The Tales Teeth Tell: Development, Evolution, Behavior

Introduction: Why Teeth?

Folks are often a little skeptical or politely suspicious when I tell them that I am a biological anthropologist who studies teeth. Perhaps fearing a quiz on their oral care, some promptly steer the conversation into seemingly more neutral territory, such as which classes I teach or whether humans are still evolving. I quickly parry that teeth contain detailed records of growth, health, and diet, as well as our evolutionary history. (And yes, we are still evolving—we’ll get to that later.) Amazingly, every day of our childhood is fossilized during tooth formation, a record that begins before we are born and lasts for millions of years. That is, if we’re lucky enough to not grind our teeth down during life. Humans wore through their choppers rapidly in the past, showing extreme wear in middle age or even by young adulthood.

So what are the tales teeth tell? The French paleontologist George Cuvier famously remarked, Show me your teeth and I will tell you who you are.¹ As the forefather of comparative anatomy, Cuvier helped to establish how the shape of a structure relates to its function. The pointed canines of lions are useful for piercing animal prey, while the flat molar teeth found in specialized seed-eating monkeys efficiently crush and grind hard objects. Paleontologists use the anatomy and behavior of modern animals to infer the diets of extinct species, thus telling what something is from their teeth alone.

Cuvier also pointed to a deeper truth about teeth, albeit unintentionally, since the details of an individual’s early life are registered in perpetuity. Teeth are unlike any other body part, recording physiological rhythms as often as every 8–12 hours, and as infrequently as every season or year. Kids today learn about annual tree rings in elementary school, yet many dentists and oral health specialists are surprisingly unaware that the focus of their livelihoods is a sophisticated time machine. Not to knock rings in trees—especially given the important clues they hold about past climates—but there is far more to discover inside our own mouths.

While this might sound like a plotline in the popular television show Bones, my colleagues and I dive inside the teeth of ancient children to establish how old they were. We employ painstaking approaches to coax secrets from dental remains formed during the eras predating birth certificates. Counts and measurements of tiny time lines are more accurate than any other aging method employed by forensic scientists. However, there is a catch, as this technique only works up to a certain age. Once root growth is complete—at around age 20 for modern human wisdom teeth—the addition of daily lines ceases, and these unique childhood records then gradually disappear with each meal or night of unconscious tooth grinding. A small group of biological anthropologists mine this wealth of information to understand how our ancestors grew up, how we evolved, and how cultural transitions prior to recorded history have affected our health. In the following chapters, we’ll explore the intimate precision, striking beauty, and integrative power of growth rhythms in teeth (figure 1).

Figure 1

Growth lines inside a 9- to 12-million-year-old fossilized ape molar. Rhythms ranging from 9 days (broad diagonal lines) to 24 hours (small, light and dark, box-like features stacked vertically) reveal that this crown took more than 2 years to form. The colorization is due to the use of polarized light microscopy, highlighting variation in mineral structure. Fossil courtesy of the Natural History Museum (London).

We will also consider the surprising records of behavior that remain on tooth surfaces for millennia. For example, the plaque our hygienists carefully remove traps food particles, bacteria, and DNA from our own cells in a sticky layer that can fossilize over time into dental calculus. While calculus doesn’t show the same faithful time records as enamel and dentine,² it captures human activity after our teeth finish growing, continuing the story of our behavior and health into adulthood and old age. We’ll learn how clues such as microscopic scratches and pits formed during chewing have spawned serious debates about the evolution of the human diet. And we’ll see how evidence from teeth may point to the uniqueness of our own species, Homo sapiens, with our long childhoods, remarkably diverse diets, and complex behaviors.

The Science of the Human Past

In 1859, Charles Darwin forever transformed biology with his theory of evolution by natural selection, detailed in The Origin of Species. Although a handful of naturalists and geologists had already come to understand that life on earth evolves, or changes through time, Darwin conceptualized an elegant mechanism for evolution that has since been subject to exhaustive scrutiny. He reasoned that evolution occurs through the greater reproductive success of individuals who are better suited for their environment than others in a population. Later dubbed survival of the fittest, this process increases the proportion of organisms with anatomical, physiological, or behavioral traits that help them survive and reproduce—provided that these adaptations can be inherited. Darwin termed the mechanism natural selection, which he contrasted with the artificial selection that occurs in horticulture or animal husbandry though selective breeding. In later chapters, we’ll explore how these cultural developments have left their own impressions in the teeth of prehistoric humans.

