Toothsayer

_27A1937.Credit.JeffreyCamden-web

Photo by Jeffrey Camden

Tanya Smith ’97 shows how simple teeth can reveal complex stories about how humans evolved.

By Lonny Lippsett

When Tanya Smith ’97 peered into the time capsules, two small children who lived 250,000 years ago came back to life. They were Neanderthals, an ancient species of now-extinct evolutionary cousins of humans, who had dwelled in what is now southeastern France. One child, Smith could make out, was born in springtime and was nursed by its mother until it was weaned two and a half years later in autumn. The second child also appeared to have weaned before age 3.

The climate back when the children lived was harsher than it is now. Winters were particularly bitter, and both children showed signs that during the cold weather, they may have suffered sicknesses, disease, vitamin deficiencies and something even more pernicious: lead poisoning. How they were exposed to lead remains mysterious, but Smith speculates that in the depths of winter, their families may have sought refuge in a cave that had natural lead deposits, where they drank contaminated water or inhaled contaminated smoke.

All this information Smith extracted from objects so small she could hold them between her fingertips: teeth.

A first molar from the younger child and a second molar from the older one were among 13 Neanderthal teeth and a fragment of skull found in a small cave above the confluence of the Rhône and Payre Rivers. What remained of the Neanderthal people who once inhabited the cave was mostly teeth.

Teeth don’t deteriorate because they are built tough to withstand the wear and tear that comes with their job of ripping and grinding. So they have long been the go-to fossils that biological anthropologists like Smith have used to reconstruct how our ancestors lived and evolved. But when Smith entered Geneseo in 1993, hardly anyone imagined that something as simple and teeny as a tooth could contain such a rich, complex bounty of forensic evidence about the person who once chewed with it.

By the time she graduated, Smith was poised to explore a scientific line of inquiry that didn’t exist when she started college. Ever since, she has been pioneering new techniques to investigate teeth, revealing vast microscopic landscapes sprinkled with clues about the lives of our early human ancestors: their growth, diets, health, behaviors — even the environments they lived in.

Smith is a leader in the field of tooth growth and human evolution. Her career has taken her around the world — from the Max Planck Institute for Evolutionary Anthropology in Germany, to Harvard University and now Griffith University in Australia. In 2018, she published her groundbreaking research on the Neanderthal children, as well as a book for popular audiences called “The Tales Teeth Tell.”

The evolution of a scientist

Smith grew up in the suburbs of Rochester, N.Y., as “one of those kids who would escape into the woods as a form of recreation and solace,” she said. “I was completely captivated by the forest, streams and animals and intrigued by the natural world. I’d catch tadpoles, bring them home and raise them. I collected skeletal remains of animal skulls to understand about life and the things left behind.”

She was inspired by the adventures of two women scientists, Jane Goodall and Dian Fossey, who conducted pioneering studies of chimpanzees and gorillas in Africa — “these strong, independent women who did incredibly brave things: go overseas, take risks and live a life in the field, studying primates in the wild.”

She came to Geneseo eager to follow in their footsteps. But two serendipitous choices bookended her college career and set her life on another trajectory. The first happened when her first-year advisor Bob O’Donnell, a Distinguished Teaching Professor of Biology, suggested she fill in her first-semester schedule with an elective in biological anthropology.

“He said, ‘I think you might like this,’” said Smith. “I didn’t know what anthropology was about. It’s fascinating to me that often one comment can change your life.”

The course was taught by anthropology professor Bob Anemone, who became Smith’s undergraduate advisor, and soon she was out in the wild, studying primates. But not live ones. She spent parts of two summers on expeditions led by Anemone to remote parts of Wyoming, where they collected 60-million-year-old primate fossils.

Microscopic looking glasses and wonderlands

In her final semester, Smith once again needed another elective. “On a whim, I took a microscopy class,” she said. “I didn’t have an idea of what I would be getting into, and again it was a class that changed my life.”

