Evolution's Bite

A Story of Teeth, Diet, and Human Origins

Peter Ungar

• Editorial: Princeton University Press
• Año: 2017
• ISBN:
  9781400884759
• Categoría: Biowissenschaften
• Categoría: Entwicklung

What teeth can teach us about the evolution of the human species Whether we realize it or not, we carry in our mouths the legacy of our evolution. Our teeth are like living fossils that can be studied and compared to those of our ancestors to teach us how we became human. In Evolution's Bite, noted paleoanthropologist Peter Ungar brings together for the first time cutting-edge advances in understanding human evolution and climate change with new approaches to uncovering dietary clues from fossil teeth to present a remarkable investigation into the ways that teeth—their shape, chemistry, and wear—reveal how we came to be.Ungar describes how a tooth's "foodprints"—distinctive patterns of microscopic wear and tear—provide telltale details about what an animal actually ate in the past. These clues, combined with groundbreaking research in paleoclimatology, demonstrate how a changing climate altered the food options available to our ancestors, what Ungar calls the biospheric buffet. When diets change, species change, and Ungar traces how diet and an unpredictable climate determined who among our ancestors was winnowed out and who survived, as well as why we transitioned from the role of forager to farmer. By sifting through the evidence—and the scars on our teeth—Ungar makes the important case for what might or might not be the most natural diet for humans.Traveling the four corners of the globe and combining scientific breakthroughs with vivid narrative, Evolution's Bite presents a unique dental perspective on our astonishing human development.

CHAPTER 1

How Teeth Work

I took my daughters to the Museum of Discovery in Little Rock when they were young. They couldn’t have been older than five and seven. In a small display cabinet at the back of the museum, we found two skulls lying side by side: a giraffe and a lion. I asked the girls, “Which one is the meat eater?” They both looked at the teeth and pointed to the lion. The giraffe has broad, flat molars for grinding leaves, and the lion has sharp, bladelike ones for slicing meat. They knew this intuitively. The differences between the teeth of a herbivore and a carnivore are obvious. What about subtler differences, say between animals that eat different parts of plants or those that eat different parts of animals? What about our teeth? What kinds of diets are they designed for?

These are important questions to paleontologists, especially to those of us who work with the fossilized remains of early humans. Our job is to document and explain the course of evolution and to reconstruct life in the past. We often have little other than teeth with which to do it. When you think of fossils, you might envision the massive skeleton of a Tyrannosaurus rex or a mammoth dominating a capacious hall at a natural history museum. You might think of the skull of a human ancestor on the cover of a popular science magazine, posed for striking effect and lit to reflect our shadowy distant past. But such fossils are actually few and far between.

There are often hundreds if not thousands of teeth for every skeleton or complete skull we find. That’s because teeth are essentially ready-made fossils. The enamel that coats ours, for example, is 97% mineral, with the rest water and trace amounts of organic material. Teeth are stronger than bones, and they are much more likely to survive the ages. Fortunately for paleontologists, they are also excellent tools for understanding life in the past. If we can reconstruct diet from teeth, for example, we can use them as a bridge to the worlds of our ancestors and other long-gone species. And the more about diet we can wring from fossil teeth, the better the resolution with which we can see those worlds.

But to get there, we need to understand how teeth work. Researchers have been grappling with that task for more than a century, and this chapter chronicles the process. How did the countless variety of mammalian tooth types, including ours, evolve from the modest peg-like structures of most fishes and reptiles? How does this relate to changing tooth function from simple biting to complex chewing actions? How do teeth actually break food? How does change in tooth shape with wear affect the process? One question has led to the next. New finds, new techniques, and new theoretical approaches have come and gone, contributions of the scientists who have stretched the limits of our knowledge built upon the foundation laid by those who came before.

EVOLUTION TO AND FROM THE MAMMALIAN MOLAR

We could begin the story with the discovery of the earliest teeth, which date back nearly half a billion years. Most were simple, pointed structures used to capture and immobilize prey, and to scrape, pry, grasp, and nip all manner of living things. These gave their bearers an advantage in the “evolutionary arms race,” as Richard Dawkins called it. The better an eater was at getting energy and other nutrients, the more babies and the more evolutionary success it had. Teeth spread quickly through the primordial oceans, and the fish lineages that had them eventually sidelined the groups that did not.