Many biology students first understand how evolution works when they’re introduced to the Galápagos finches that Darwin encountered during his world travels. Long-term study has shown that finches with particular beaks have been able to colonize different islands and survive periods of environmental change. Beaks with sizes and shapes best suited to the local food supply are an adaptive trait that allows individuals who possess them to contribute their genes to the next generation. Birds with really mismatched beaks simply don’t make it. Thus, populations of Galápagos finches evolve when individuals with genetically-determined adaptive features become more numerous than they were in preceding generations.

Darwin’s friend and loyal defender Thomas Huxley extended this theory to explain human evolution in the controversial 1863 classic Evidence as to Man’s Place in Nature, which Darwin followed in 1871 with The Descent of Man, and Selection in Relation to Sex. These books effectively launched paleoanthropology, a branch of contemporary biological anthropology that investigates human origins and evolution. Incredibly, Huxley and Darwin developed their ideas without much direct evidence of human ancestry. The fossilized remains of several hominins—humans and their extinct relatives—had been recovered earlier, but their discoverers, ignorant of the concept of evolution, regarded them as modern humans and relegated them to museum storage. The first fossils were unearthed from a Belgian cave during the winter of 1829–1830.³ This discovery included a massive skull, a tiny upper jaw, and teeth from an infant Neanderthal, our brawnier, Northern-dwelling cousins (figure 2). We’ll return to the story of this infant later, since cutting-edge imaging of its teeth has revealed exactly how old it was when it died. (Spoiler alert: it’s younger than anyone expected!)

Figure 2

Infant Neanderthal upper jaw and associated baby and permanent teeth. Individuals from this Belgian site (commonly known as Schmerling Cave or the second Engis cave) were the first fossil hominins ever discovered. Fossil courtesy of the University of Liège (Belgium).

Paleoanthropologists who carefully pore over the teeth of ancient hominins are lucky compared to those who study other parts of their skeletons. Thousands of hominin teeth have been discovered and deposited in the natural history museums of Europe, Asia, and Africa over the past century. In some instances, fossil teeth are all that remain of long-extinct forms.⁴ This is due to their highly resilient nature—more than 95% of the enamel cap is composed of mineral. It’s not too much of a stretch to regard them as fossils in our mouths, although anyone who’s had a root canal knows how alive our teeth are! Ongoing paleontological fieldwork uncovers new fossils each year, and this sometimes includes hominin children with pristine jaws. In 2006, my friend Zeray Alemseged vaulted to rock star status with his announcement of the discovery of a 3-million-year-old baby from Ethiopia (figure 3).⁵ He had spent years carefully freeing the remarkable skeleton from a hard sandstone block, which the press dubbed Lucy’s Child—a reference to the iconic female skeleton popularly known as Lucy from the same species, Australopithecus afarensis.

Figure 3

Infant skull from the most complete australopithecine child discovered. Upper image is the original fossil; lower image is a virtual model showing its erupted baby teeth after sandstone removal. Fossil courtesy of the National Museum of Ethiopia (Addis Ababa). Image credit: Zeray Alemseged and Paul Tafforeau.

Scientists today have an impressive toolkit available for the study of rare fossils, and Zeray turned to my collaborator Paul Tafforeau for help peering into this child’s history. Paul is an expert in using high-powered X-rays at the European Synchrotron Radiation Facility in Grenoble, France. We have been working together for more than a decade to image the tiny structures in teeth without needing to slice fossils open. Armed with many terabytes of X-ray data and cutting-edge engineering software, our team spent months working to determine how old Lucy’s Child was when it died. We’ll return to the story of how state-of-the-art synchrotron imaging has revolutionized the study of hominin development in the following chapter.

