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Teeth, Mollusks: Design in Biominerals

Evolution News

Minerals created by diverse life forms show optimized molecular structure. Rather than indicating convergent evolution, do they originate from divergent foresight?

Two recent papers explore the crystal structure of biominerals, which are found in everything from mollusk shells to the teeth in our mouths. These structures are fascinating for their exquisite properties, but the subject of biomineralization also allows comparing the explanatory power of design over Darwinian evolution. 

The structures inside biominerals are so exquisitely optimized, engineers study them for ideas on how to create superior materials. But on the flip side, one might wonder why an intelligent designer would use such different mechanisms to achieve the desired properties of strength and fracture-resistance in diverse creatures. Is this evidence of convergent evolution, or foresight?

Human Teeth

“The Hidden Structure of Tooth Enamel,” written by seven scientists in Nature Communications, introduces a fascinating topic we can all relate to. Many of us may not pay much attention to our teeth until we go to the dentist, but it is quite remarkable that these natural utensils, subject to severe biting and grinding forces and chemical attacks almost every day, for many people can last a lifetime. The authors of the open-access paper sure found tooth enamel fascinating:

Dental enamel is the most highly mineralized tissue in the human body. Its outstanding mechanical properties combine the extreme hardness and stiffness with exceptional resilience, which enables it to withstand hundreds of masticatory cycles with biting forces of up to 770 N, in the harsh environment of the oral cavity, which also undergoes extreme pH and temperature fluctuations within the human body. Despite the fact that it does not remodel or repair, it lasts decades without catastrophic failure. [Emphasis added.]

What does 770 N mean? A newton (N) is the force that produces an acceleration of one meter per second per second on a mass of one kilogram. That’s a non-trivial force. Now multiply it by 770 to consider what your teeth go through!

Like the head of a hammer, enamel is the outer portion of a tooth that faces this harsh environment. Unlike bone, which the body can repair, enamel has to endure this treatment for life. Naturally, the scientists were eager to explore how it survives the amount of wear and tear that would put many artificial materials out of commission in far less time. A summary of the paper on Phys.org explains how the team used advanced imaging methods called polarization-dependent imaging contrast (PIC) mapping to figure out what makes the biominerals so durable. 

Tooth enamel is organized in micron-length rods made up of long, skinny crystals of hydroxyapatite. [Pupa] Gilbert and her group at UW–Madison applied PIC mapping to several human tooth samples and measured the orientation of each crystal in tooth cross sections.

“By and large, we saw that there was not a single orientation in each rod, but a gradual change in crystal orientations between adjacent nanocrystals,” Gilbert says.

It’s the slight misorientation between rods that stops cracks from propagating through the interface, they found. Next, they wanted to know if the degree of the misorientation angle makes a difference. 

“I started wondering, is there an ideal misorientation angle that is most effective at deflecting cracks?” Gilbert recalls. “The experiment to test this hypothesis could not be done at the nanoscale, nor by simulations, so I started thinking, okay, we trust evolution. If there is an ideal angle of misorientation, I bet it’s the one in our mouths.”

The thinking of evolutionists is that Darwinian selection always trends toward an optimum. Design theorists know that is nonsense and may find the remark humorous. The point to chew on, though, is that the nanostructure of tooth enamel is optimized for durability. It works so well, the authors thought about “how it can be applied to synthetic materials.” The paper concludes:

This structure, previously hidden, contributes to making enamel extraordinarily resilient, as it endures hundreds of mastication cycles per day, with hundreds of Newtons of biting force. This structure prevents catastrophic failure of enamel by deflecting cracks inside rods, and keeps it functional for our entire lifetime.

Biominerals in Mollusks

The phylum Mollusca, which dates to the Cambrian explosion, contains exquisite examples of biomineralization. Some of the most beautiful shells of all, including the chambered nautilus with its Fibonacci Series-Golden Ratio geometry, as well as snails, oysters, and scallops fit into this group. A team of six scientists writing in PNAS sought “to expand our understanding of the macroscopic morphospace of possible molluscan shell shapes to the level of possible ultrastructures that comprise them.” Evolution weighed heavily on their mind.

Molluscan shells are a classic model system to study formation–structure–function relationships in biological materials and the process of biomineralized tissue morphogenesis. Typically, each shell consists of a number of highly mineralized ultrastructures, each characterized by a specific 3D mineral–organic architecture. Surprisingly, in some cases, despite the lack of a mutual biochemical toolkit for biomineralization or evidence of homology, shells from different independently evolved species contain similar ultrastructural motifs.

Of note is the authors’ reliance on the concept of morphospaces, developed by the late David Raup, a Darwinian paleontologist who attended the 1993 gathering of ID scientists at Pajaro Dunes. In his remembrances of Raup, Paul Nelson respected Raup’s openness to criticisms of evolution and his “courage in engaging in thoughtful discussion with those of differing viewpoints.”  

It’s lamentable that these scientists did not follow that aspect of Raup’s character. In their strict Darwinian thinking, the similarities in biominerals between scallops, nautiloids, and snails must be due to convergent evolution. They conclude that the environment somehow constrains biominerals to take on similar motifs. Creatures “discover” these motifs among what is thermodynamically possible in the morphospace of mineral construction, given the materials available. The team says:

In Raup’s classic works, elegant parameterization of the macroscale geometry of molluscan shells defined a morphospace that permitted a theoretical exploration of the wide diversity of shell morphologies. This work allowed a mathematical analysis of molluscan shell shapes and the theoretical exploration of limits of their evolution and thus, provided insights on why only certain morphologies exist in nature, in molluscs, and other species. For example, the similar spiral morphology seen in receptaculitids and bryozoans, despite very different methods of growth, reflects an architectural constraint on the fabrication of a growing spiral with physically constrained units.

Constraints, however, cannot explain the beauty of a nautilus shell. There is nothing about a constraint that can force elegant design. That’s like saying that highways constrain what cars are possible. “Well, yeah, but…” Perhaps a highway’s dimensions constrain the width of a car, but isn’t there a lot more to a car’s design than a series of constraints?

The evolutionary storytelling need not concern us now. More interesting are the facts about these wonderful minerals constructed by living organisms: in this case, mollusks. It’s refreshing to hear rare pearls of wisdom in the paper, such as this line in the concluding discussion: “molluscan shell biomineralization and morphogenesis is an extracellular process that proceeds under genetic control and is remotely orchestrated by the cellular tissue.” They add, “The main distinction between the growth of the various biomineralized structures in nature are the driving forces set by the organisms that ultimately regulate the nucleation, manipulate the shape, and assemble the mineral components.” I.e., they do not self-organize, nor do the constraints of physical laws design the shells. The shells are driven by a genetic code. 

It is certainly worth asking how that code got there. Every code we know of that regulates complex structures was designed. Science will improve when the constraints of Darwinism are lifted.

Photo: Human teeth from Qesem Cave, Israel, between 200,000 and 400,000 years old, by Israel Hershkovitz, Tel Aviv University, via EurekAlert!