But to understand the nuances of how our teeth work, we need to start much later,¹ with the earliest mammals and their immediate predecessors. We need to start with the transition from teeth used mainly for obtaining food to those used for chewing it. Chewing separates food into pieces small enough to swallow, breaks open protective casings that would otherwise pass through the gut undigested, and fragments morsels to expose more surface for digestive enzymes to work on. This gave early mammals the extra fuel they needed to generate their own heat, which meant they could be active at night and live in colder climates or those with more fluctuating temperatures. They could sustain higher levels of activity and travel speeds to cover larger distances, avoid predators, capture prey, and produce and care for offspring.

The ability to chew opened a whole new world of possibilities for the earliest mammals. And today they come in a dizzying variety of shapes and sizes, from the tiny bumblebee bat, at less than a tenth of an ounce, to the behemoth blue whale, at nearly 200 tons. They are found in an incredible variety of habitats, from Arctic tundra to Antarctic pack ice, high-altitude mountaintop to deep ocean water, and desert to rainforest. We are members of a remarkably successful biological class of animals. That success is due, in no small measure, to the ability to heat the body from within, and the chewing that gave our common ancestor the fuel it needed for the task.

Pay attention the next time you eat something. Your jaw, tongue, and teeth act in concert with sensory feedback. The alignment and movements of opposing teeth are precise to 1000ths of an inch as you generate, direct, and dissipate the forces needed to break food. You position and hold objects in your mouth and keep air and food passages separate to prevent choking. All this is tightly coordinated, with the various parts working together in symphony and synergy.

How could such an amazing system evolve? The transition is detailed in a fossil record of mammal-like reptiles and early mammals that spans more than 100 million years. The changes to the jaw and mobility of its joint; reorganization of the muscles that move that jaw; development of a bony palate to separate air and food passages; and differentiation of teeth into incisors, canines, premolars, and molars that fit together precisely enough for chewing are all there. The transformation culminated with a new and very special kind of molar tooth, the one from which ours and those of other living mammals ultimately evolved. It’s with this molar that our story really begins.

The Missing Link between Reptilian and Mammalian Teeth

The basic model for the evolution of mammalian molar teeth was worked out by Edward Drinker Cope, one of the most productive and colorful characters in nineteenth-century paleontology.² Cope was born in 1840 just outside of Philadelphia, the son of a wealthy merchant. His father had expected him to become a gentleman farmer, but Cope was much more interested in natural history. While his formal higher education was limited to a single course in comparative anatomy at the University of Pennsylvania, he racked up countless hours of practical training, cataloging the reptile collection at the Academy of Natural Sciences, with frequent visits to the Smithsonian in Washington, DC, for comparative study. Cope also visited natural history collections around Europe, and called on some of the most important naturalists of the day there. His father, a devout Quaker and pacifist, had sent him to Europe in 1863, in part to keep him from enlisting or being drafted into the American Civil War.

Cope met Othniel Marsh in Germany. Marsh was an American graduate student studying paleontology at the University of Berlin at the time. Their relationship started amicably enough, and both returned after a time to the United States to begin working as paleontologists. Marsh’s wealthy uncle, international merchant and financier George Peabody, founded the Museum of Natural History at Yale at Marsh’s behest. Marsh became its first director. Cope went to teach at Haverford College as professor of zoology,³ though he quit his job there after just a couple of years, evidently to focus on research. In fact, Cope moved his family to Haddonfield, near important dinosaur beds in western New Jersey. But trouble started when Marsh came to visit in 1868 and made a deal with quarrymen there to send whatever new fossils were found to Yale. Cope was incensed. To make matters worse, Marsh soon after called Cope out in public for reconstructing the skeleton of an extinct marine reptile incorrectly. To his embarrassment, Cope had put the head at the end of the tail rather than on top of the neck.