What Makes Humans Human

In 1925, the young anatomist Raymond Dart announced a new ancient hominin, Australopithecus africanus, which means southern ape of Africa.⁶ Miners had recovered a child’s skull from limestone cliffs at Taung, South Africa (figure 4), and Dart’s interpretation of the fossil launched an intense scholarly debate. When scientists formally describe fossils, they employ Cuvier’s comparative approach to identify similarities and differences with other closely related species. Dart compared the Taung Child to chimpanzees, gorillas, and humans, concluding that it was in an intermediate evolutionary position between living apes and humans. This was met with great suspicion by the leading anatomists and anthropologists of the time, who regarded the skull as much more ape-like than human-like—dismissing its position as a human ancestor. While other hominins had been recovered prior to the Taung Child, this was the first fossil that was different enough from living humans to justify placing it in its own genus, Australopithecus. Given this unique position, it is little wonder that scientists initially struggled to understand its evolutionary relationship to living humans and great apes. In time, after considerable polarization of the scientific community, Dart’s assertion was accepted. Numerous australopithecine species have since been found in fossil deposits. The acceptance of Dart’s discovery also validated a radical proposition by Charles Darwin that the ancestors of living humans would be found in Africa. I’ll discuss this further in chapter 5, where we’ll encounter fossils that are even more ancient than Australopithecus.

Figure 4

The Australopithecus africanus child studied by Raymond Dart. The upper and lower jaws have a full set of baby teeth along with the permanent first molars. Fossil courtesy of the University of the Witwatersrand (Johannesburg).

Dart’s description of the Taung Child includes an important observation about its teeth: The specimen is juvenile, for the first permanent molar tooth only has erupted in both jaws on both sides of the face; i.e., it corresponds anatomically with a human child of 6 years of age.⁷ Dart is correct; first molars in modern humans tend to erupt by about 6 years of age. Importantly, he identified a developmental yardstick—eruption of the first molars—that can be compared with other juveniles. The tricky part is determining whether the Taung Child died at the same chronological age as an ape or human. Think of the old proverb that 1 year of a human’s life is equal to 7 years of a dog’s life; most mammals grow up rapidly, achieving reproductive maturity and other life stages at younger calendar ages than do humans. Our first molars erupt around 6 years of age, while they emerge in African apes around 3 years of age. If the Taung Child was 6 years old when it died with its first molars newly in place, it had a developmental pattern like a modern human. But if it was closer to 3 years old, it more closely resembles the great ape pattern of dental development. Knowing this fossil’s precise age would help to determine how rapidly its skull and brain grew, as well as whether it was likely to have had a long childhood and late maturation. Scholars debated both sides of this developmental conundrum heartily, and as we’ll see in the following chapters, it took 60 years before two up-and-coming biological anthropologists settled the issue using the tiny time lines in teeth.

The Taung skull was also recovered with an exceptionally preserved replica of its interior surface, which has fueled studies of the evolution of the human brain. Our brain is often described as a triune structure, based on three key evolutionary transitions. An ancestral reptilian core controls our reflexive fight-or-flight instincts, our deep mammalian centers facilitate the emotional attachment and regulation essential to parenting and group living, and our outer primate neocortex gives us the cognitive acumen to master complex social behaviors. It is this last evolutionary milestone that drives biological anthropologists to ask why humans are the way they are and when we became this way. Today, modern science has capitulated to the originally blasphemous ideas of Darwin and Huxley, embracing evolutionary explanations for our anatomy and behavior. Ideas about human evolution have also captured the imaginations of barefoot runners and inspired the popular Paleo Diet. The nascent field of evolutionary medicine has helped to explain health problems such as obesity and diabetes in the context of our evolutionary history. This is encouraging to those of us who’ve been confounded by deep resistance to the idea that humans descended from simian ancestors.