The class was led by Harold Hoops, professor of biology, who guided the class through the painstaking technical procedures of preparing specimens and gave them access to the transmission electron microscope — “unquestionably the most sophisticated machine I’d ever been trusted to operate,” she wrote — in the bowels of the pre-renovated Bailey Hall science building.

It was an epiphany.

The microscope unveiled a spectacular cellular landscape of membranes, nuclei, mitochondria and organelles.

“I fell in love with the experience of being in another world,” she said. “When you’re immersed in uncovering a landscape that you can’t see without these tools, it’s like going into the woods. It’s magical, ethereal,otherworldly.”

To complete Hoops’ course, students had to do an independent project. Anemone tossed her a suggestion: Why not investigate what the electron microscope could reveal inside one of the fossil primate teeth they collected in Wyoming?

Since the 1930s, scientists had known that as teeth grow, they add microscopic layers of enamel in a regular rhythm that reflects and records whatever is going on in the body — much the way tree rings form within trees. Several decades later, anthropologists speculated that they could extract useful information about ancient species’ growth — if they could find these layers in fossil teeth.

Roots and rhythms

Smith didn’t really know what she was looking for. But after several long hard days in the lab, “I had an incredible moment of discovery, something literally coming into focus,” she said. The growth lines appeared, stretching out in a series of wavy, closely layered bands like contour lines on a topographical map.

Smith went to SUNY Stony Brook for graduate school, where — despite her forays into teeth and microscopy — she was still set to pursue Ph.D. research on lowland gorillas.But in her first semester, another twist of fate occurred: anthropology professor Lawrence Martin, who specialized in ape evolution and was intrigued with this nascent line of inquiry about growth lines, needed a research assistant to help prepare and analyze samples.

“Here was somebody at Stony Brook doing work on something I was working on a year before that I didn’t think anyone actually did as a job,” Smith said. “The field was still in its early days. There were a lot of debates about whether there really were true timelines in teeth and how accurate they were, and people were pushing back and saying, ‘No, they don’t represent real biological rhythms.’”

Analyzing countless specimens of fossil and modern primate teeth, Smith demonstrated the techniques to identify growth lines. Her Ph.D. dissertation was a breakthrough, convincing skeptical scientists once and for all that the lines were laid down to the steady rhythm of the body’s daily hormonal and metabolic cycles. Stressful events that disrupt those rhythms — illnesses, diseases, traumas, malnutrition, or weaning suddenly from breast milk to solid food — can disrupt the daily secretions of enamel and cause visible disruptions in growth lines.

Being born is a stressful event for babies, and since teeth begin calcifying in utero, the growth lines on primates’ birth days stood out like neon signs. That started the calendar allowing scientists to count growth lines to determine ages when other events, natural or unnatural, occurred in their specimens’ lifetimes.

“Every day of our childhoods is fossilized in our teeth,” Smith said.

Dental detective work

On a macro level, teeth have long provided clues to human evolution. In “The Tales Teeth Tell,” Smith outlines some of this history. Pointy canine teeth like those in chimpanzees and gorillas are smaller and less daggerlike in the fossils of our human ancestors (hominins), because as they became bipedal, they used arms, hands and weapons to fight instead of teeth. Hominins also developed what Smith describes as “an oral Swiss Army knife” of incisors to bite fruit, canines and premolars to pierce and tear meats and plants, and molars to grind hard nuts and seeds — indicating omnivorous evolutionary developments in their diet. That gave hominins a wide array of easily digestible food to fuel the high energy needs of bigger brains.

Hominin fossil teeth and jaws have also shrunken over evolutionary history. Anthropologists theorize that the invention of fire, tools, agriculture and more recently industrialization all led to softer foods that didn’t require strong, large jaws — leaving more room perhaps for bigger skull cases for larger brains. The fossil record shows that people today are five times more likely to have impacted wisdom teeth than our ancestors because smaller jaws leave less room for them.

