Genetic Roots of Tooth Enamel Reveal Distinctive Design by Dr. Elizabeth Mitchell

ScienceDaily: Getting to the root of enamel evolution: Connecting genes to hominin teeth shows evidence of natural selection

The enamel on your teeth tells a story. But can it trace your evolutionary past? Researchers based at Duke University compared several primate genes responsible for producing enamel. They believe they have identified two genes on the path from ape-like ancestor to human. A Duke press release states that the study just published in the Journal of Human Evolution “offers insight into how evolution shaped our teeth, one gene at a time.”1

Tales Told by Teeth

Because tooth enamel incorporates2 elemental isotopes and minerals from your diet as well as scratches and residue from hard foods you grind with your teeth, it may reveal where you’ve lived and what you’ve eaten over time. Permanent teeth, designed to last a lifetime, contain below their surface a microscopic record of their growth, analogous to tree rings. “Teeth also preserve their growth bands,” explains Duke evolutionary anthropologist Christine Wall. “So in terms of understanding fossils, teeth can tell you how old a juvenile was when it died, or how long it takes for teeth to develop—so you can compare between living and extinct species.” But can teeth tell more?

Wall’s team believes the genetic roots of tooth enamel production will teach us about humanity’s evolutionary past. “The fossil record is always the most complete for teeth," says Wall. “And enamel thickness has long been a key trait used to diagnose fossil hominins3 and reconstruct their diets and life histories.” Comparing enamel-associated genes in humans and five primate animals, they believe they have found a way to trace human evolution through selective dietary innovations in the lives of ape-like ancestors.

Enamel thickness, though determined long before birth when enamel-producing cells build a precisely oriented protein framework for its mineral structure, does vary with a species’ typical diet. Human tooth enamel is, generally speaking, thicker than apes’. Many evolutionary paleontologists believe dietary changes contributed to the evolution of bigger brains and thus to human evolution. The authors of the recent study think that the genetic distinctives that help make human tooth enamel thick are the footprints of selective changes that led to the evolution of modern humans. This study is part of a larger project to find evolutionary connections between human anatomy, genetics, and diet.

Building Enamel

The team compared corresponding enamel-associated genes in humans, gorillas, chimpanzees, orangutans, gibbons, and rhesus macaques. Of these, humans typically have the thickest enamel, gorillas and chimps the thinnest, and the others somewhere in between. The thin enamel of chimps and gorillas is sufficient to protect their teeth from their diets of fruit and leaves. The orangutan, having much thicker enamel, is equipped for its more omnivorous diet. But the enamel thickness that must protect teeth throughout a lifetime of chewing is determined not by diet but in the womb.

Enamel is the hardest, most mineral-rich substance in the human body. Covering a tooth’s crown long before it erupts from the gum, human enamel is usually thickest on the top. Several proteins are found only in tooth enamel. They form a framework for the minerals that make enamel hard. Once a tooth’s enamel is mature, the cells that produce these proteins disappear. Additional minerals get incorporated into enamel, but worn or decayed enamel cannot be replaced.

“We decided to look just at genes that have a known role in tooth development,” explains Duke University biology professor Greg Wray. Four proteins involved in tooth formation are enamelysin, amelogenin, ameloblastin, and enamelin. Mutations in the genes controlling production of these proteins are associated with abnormal enamel formation. The team compared genes in humans and five primate animals that they believe all share a common ape-like ancestor. They believe differences between them trace the evolution of each species, and they also believe that the greatest differences indicate the genes most favored by natural selection in the course of evolution over millions of years. “That's when we know a gene is under positive selection,” says lead author Julie Horvath.