Scholars apply a similar evolutionary lens to understand ailments that befall our teeth. As fossil members of our genus, Homo, adopted tools and behaviors to process food, eating became easier. Because our muscles and bones respond to stress and strain, a reduction in chewing forces led to the development of smaller jaw muscles, and their bony supports followed suit by shrinking across generations. During the transition from hunting and gathering food to agricultural production 10,000–15,000 years ago, lower jaws became even shorter and broader.⁸ This decrease in the size of our faces and jaws is frequently offered as an explanation for high rates of third molar impaction, driving the need to surgically remove them. Studies highlighted in chapter 3 reveal that molar impaction became more common over the past 10,000 years in small-jawed individuals. This potentially dangerous condition affects nearly one in four people living today.

Another lens through which we view human uniqueness relates to our behavior. As our recent ancestors perfected ways to cut, tenderize, and cook their food, they no longer required such perfectly attuned choppers to meet their daily caloric needs. This is particularly evident from skyrocketing rates of malocclusion, or misaligned contacts between chewing surfaces, in modern populations that rely on heavily processed foods. Teeth have also been probed for insights ranging from our prehistoric social groupings and stratification to our predilection for symbolic communication. In the final section of this book, I’ll trace the origins of dentistry and highlight the handiwork of the first dental artisans. It turns out that modern humans aren’t alone in their urge to floss or pick their teeth. Nor is a fascination with tooth necklaces unique to us, and in the past nothing barred some grisly ways of acquiring dental bling. One type of behavior that does distinguish us from living and fossil primates is the worldwide human phenomenon of altering our own teeth—coloring, filing, and even removing them to fit in, or perhaps to stand out!

I taught at Harvard University for a number of years, and my favorite class was an annual research seminar that enrolled a small group of dedicated students. We’d spend three months using evidence from teeth to investigate the development, evolution, and behavior of humans and their primate relatives. Perhaps more importantly, my students were empowered to follow their own curiosity by performing research—a first for many of them. They would start with an exploration of living primates from the Harvard Museum of Comparative Zoology’s impressive collections, examining oddities featured in chapter 4, like the tooth combs used for grooming and gouging bark. From this vantage point I would explain how tooth size tends to go hand in hand with diet and body size. Primates preferring a diet of insects, vertebrates, and tree sap tend to be small, while those above a certain weight rely on fruits, leaves, and seeds. The reason for the difference is a matter of metabolism and digestion. Small primates require high-energy diets that can be processed by short digestive tracts, while large primates bulk up on lower-energy items that they process more slowly in elongated digestive systems.

Humans, with our incredibly large, slow-growing, and metabolically demanding brains, have taken yet another tack. Hungry brains and an active lifestyle require a high-energy diet, and we can ingest and digest a dizzying array of foods. Our impressive set of teeth—an oral Swiss Army knife—includes a suite of tools adapted from our mammalian ancestors. Incisors help us bite into fruit, pointed canines and their moderately crested posterior neighbors pierce and tear meat and plants, and low-crowned molars help us grind up hard foods, such as nuts and seeds. The top selling point for our dentition is that it represents a balance between efficiency and overspecialization. Human incisors aren’t as broad and shovel-like as primates that specialize in fruit; our premolars teeth aren’t as crested as those that seek out leaves, insects, or animal prey; and our molars are not as flat and basin-like as primates that mainly rely on seeds. This dental blueprint suggests that, in keeping with our status as omnivores, we’ve hedged our bets evolutionarily.

In the following pages, we’ll explore the human odyssey from a unique angle. Beginning with our development, continuing with our evolution, and culminating with our behavior, you’ll learn how teeth illuminate our history like no other part of our anatomy. At times current and personal, these tales will engender a new appreciation of our remarkable evolutionary journey.

Notes

1. Quoted on p. 1 of Simon Hillson, Dental Anthropology (Cambridge, UK: Cambridge University Press, 1996).

2. The term dentine was coined by the British anatomist Richard Owen but is often spelled dentin in the United States. I prefer dentine in homage to my British doctoral advisors and mentors, Lawrence Martin and Donald Reid. For more information see Michael John Trenouth, The Origin of the Terms Enamel, Dentine and Cementum, Faculty Dental Journal 5 (January 2014): 27–31.