Fossils also reveal another modern malady: cavities. Fossilized tartar (the stuff your dental hygienist works hard to scrape away) often preserves traces of microbial communities in mouths. Scientists have found that cavities were rare in hunter-gatherers, but a certain previously undetected species of bacteria, Streptococcus mutans, suddenly began to bloom in humans’ mouths when agriculture began and people ate more carbohydrates. S. mutans also love to eat carbohydrates. A byproduct of their metabolism is lactic acid, which dissolves enamel and creates cavities. The trend has accelerated since the Industrial Revolution and the accompanying glut of processed foods such as sugar.

But seeing into teeth on a microscopic level, as Smith demonstrated, opened up entirely new vistas.

“In science, a discovery or a new set of tools comes along, and you can jump the curve and pursue something you couldn’t before,” she said. Another curve-jump occurred when Martin introduced her to Ph.D. student Paul Tafforeau, who was experimenting at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.

As big as a stadium, the ESRF has powerful magnets that bend electrons accelerating at nearly the speed of light around a racetrack-shaped tunnel 844 meters in circumference. It creates high energy beams that Tafforeau discovered could discern microscopic growth. The ESRF allowed Smith and Tafforeau to detect daily growth lines without cutting open the teeth to prepare samples, which meant museums would provide them with samples of rare hominin tooth fossils.

In 2005, they used the synchrotron to examine a chip of enamel from a fossil hominin jaw found in North Africa that was later determined to be 300,000 years old. The growth lines showed that the hominin had not matured as fast as apes, but had a long, slow childhood.

“It grew up like we did,” Smith said. “Everything that preceded it was different than humans today. It turned out to be the oldest known Homo sapiens individual. It was exciting to have access to the inner lives of ancient hominins.”

Chemical cues

After earning her Ph.D., Smith went to Leipzig, Germany, to work at the Max Planck Institute for Evolutionary Anthropology, and in 2008, she joined the faculty of Harvard. In 2016, she became a professor at Griffith University in Brisbane, Australia — coaxed by the opportunity to have ready access to another curve-jumping instrument, a sensitive high-resolution ion microprobe, or SHRIMP, which can measure minute amounts of elemental isotopes.

Different elements are woven into dental growth lines, each illuminating a thread in a person’s story. In the Neanderthal youngsters’ teeth, spikes in lead concentrations revealed their exposure to environmental contamination. Measurements of heavier and lighter isotopes of oxygen showed when it happened — only in winter, because warmer weather and climates lead to more evaporation, which leaves behind more O18 isotopes in the water. When Neanderthals began to suckle, tiny amounts of barium from their mothers’ skeletal reserves began to infuse their growth lines and continued until they weaned.

And there are even more elements to plumb. Different types of plants have different ratios of carbon isotopes, giving scientists clues to which plants hominins ate. By comparing strontium isotopes in rocks in different areas and in fossil teeth, scientists can reconstruct where hominins migrated during their lifetimes.

“I find myself getting lost in the intimacy of the information I’m gathering,” Smith said. “Just to look at the first few years of somebody’s life who lived tens of thousands of years ago — there’s a sense of real gravitas. It’s a privilege to bring their stories to life, to help illustrate what it was like for them growing up.”

Smith is particularly excited to shed light on a key step in human evolution: nursing behavior.

“It’s a hotly debated question,” she said. “When did we go from an apelike to a more humanlike strategy, which has allowed our populations to swell? We don’t nurse our children for five to eight years like chimps or orangutans. Because human mothers don’t invest that long in each child, we can reproduce more rapidly and recoup from losses more easily. That’s one of the reasons there are billions of us, and not so many apes.

“And changing environments plays a key role,” she continued. “If you are in a resource limited environment, you only have children at the best times when you have sufficient food, and you’re going to wean them when there are adequate resources, so they’ll be all right on their own.

“Twenty years into this, I’m still finding neat and innovative ways to better understand what life was like in the past. I’m still learning, still encountering things I’ve never seen before.”

Author: geneseoscene

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