These events touched off the “Great Bone Wars,” perhaps the greatest rivalry in the history of science. Competition between the two men was fierce as each jockeyed for position in the field and worked to destroy the other’s career. A dark cloud hovered over American paleontology in the late nineteenth century, but there was a silver lining. Cope and Marsh were both incredibly productive as each poured his energy and resources into besting the other. Cope alone published more than 1400 scientific works and amassed 13,000 new fossil specimens, despite his untimely death at 57. Most important for us, this all-consuming rivalry led to the discovery of how mammalian teeth evolved.

In 1872 the US Congress authorized a series of expeditions to map parts of the country west of the 100th meridian. Cope joined the survey two years later, ostensibly to chart geology. The Bone Wars were on, and this would be a great opportunity to search for new fossil sites in an area Marsh had not worked. Among Cope’s many finds was a series of layers of lime- and clay-rich rock in badlands near the town of Cuba in northwestern New Mexico. He separated the layers into two formations he named the Puerco and Torrejon and recognized them to be just younger than underlying deposits known to contain dinosaur fossils. These are today combined into the Nacimiento, which formed early in the Paleocene epoch, between about 65 and 58 million years ago,⁴ just after the rock fell on the Yucatán to end the reign of the dinosaurs. Cope didn’t find any fossils in the formation at the time, but the Nacimiento would later become known as, in the words of his junior colleague Henry Fairfield Osborn, “the most unique and important palaeontological discovery of his life.”

Shortly after Cope published his study of the geology from the survey, Othniel Marsh hired a frontiersman named David Baldwin, who had also participated in the mapping expeditions, to collect fossils in the area. Baldwin and his more-or-less faithful burro wandered northwestern New Mexico on and off over the next four years in search of specimens for Marsh.⁵ He was untrained and wrote poorly, but was very good at following rock strata and finding fossils. Baldwin found and sent dozens of boxes of weathered bits of bone and teeth to Yale. But Marsh wasn’t impressed with the scraps and, after a dispute over payment for his efforts, Baldwin quit. He began working for Cope instead, and continued to do so for the next eight years.

While Marsh hadn’t recognized the importance of Baldwin’s finds, Cope did, especially those from the Nacimiento Formation. These were the first Paleocene-age land animals found in the Americas and the very best record anywhere of primitive mammals alive just after the dinosaurs. Nearly everything Baldwin found was new to science, and Cope described more than 100 new species of vertebrates from the Puerco series. They were incredible finds—and Marsh had missed them. These bits of bone and teeth turned out to be more important, at least to those of us who care about fossil mammals, than all the big, beautiful dinosaurs the Bone Wars ever produced. Baldwin had found a link between the humble cone-shaped teeth of reptiles and the myriad forms of molars adorning the mouths of living mammals. Cope had been trying to understand how complex teeth evolved from simple cones for more than a dozen years. Baldwin’s new fossils gave him the key to deciphering the evolution and diversity of mammalian molar types.

1.1. The geological time scale. Dates from the International Commission on Stratigraphy, “International Chronostratigraphic Chart v2015/01,” http://www.stratigraphy.org/ICSchart/ChronostratChart2015-01.pdf. “Myr” refers to millions of years ago, and “Kyr” refers to thousands of years ago.

Cope described the teeth at a meeting of the American Philosophical Society in 1883. Most of the upper molars had triangular-shaped biting surfaces with three main cusps, or tubercles: two on the cheek side and one on the tongue side. He called these teeth tritubercular. A row of upper molars is basically several of these triangles lined up end to end so their bases are continuous and their tips all face the same direction. The cusp at the tip of each tooth is connected by a sharp ridge to the one in front and the one behind, forming a “V” that points in toward the tongue. When you line up the teeth, the crests form a continuous zigzag-shaped blade running the length of the row, kind of like a jagged cookie cutter (e.g., VVV).