3. Michel Toussaint and Stéphane Pirson, Neandertal Studies in Belgium: 2000–2005, Periodicum Biologorum 108 (2006): 373–387.

4. Teeth are estimated to make up 70% to 90% of all fossils recovered from several hominin sites in Africa, for example: Bernard A. Wood, Tooth Size and Shape and Their Relevance to Studies of Hominid Evolution, Philosophical Transactions of the Royal Society of London B292 (1981): 65–76.

5. Zeresenay Alemseged et al., A Juvenile Early Hominin Skeleton from Dikika, Ethiopia, Nature 443 (2006): 296–301.

6. Raymond A. Dart, "Australopithecus africanus: The Ape-Man of South Africa," Nature 115, no. 2884 (February 1925): 195–199.

7. Quote from ibid., 196.

8. Noreen von Cramon-Taubadel, Global Human Mandibular Variation Reflects Differences in Agricultural and Hunter-Gatherer Subsistence Strategies, Proceedings of the National Academy of Sciences USA 108 (December 2011): 19546–19551.

I Development

I believe that much more of the intimate history of the individual is revealed to the microscopist by a study of the enamel than has been generally understood.

—Alfred Gysi, DDS, 1928 (as related by George Wood Clapp, DDS in Metabolism in Adult Enamel, Dental Digest 37 (1931): 664–665)

1 Microscopes, Cells, and Biological Rhythms

Chewing over a few facts about dental development brings the fossil record to life in new ways. I learned this firsthand through studies of biological anthropology in college—as well as during my immersion in the field of microscopic imaging. Much of what we understand about teeth is due to the historic development of microscopes, which illuminate tiny tooth-forming cells and their mineralized secretions. Microscopic imaging has also taught us that teeth contain biological rhythms—internal clocks that mark each day of our childhood. This in turn has opened the door for anthropologists to calculate the age of ancient youngsters from their fossilized dental remains. In this chapter, I’ll highlight several approaches to landscapes that are invisible to the naked eye—culminating in high-powered synchrotron X-ray imaging. This relatively new technique has allowed breakthroughs in fields ranging from art conservation to engineering. Learning how teeth grow will also elucidate numerous tales of our evolution and behavior in the pages that follow. For example, in chapter 6 we’ll combine knowledge of dental development and X-ray imaging to unpack the origin of our exceptionally long childhood.

Geeking Out on Microscopes

Many of us met our first microscopes in an elementary or high school biology course, employing them for mundane explorations of onionskin or cells from a swab of our own cheek. Perhaps we were lucky enough to witness a fresh-water hydra regenerating, or the erratic movements of a protozoan zipping through a droplet of pond water on a glass slide. Like most teenagers, I took the objective lenses and eyepieces of the compound microscope for granted. I dutifully memorized its parts in order to pass my lab quiz without giving much thought to their significance. I couldn’t have imagined the three-dimensional microscopic world that I would enter a decade later, nor the exhilaration of being the first person to explore the childhood growth of fossil primates long since departed.

During the seventeenth century, these simple tools supercharged the nascent field of biology.¹ The term microscope, meaning small viewer, was introduced by a group of Italian scholars that included Galileo, who rose to fame through astronomical observations with his far viewer, or telescope. A few decades later the British microscopist Robert Hooke proposed the word cells to describe the tiny units of life he encountered in the tissues of plants and animals. In his artful 1665 classic Micrographia, Hooke dramatically illustrated the jagged teeth of a snail as seen with a microscope, noting: the animal to which these teeth belong, is a very anomalous creature.² Modern science has since confirmed that snails are unusual, as they continuously form new generations of teeth and modify the shapes of these replacements in response to changes in their diet.³ Even more impressive are aquatic snails known as limpets, which grow tiny teeth made of the strongest biological material yet discovered. This design allows them to grind up rocks while feeding on particles that adhere to them. Engineers are studying these resilient teeth to learn how to create stronger artificial materials.

