Peter Ungar on Evolution’s Bite

UngarWe 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 recently took the time to answer some questions about his new book.

Why do paleontologists care so much about teeth? What makes them so special?

PSU: Paleontologists care about teeth because oftentimes, that’s all we’ve got of extinct species to work out details of life in the past. Teeth are essentially ‘ready-made fossils,’ about 96% mineral, so they survive the ages much better than other parts of the body. They are special because they come into direct contact with food, and can provide a bridge to understanding diet in the past. We can tease out the details by studying their size, shape, structure, wear, and chemistry. Teeth connect us to our ancestors, and them to their worlds. I like to think of nature as a giant buffet of sorts. I imagine animals bellying up to the sneeze guard on this biospheric buffet with empty plate in hand. Teeth can teach us about the choices they make; and it’s those choices that help define a species’ place in nature. As the old adage goes, you are what you eat. Teeth are important because they can help us understand relationships between animals in the past and the worlds around them, and about their—and our—evolution.

Why do we have so many problems with our teeth today? Why do we get cavities, require braces, and have impacted wisdom teeth?

PSU: Think about how extraordinary your teeth are. They have to break food, without being broken themselves, up to millions of times over your lifetime. And they have to do it built from the very same raw materials as the foods you are eating. Nature is truly an inspired engineer, and it’s remarkable they last as long and function as well as they do. But they’re not perfect. Most of us today get cavities, and many of us have crooked front teeth, and impacted wisdom teeth. This is largely because of our diets. We eat mostly soft foods, loaded with highly-processed carbohydrates, especially refined sugars. Cavities form by erosion from acids produced by plaque bacteria. Feeding those bacteria diets high in carbohydrates, especially sugars, means more cavities. Also, when we eat soft foods as children, we don’t exercise our jaws enough to stimulate the growth they need to make room for all our teeth. The result is crowded lower incisors, uppers that jut out over the lowers in the front of the mouth, and impacted third molars in the back. It’s not that our teeth are too big for our jaws, it’s that our jaws don’t grow long enough to accommodate all our teeth. Most traditional foragers that eat tougher or harder foods have longer jaws, and so don’t suffer the sorts of orthodontic problems the rest of us have.

Do other species have these problems? If not, why are we so different?

PSU: I’ve seen cavities and evidence for gum disease in some non-human primates, particularly in species that eat a lot of fleshy, sugary fruit, but they’re much rarer than in us. There are very few early human fossils that provide evidence of dental disease in our distant past either. Again, it seems to be a mismatch between our diets today, and the foods that we evolved to eat. Our teeth are not designed for hamburgers and French fries, nor to be bathed in milkshake. If you want to see evidence of that mismatch, just smile and look in a mirror.

What was your motivation for writing a popular science book?

PSU: My PhD dissertation was 654 pages, mostly focused on a quarter of a square millimeter of the surface of some incisor teeth. Most academics are so narrow in their research focus that it can be difficult to see the forest for the trees. I wrote this book to give myself the big picture, to give me an appreciation of the larger context into which my own work fits. Also, no more than half a dozen people actually read my dissertation cover to cover, and that includes my mother. Academics often feel like they’re speaking, but no one is listening. I wanted to reach a larger audience. This book at first glance seems to be about teeth – but it’s really about the biospheric buffet, and how environmental change over deep time swapped out items and choices available to our distant ancestors. The take-home message is that large-scale climate swings winnowed out the pickier eaters among us, and drove our evolution. Teeth are our window through which to see it. The most important message here is that climate changes, and species have to change to accommodate or die. That’s why we’re here. It’s a timely, important lesson.

As a scientist who has spent the last three decades studying evidence for the evolution of human diet, what do you think of today’s “Paleolithic diet” trend? And what was the ancestral human diet, anyway?

PSU: I’m not a fan. I like pizza and bagels too much. Still, there’s little doubt that our ancestors did not eat such things; so it makes sense that a discordance between the foods we evolved to consume and what we fuel ourselves with today can wreak havoc on our bodies. Try putting diesel in a car built to run on regular gasoline (actually, don’t). And people do lose weight when they cut refined carbohydrates and processed sugars from their diets. We could well benefit from eating more like our Stone Age ancestors, with menus like those in some popular diet books—you know, spinach salads with avocado, walnuts, diced turkey and the like. I am not a nutritionist, and cannot speak with authority about the nutritional costs of benefits of Paleolithic diets—but I can address their evolutionary underpinnings. Think about it this way. Any diet that drains the body of fat reserves means not meeting daily caloric needs. It is difficult to believe that nature would select for us to eat only foods that don’t provide the nutrients required to maintain the body. In fact, the whole idea of the Paleolithic diet is problematic. Even if we could (and we can’t) reconstruct the glycemic load, fatty acid, macro- and micronutrient composition, acid/base balance, sodium/potassium ratio, and fiber content of foods eaten at a moment in time in the past, the information would be meaningless for planning a menu. All these nutrients varied with food availability over space and time, as items on the biospheric buffet table were swapped in and out, so focusing on a single point in our evolution is futile. We’ve been a work in progress for millions of years. What was the ancestral human diet? The question itself makes no sense.

Peter S. Ungar is Distinguished Professor and director of the Environmental Dynamics Program at the University of Arkansas. He is the author of Teeth: A Very Short IntroductionMammal Teeth: Origin, Evolution, and Diversity and Evolution’s Bite: A Story of Teeth, Diet, and Human Origins.

Anurag Agrawal: Monarchs vs. Milkweed

by Anurag Agrawal

Coevolution is a special kind of evolution. And monarchs and milkweeds exemplify this special process. In particular, what makes coevolution special is reciprocity. In other words, coevolution is one species that evolves in response to the other, and the other species evolves in response to the first. Thus, it is a back-and-forth that has the potential to spiral out of control. In some arms races, the two organisms both benefit, such as that between some pollinators and flowering plants. But coevolution is more common among antagonists, like predators and their prey.