MMP20 and ENAM, the genes for two of the proteins, enamelysin and enameliin, respectively, had significant interspecies differences in the regulatory regions that control their transcription. The protein-coding parts of the genes did not differ significantly between species, and the genes for the other two proteins did not differ. Therefore the team concludes that in the course of human evolution changes in the regulation of transcription of these two genes contributed to us becoming what we are, selectively favoring us to eat the kinds of foods we do and ultimately to evolve our bigger brains. They write, “We expect mutations affecting gene expression to comprise an important part of the genetic basis for dietary adaptations during human evolution.”4

Enamel consists of enamel rods within a matrix. The rods fracture if they are not properly oriented with the underlying dentin structure. supported by the tooth’s dentin underneath. The enamel rods are packed with ribbons of crystalline hydroxyapatite. Enamel formation, at the molecular level, is a complex multi-step process. During the early phases of enamel formation, enamelysin is continuously secreted by ameloblast cells into maturing enamel rods to clear space for enamelin. Enamelin surrounds the crystalline ribbons within these rods to maintain space for the eventual deposition of minerals. Enamelin is also highly concentrated at the enamel-dentin boundary. Enamel thickness as well as the shape of the enamel-dentin boundary vary not only between species but within species, so it is not surprising to find interspecies genetic differences affecting the production of enamelysin and enamelin.

Translating Tales Told by Teeth

Enamel incorporates, either chemically or physically, some elements to which it is exposed. For instance, the ratios of carbon and oxygen isotopes from the food and water a person or animal has been consuming over time can be measured in the carbonate and phosphate components of tooth enamel. Measuring the proportions of such isotopes and comparing them to available food and water sources can provide clues about the place where a person or animal has—or had—been living. The scratches on a fossil tooth’s enamel, its thickness, and the material embedded in the enamel surface can also provide clues about a person or animal’s accustomed diet. For example, recent research on enamel has shown that the extinct ape Paranthropus boisei (aka “Nutcracker Man”) probably spent his days scrabbling under sedges and chewing for hours on the tiger nuts he found there, much like baboons do today. These are the sorts of “stories” that scientific observations of tooth enamel and dental calculus can tell us.

Using enamel thickness to identify the original owner of a tooth is more problematic since the thickness of enamel varies from species to species but also within a species. Enamel thickness and the shape of the enamel-dentine junction vary with tooth and gender and also among populations of humans regionally and temporally. “It is unclear to what degree these differences are due to environmental or genetic factors,”5 write the authors of a 2006 study of modern human enamel, published in Archives of Oral Biology.

Paleoanthropologists, however, often cite the thickness of tooth enamel as a reason for declaring an extinct ape fossil to be a human ancestor. Thus, even with evolutionary considerations aside, the variability within humans as well as the variation in other species make enamel thickness a very crude and imprecise way of identifying the owners of fossil teeth. The authors of the 2006 study write, “These factors must be considered in the categorization and comparison of ape and human molars, particularly when isolated teeth or fossil taxa are included. Human relative enamel thickness encompasses most values reported for fossil apes and humans, suggesting limited taxonomic value when considered alone.”5

Comparing Common Designs

What the researchers in this study observed are the two areas of the enamel-producing blueprint that differ between humans and primate animals. Since enamel tops teeth of both, it is no surprise that these human differences occur in the regulatory parts of these genes, the parts that control transcription, rather than the genes that code for the enamel-building proteins themselves. Furthermore, since the difference between human tooth enamel and that of most apes is thickness, the researchers assume—probably correctly—that these regulatory differences are those associated with building more enamel to make the final product thicker.

The problem with this research is not the comparative genomics. Comparative genomics can be revealing. As in this study, it has revealed one small aspect of the countless differences between God’s design for humans and many of the primate animals. But comparing the enamel-producing genes of humans to those of animals with which we share some physical similarities does not reveal an evolutionary path that produced those differences. That is an evolutionary interpretation imposed upon the data by those that are convinced—despite the lack of experimental evidence for molecules-to-man evolution—that apes and humans share a common ancestor.

God our Common Designer used many common designs and variations as He created humans and animals, but similar design does not demonstrate common evolutionary ancestry. God created Adam and Eve, the first humans, in His own image on the same day He created land animals. We learn this from His eyewitness account recorded in the Bible’s book of Genesis. Therefore we know that no evolution from ape-like ancestors was involved. Furthermore, what we observe in the scientific study of living things confirms that living things all reproduce and vary within their created kinds, an observation that is consistent with what we read in the Bible. This study has taught us more about God’s design for teeth, not about the ascent of and divergence of teeth up the evolutionary tree of life.