Hooke’s Dutch contemporary Anthony Leeuwenhoeck may have been the first person to train his microscope on the structure of tooth enamel, reporting the presence of fine transparent pipes that look like small spheres when viewed end on. In the scholarly language of his time, he reports: Six or seven hundred of these Pipes put together, I judge exceed not the thickness of one Hair of a Man’s Beard.⁴ Leeuwenhoeck’s description of enamel prisms is quite accurate, and all the more impressive given the rudimentary microscope he employed. These long, rod-like structures are circular in cross-section, measure about 0.005 millimeters in diameter, and are nearly transparent due to their high degree of mineralization. I’ll return to these important building blocks during our exploration of how teeth are built to last.

As someone deeply fascinated by the natural world throughout my childhood, I decided to pursue a bachelor's degree in biology at the State University of New York at Geneseo. My freshman-year advisor suggested that I enroll in a biological anthropology course, which I did, despite never having heard of the subject before. Exploring human and primate biology from an evolutionary perspective was immediately engrossing, and I realized that I had found an intellectual home. Like many young women taken with the biographies of Dian Fossey and Jane Goodall, I dreamt of going to Africa to study the behavior of mountain gorillas, and I diligently prepared to do so. Over the next few years I studied wild howler monkeys in Central America, tracked the secretive lemurs of Madagascar, and took numerous courses in biology and biological anthropology—an academic journey that led me back to the microscopic world I had prematurely dismissed.

Why would an adventurous woman give up her dream of studying gorillas in the wild to spend countless hours peering into microscopes in dimly lit labs? Like Robert Hooke and Anthony Leeuwenhoeck, I was quickly taken with a vivid world unseen by most. The romance began during my final semester of college, when I naively enrolled in an elective course on electron microscopy that happened to fit into my schedule. Led by the offbeat and enthusiastic Harold Hoops, a balding microscopist who actually wore a white lab coat to every class, I joined a small group of students learning to prepare, magnify, and photograph the internal structure of mouse cells. Hoops carefully walked us through the application of toxic chemicals that paradoxically preserve organ tissues for sectioning and staining. The process of creating our own glass knives and shearing ultrathin slices of rubbery blackened tissue intrigued me. We gently placed these slices in a transmission electron microscope, which was unquestionably the most sophisticated machine I’d ever been trusted to operate. As the beam of electrons passed through the tiny sliver of mouse spleen, something began to come to life inside of me.

The microscope monitor revealed a grainy black and white world that I’d only caught impersonal glimpses of from biology textbooks. Cell membranes, darkly stained nuclei, mitochondria, and other visual wonders populated the screen. I captured dozens of images of cellular landscapes, and then immersed myself in the art of creating prints in a darkroom. This introduction to microphotography sparked my deep appreciation of tiny biological structures. A faded black-and-white picture of a mouse cell that I magnified 78,900 times has hung beside my college degree for the past 20 years. As the first member of my family to finish college, I can’t explain to them why this print means more to me than the diploma, nor why I’ve been chasing the perfect image through an objective lens ever since. The truth lies in that sweet territory where the boundaries of science and art blur, a private marriage between the left and right hemispheres of my brain.

Before I could don my black cap and gown to celebrate graduating with my college peers, Professor Hoops challenged us to design and execute an independent research project using electron microscopy. My mentor in the anthropology department, Bob Anemone, suggested that I investigate the microscopic structure of fossil teeth he had collected in Wyoming. It was no small task. I had no idea how to prepare the teeth for imaging, nor did I really know what I was looking for. At the time, I was vaguely aware that primate teeth had microscopic lines representing growth rhythms. Yet little was known about the structure or development of the teeth of other mammals, much less 60-million-year-old insect-eaters with teeth the size of pinheads. I passed many hours in the deserted basement of the biology building, searching for elusive growth lines at the helm of an electron microscope.

After days of scanning tooth surfaces at a maddening magnified crawl—hoping for some sign of their development—I brought an unusual abraded spot into focus. I immediately noticed long, thin, enamel prisms, and cross-cutting these prisms were parallel lines that resembled the daily growth lines of primate teeth (figure 1.1). I had done it! Accompanied by the drone of a vacuum pump and an electron gun, I had encountered a secret world in that windowless room. I captured