability to maintain a low concentration of 100 nanomolar, to raise the concentration only temporarily when needed and use it as a signal, and then to lower it quickly again. This is the basic ability of every cell. By the Cambrian, some animals extended this ability to the outside of the cell. They began to make calcium-based structures, that is, shells and skeletons, outside the cell. This was a new chapter in the ability to control calcium. Beyond the defensive use of maintaining homeostasis within the cell, it was a shift to the offensive use of building the structure of the body using calcium. This ability provided animals with a variety of advantages: defense, support, and storage.
But the Cambrian revolution is something that happened in the sea. The sea is an environment rich in calcium. Obtaining calcium from seawater is relatively easy. In the next chapter we will look at the new challenge that life faced when it advanced onto land. The land is an environment entirely different from the sea. Calcium is scarce, and although there is ultraviolet light that can synthesize vitamin D, calcium cannot be absorbed through gills as it is in water. To survive in an environment where calcium is scarce, the calcium reservoir called bone had to be managed even more precisely. A completely new system of calcium control was needed. This is exactly the reason why the parathyroid hormone and vitamin D systems evolved.
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