Dental Exam

By Shara Bailey

The Tales Teeth Tell aims to introduce readers to the wonderful world of teeth and what they can tell us about human evolution. In the field of evolutionary anthropology, teeth have long played second fiddle to the skeleton, and the skull in particular, but things are changing.

In the past five years, at least three books have tackled the topic of teeth and human evolution: two by the paleoanthropologist Peter Ungar (Teeth: A Very Short Introduction and Evolution’s Bite) and one by the biological anthropologist Debbie Guatelli-Steinberg (What Teeth Reveal about Human Evolution). And it’s no wonder: not only do teeth preserve better than bone, making them the most abundant human remains in the fossil record; they also provide a more accurate picture of an individual’s underlying genome. Much of what we know about human evolution comes from teeth, and with the advent of new technologies and the discovery of important new fossils, researchers are learning more every year.

This book’s author, the human evolutionary biologist Tanya Smith, is an expert on dental microstructure, the complex, microscopic architecture of teeth. As such, she is able to offer insights into how the study of microstructure informs our understanding of the development, evolution, and behavior of humans and our fossil ancestors. But Tales Teeth Tell does not focus primarily on the microscopic aspects of dentition. Instead, Smith provides a broad perspective on human evolution, richly informed by the study of human teeth.

Comprehensively researched and well-written, the book covers three important topics—human development, evolution, and behavior—with each comprising a section of the book. In the first section, Smith explores the microscopic world of tooth growth, including how enamel (the hard, mineralized outer layer) and dentin (the tough, fibrous tissue below the enamel) are laid down; how teeth erupt, or visibly emerge from the gums; and how biological markers become apparent with microscopic observation. For example, the dark neonatal line visible in enamel and dentin coincides with an individual’s birth, and it is used as a starting point to accurately calculate the age of fossilized hominin children—that is, children of the evolutionary group that includes modern humans and now-extinct bipedal relatives.

Smith invokes useful metaphors to help illustrate otherwise difficult concepts. Incremental lines in teeth, for instance, are like the annual growth rings in trees, and help to determine the age and health of an individual. Her exploration of dental microstructure leads to a later discussion of the evolution of unique human characteristics such as our extended childhoods and post-reproductive lifespan. She describes the ways in which dental anthropologists can use microscopic information to observe stress lines in teeth and to tell when, and perhaps why, individuals were stressed, physiologically or psychologically, during growth. She also provides an informative discussion on how a mismatch between what humans evolved to eat (foods such as fruit, nuts, and meat) and what we actually eat today (highly processed, sugar-laden foods) has left modern humans suffering from common dental problems such as impacted wisdom teeth, cavities, and periodontal disease.

In the second section, Smith tackles human evolution. She starts at the beginning—the evolution of teeth—and does a respectable job discussing some important debates in a way that nonscientists will be able to understand. Whether teeth originated externally as pointy cheek scales around the lips or internally from structures in the pharynx, for example, is a controversial question among evolutionary biologists. How and when teeth evolved has important implications regarding whether teeth evolved once or many times independently during the course of evolution.

Following her discussion of tooth origins, Smith touches on the major aspects of human dental evolution: from single- to multi-cusped teeth to our apelike dental pattern, the reduction of our canine teeth, and the evolution of smaller jaws and smaller teeth to fit into them. This is by no means a comprehensive discussion of human evolution, but that is not Smith’s intent. Rather, this section provides a good introduction to why teeth are shaped the way they are and how they got that way.

Smith concludes her discussion of evolution with the topic of growth and development, one that is near and dear to her heart. She circles back to the microscopic lines in teeth that tell researchers how fast or slow an organism has grown, explaining how these lines have been used to discover when the particular growth pattern of humans evolved. Modern humans grow slowly and live a long time, with the longest juvenile dependency period and longest lifespan of any primate. There has been significant debate in paleoanthropology about when the uniquely slow growth of Homo sapiens began and how fast or slow Neanderthals, by comparison, grew. Although Smith’s research indicates that the earliest H. sapiens grew slowly compared with Neanderthals, she does not offer a one-sided view. Instead, she provides an important discussion about difficulties in knowing the answer to this question. She also explains why scientists sometimes get a mixed signal from the fossil record.

In the third section, Smith provides a dental perspective on human behavior. Here, behavior includes diet, the use of teeth as tools and as social signals, and the ways humans have intentionally and unintentionally modified their teeth. A variety of approaches are available for studying these topics, such as examining stable isotope ratios, tooth wear, and ancient DNA, and Smith seamlessly ties the results together. One thing that comes through clearly is that there is still much debate about how our fossil relatives behaved: whether the earliest hominins lived in monogamous pairs or in groups that included numerous males and females, for example, or whether the extinction of Neanderthals was related to a lack of dietary breadth. Smith cautions readers not to give up; despite a number of open questions, it is the nature of science to be revised and refined. Each time scientists discard an old hypothesis in favor of a new one, they learn something along the way.

Smith’s final chapter illuminates the future of dental research. Exciting stem cell research is making it possible to regrow lost teeth. Humans continue to mature earlier and live longer, even compared with just one hundred years ago. She also touches on the fascinating topic of the human microbiome, the trillions of microorganisms that live in the human body, some of which are preserved in dental tartar.

Tales Teeth Tell is meant to be a popular science book, but Smith also wanted to make it meaningful for people in the field. In both respects she succeeds. Smith writes well and the book is effectively organized. Her occasionally breezy style (Smith often refers to teeth as “choppers,” for instance) is meant to engage rather than intimidate the reader. The book is illustrated with copious figures, including high-quality images to help readers understand the topic. Laypeople and dentists will certainly learn new, interesting information about teeth and will appreciate the many illustrations. But even specialists in dental anthropology and human evolution will glean at least a few nuggets of new information. Smith undertook an extensive literature review of every topic she tackles in the book; she uses plenty of endnotes to draw readers’ attention to citations and further reading; and she does an excellent job of summarizing the state of the art of dental anthropological research. All the information is current and not all of it is mainstream. For instance, most readers may be unaware that our hominin relatives were using toothpicks more than two million years ago, and that we can tell this from looking at wear patterns on their teeth.

There are several aspects of this book that especially impressed me. First, Smith has made an effort to tune into specific questions that nonspecialists have about teeth: Why are modern humans “losing” our wisdom teeth? (Soft diets led to underdeveloped jaws, so we lack the room to erupt them.) Should we be eating a so-called paleodiet—that is, a diet imagined to be similar to that of our early human ancestors? (Probably not: hominin diets were much more varied than what’s presented in the popular press.) What will humans look like in the future? Readers will certainly appreciate having answers to these questions, or at least knowing why some—such as this last one—may be unanswerable.

Second, Smith has taken many, sometimes disparate, topics and seamlessly woven them together. Transitioning between topics is no easy task, and Smith is to be applauded for her efforts. At the end of each chapter she provides a brief summary of what was covered and how it relates to what is to come. I found this to be a valuable addition, rather than redundant. She covers a lot of information in each section, so a one-paragraph summary of the main points is useful. The summary also provides a transition from one section to the next.

Finally, throughout the book Smith provides a balanced perspective on what are often heated debates within dental anthropology and paleoanthropology. In doing so she acknowledges that this might be frustrating to some readers, who just want to know answers. However, by providing multiple views, Smith provides something closer to the truth: we know a lot, but there is still plenty to learn. In providing a wealth of information, much of which will be new even to dental anthropologists, Smith lays the groundwork for those who might be interested in picking up some of these ideas in the future.