The uppers are paired with lowers that have matching triangles facing the opposite way (e.g, ΛΛΛ), with two cusps on the tongue side and one on the cheek side. The lower triangles fit between the uppers, so that opposing blades slide or shear past one another. Baldwin’s Paleocene teeth were amazing slicing and dicing machines that could give the average late-night infomercial food processor a run for its money. But wait, there’s more! The lower teeth also had a low shelf that formed a basin to oppose the inner cusp of the uppers. Food could be crushed and sheared at the same time. The tritubercular tooth was a dazzling feat of engineering, and a link that indeed allowed Cope to connect the chain between the primitive cones of our reptilian ancestors and the specialized teeth of mammals that followed.

Cope speculated that early ancestors of the mammals started out with cone-like teeth, and that small cusps were added in front and back. Over time, nature rotated the new cusps out of line from the original cone, outward for the uppers and inward for the lowers, to form the reversed triangles of opposing rows. A shelf set on the back end of the lowers completed the effect. Cope argued that it was easy to build the teeth of today’s mammals from this basic form. Straighten the crests and take out the shelf, and you’ve got the bladed teeth of cats and dogs. Add a fourth cusp to square off the triangles and raise the shelf, and you’ve got our molars. A few additional tweaks get you to a horse tooth or a cow tooth. Step by step, then, Cope traced the path of evolution from reptilian tooth cones to the ancestral mammalian molar to the myriad forms we have today. His was a brilliant theory, based on intuition, reason, and fossil evidence.

More than half a century later, paleontologist William King Gregory would write, “One can only wonder at Cope’s amazing insight into the problem of the evolution of the dentition. All that has been done since is practically only an amplification and verification of this prophetic passage.”⁶ Imagine trying to assemble a puzzle with no picture to guide you, most of the pieces missing, and no idea of how those you have fit together. This was harder. And Cope got it basically right. His model was soon confirmed by his junior colleague, Henry Fairfield Osborn, then professor of comparative anatomy at Princeton. Osborn studied even older, more primitive teeth of mammals from the Mesozoic era, the age of the dinosaurs. This also allowed him to fill in some of the details Cope’s model was missing. Today we can quibble about a few of the specifics of the Cope-Osborn model, particularly some of those introduced by Osborn, but it remains the foundation upon which our understanding of the origin and evolution of mammalian molar teeth is built.

1.2. The tritubercular, later referred to as the tribosphenic, molar. A. stylized reverse-triangle configuration of upper and lower molar teeth; B. illustration of upper and lower molars, modified from Henry Fairfield Osborn, Evolution of the Mammalian Molar Teeth to and from the Tribosphenic Form (New York: Macmillan, 1907).

THE ROLE OF TEETH IN CHEWING

Neither Cope nor Osborn spent much time worrying about the details of how fossil teeth worked in life, or what sorts of foods they were used on. The early days of paleontology were about digging, describing, and naming fossils. Then came arranging species into groups, figuring out relationships between them, and tracing the evolution of specific anatomical forms, like the tritubercular molar, through time. But identifying that molar type, and working out how it developed from a simple cone and then evolved from there, is integral to the story. Without those details, researchers could not have begun to understand the role of teeth in the evolution of mammalian chewing. Nor could they have figured out why teeth work as they do today.

Fossils as Animals Alive in the Past

George Gaylord Simpson knew this intuitively, even back when he was a graduate student at Yale in the 1920s. His adviser, Richard Swann Lull, was the new director of the Peabody Museum there and a former student of Osborn’s. Like his academic grandfather before him, Simpson chose to work on Mesozoic mammals. There was a great collection assembled by the museum’s first director, none other than Othniel Marsh, during the Great Bone Wars. Many of the teeth and jaws had never been properly cleaned, prepared, or described, and Simpson wanted the job. Lull was initially not happy with the idea, but he acquiesced after several months, and Simpson wrote a series of papers on the specimens, many while he was still in graduate school. It was an interesting twist of fate, Simpson building on Cope’s legacy by working on Marsh’s fossils!

The fourth paper in the series was especially important. Simpson introduced a whole new way of looking at fossils. In his own words, it was an “attempt to consider a very ancient and long extinct group of mammals not as bits of broken bone but as flesh and blood beings.”⁷ The approach seems obvious to us today. Fossils are the bones and teeth of animals that were alive...