How Cells Build Hard Parts
You have rocks in your head, and it’s a good thing, or you would die of starvation and imbalance. Living things have need of inorganic structures for various functions. Can you name the mineral structures in your body? The answer is: bone, dentin, enamel and otoliths. The last three are specific to your head. Dentin and enamel help us chew our food, and otoliths help us know which way is up (see 10/10/2003 headline). Vertebrates have bones and teeth, birds lay eggshells of calcium carbonate, and many marine and terrestrial animals build mineral shells. Scientists and engineers are drawn to the skill organisms exhibit in the construction of hard parts (called biomineralization), and they want to imitate it. We’ve drawn attention to the amazing capabilities of the conch shell (see 06/26/2003 headline) and diatoms (see 07/21/2004 headline). Two recent articles in science journals discuss the human fascination with biomineralization.
A book review in Science last week1 opens with praise for the lowly diatom:
The abilities to design and construct inorganic materials with specified atomic structure, size, shape, orientation, and number of defects and to integrate these architectures into functioning devices form the foundation for advances in technologies that rely on the devices’ electrical, optical, magnetic, and chemical outputs. However, assembly methods that allow simultaneous control of these features at lengths from the nanometer scale to the macroscale continue to elude scientists and engineers….
What if there were constructors that could sequester inorganic ions from water, accumulate and concentrate them to produce architectures controlled over length scales from nanometers to tens of centimeters, and do all of this in a matter of hours at ambient temperatures? Such constructors are not inventions of science fiction novels but rather single-cell plants called diatoms…. Biomineralization processes can form structures that are the envy of all of us who strive to understand molecular mechanisms of the assembly of inorganic materials.
The book Mark E. Davis is reviewing is Biomineralization by the Mineralogical Society of America and Geochemical Society, 2003. He was especially impressed by the complexity of the molecular mechanisms organisms use to build their hard parts, mechanisms that show mastery of molecular biology, protein chemistry, nucleation thermodynamics, and crystal growth. Some organisms build minerals inside cells, outside cells, or between cells. Davis found one example particularly attractive to the materials scientist:
Nacre, the mother-of-pearl layer found on the inner surface of shells, has a fracture toughness approximately 3000 times that of the synthetic analogue aragonite (calcium carbonate). Nacre is composed of thin (circa 30 nm) layers of a protein-polysaccharide intercalated between 0.5 micrometer-thick layers of aragonite tablets. The weak interface between the organic and inorganic layers is thought to dissipate the energy of crack propagation and thus strengthen the composite structure. This sophisticated architecture provides clues as to how man-made structures could be improved.
How could such capabilities evolve? “The evolution of mineralized tissues has been enigmatic for more than a century,” says a team of three Penn State scientists writing in PNAS2 on the subject. Feeling that comparative genetics could help solve the enigma, they undertook a search for homologous genes and proteins between disparate groups. “Mineralized tissue is a critical innovation in vertebrate evolution,” they begin, “offering the basis for various adaptive phenotypes: body armor for protection, teeth for predation, and endoskeleton for locomotion.” Certain “primitive” fish have dentin-like body armor covered with an enameloid substance that the team believes evolved into fish scales. Their previous work suggested that mammalian teeth and agnathan body armor are homologous. This time, they examined the genome of a teleost fish and failed to find any homologous proteins for mammalian tooth enamel. Though dentin in teeth seems homologous with body armor that formed on skin collagen of fish, their analyses “suggest that mammalian enamel is distinct from fish enameloid.” Instead, they believe “Their similar nature as a hard structural overlay on exoskeleton and teeth is because of convergent evolution.”
1Mark E. Davis, “How Life Makes Hard Stuff,” Science, Vol 305, Issue 5683, 480, 23 July 2004, [DOI: 10.1126/science.1099773].
2Kawasaki, Suzuki and Weiss, “Genetic basis for the evolution of vertebrate mineralized tissue,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0404279101, published online July 22, 2004.
These two articles illustrate the disparity between hard science and soft, mushy, slippery Darwinian scientism. It goes like this: (1) The organism excels at an engineering feat. (2) It must have evolved, but we don’t know how. To the extent the organism elicits admiration, the Darwinian explanation elicits disgust.
The PNAS article is a useless hodgepodge of storytelling, attempting to force uncooperative facts into a predetermined plot. In one place, they “calibrate” their Darwinian tree based on Darwinian assumptions. When that produces anomalous results in another part of the tree, they simply adjust the rate of evolution on that branch. When another branch has trouble, they rearrange the branches and invoke the magic trick of “convergent evolution” to explain similarities that did not appear to have a common ancestor. All through, there are wiggle words like must have, might have, quite possible, suggests, possible, co-opted, although there is no direct fossil evidence to date… may not have, probably, assumed to etc. The data are only secondary props in this tweakfest to keep Charlie as the national idol. Do they ever explain how multiple genes produced multiple proteins by accident that work biomineralization wonders? No; it is all an exercise in reassuring the reader that the Darwin Party is not really lost. For baloney detectors who are not intimidated by the bluffing of technical jargon or prestigious journals, it makes no sense. Try this howler for fun:
Together these facts make it likely that the developmental mechanism of mammalian tissue mineralization was elaborated during bony fish evolution in actinopterygians or sarcopterygians. Although the genetic tools of tissue mineralization are totally unknown for chondrichthyans, it is quite possible that they have developed their own tools through independent gene duplications and functional selection histories.
What a total whitewash; do you see what they did? They just swept a huge problem under the rug. When the data were missing or contrary, they ascribed it to evolution anyway. They personified fish, turning them into materials engineers and tool inventors. And that ending phrase, “independent … functional selection histories,” should be framed as a classic euphemism for Darwinian dogma.