When biologists first described coevolution, they likened it to an arms race. An arms race, such as that between political entities, occurs when two nations reciprocally increase their armament in response to each other. So how does an arms race between monarchs and milkweeds, or between cats and mice, or between lions and wildebeest, or between plants and their pathogenic fungi, proceed? When coevolution occurs, it proceeds with “defense” and “counter defense.” And one of the few rules of coevolution is that for every defense that a plant or prey mounts, the predator mounts a counter defense, or an exploitative strategy to overcome the defense.

Once a monarch butterfly lays an egg on a milkweed plant, the natural history of coevolution unfolds. For every defense that the plant mounts, milkweed mounts a counter defense. Once the caterpillar hatches, it must contend with a bed of dense hairs that are a barrier to consumption of the leaf. But monarchs are patient, and have coevolved with the milkweed. So their first strategy is to shave that bed of hairs such that the caterpillar has access to the leaves that lie beneath.


For every defense there’s a counter defense. But next, when the monarch caterpillar sinks its mandibles into the milkweed leaf, it encounters a sticky, poisonous liquid called latex. In this video we will see how the monarch caterpillar deactivates the latex bomb that the milkweed puts forward.

And so the arms race continues, with reciprocal natural selection resulting in coevolution between monarchs and milkweeds. In my book, Monarchs and Milkweed, I outline the third level of defense and counter defense between these two enemies. Milkweed next mounts a remarkable and highly toxic defense chemical called a cardiac glycoside. But, yes, again the Monarch has evolved the means to not only not be poisoned by the cardiac glycoside, but to sequester it away and put it to work in defense of the Monarch itself from its enemies, such as predatory birds. For more on the Monarch – Milkweed arms race see this video, filmed in Ithaca, New York outside of Cornell University where we conduct our research.

AgrawalAnurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He is the author of Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.

Anurag Agrawal: The oldest butterflies?

by Anurag Agrawal

It’s unclear when humans became humans. Presumably it was a gradual growth of our consciousness over the eons. There are some things, however, that appear to distinguish us from most other animals. For example, our artistic depictions. From the deepest, darkest caves have emerged pictures of humanity from thousands of years ago. And in an Egyptian tomb, that of Nebamun, on a painting called “Fowling in the marshes” (from around 1350 BCE) comes one of the oldest human depictions of butterflies. It happens to be of the African Monarch, Danaus chrysippus, sometimes called the plain tiger, a close relative of our beloved North American Monarch butterfly, Danaus plexippus.

I stumbled on this lovely scrap of history when a friend and colleague, Harry Greene, gifted me a book: Nabokov’s Butterflies (2000), a collection of unpublished and uncollected writings. Some explanation is in order. Harry is an extraordinary naturalist and big thinker in ecology and evolution. Like many senior scholars, his predicament was the lack of shelf-space in his office. And so I was the beneficiary of Nabokov’s Butterflies. Vladimir Nabokov, a Russian-American author, and noted entomologist, was most famous for his writings, for example, Lolita, and his celebrated translation of Pushkin’s novel in verse, Eugene Onegin. His ideas about biology were diverse, he was a passionate lepidopterist, and he often intermixed his literary writing and entomological excursions. Lolita is said to have been written primarily on butterfly collecting trips in the American west. Nonetheless, Nabokov also clung on to other ideas that held little merit in the scientific sphere. Most prominently, Nabokov rejected evolution by natural selection as a driver of certain organismal traits that he deemed ‘coincidental, miraculous, or too luxurious.’


Nabokov was a professor at my own Cornell University in the decade following WWII. Although he taught literature and had well-known students at Cornell (including U.S. supreme court justice, Ruth Bader Ginsburg), his entomological interests continued. In fact, after he retired from Cornell in the mid-1960s, Nabokov had sketched out an outline of a book: The Butterflies of Europe. And although the book never came to be, the outline was recapitulated in Nabokov’s Butterflies. Flipping through the book, I stumbled on his entry for Danaus in which he wrote, “This butterfly has the distinction of being the oldest known to have been represented by man. Seven specimens of it (with typical white-dotted Danaus body but somewhat Vanessa cardui like wingtips) are shown flitting over the papyrus swamp…” (page 603).

I later asked another friend, Harvard’s Lepidopterist, Naomi Pierce: did Nabokov have it right? On the money, she independently pointed to the similarity of Danaus chrysippus and the painted lady, Vanessa cardui, wondering if the butterflies on this three thousand year old tomb painting were Danaus or Vanessa. She concluded, as did Nabokov, that the African Monarch ruled. Detailed assessment of the color patterns on the wings were informative to both entomologists. The oldest human depiction of a butterfly? Perhaps not. Naomi mentioned some evidence of butterflies in Minoan artifacts from Crete, a thousand years earlier than Nebamun, and likely in Pyrenees cave paintings, some 10-30 thousand years earlier!

Of course, there is nothing special about being the oldest depiction of a butterfly by Homo sapiens. But suffice it to say, butterflies, metamorphosis, wing patterning, and the beauty of nature have been on our minds for a very long time. Thanks Harry and Naomi! And thanks Nabokov. Who knows what becomes of those side hobbies and obsessions we all hold.


AgrawalAnurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He is the author of Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.

Anurag Agrawal: The migration patterns of the monarch butterfly

by Anurag Agrawal

The plight of monarch butterflies if often in the news: many scientists around the world are working hard to understand their annual migratory cycle. How do the monarchs produced during summer in the northern reaches of America contribute to the overwintering population in Mexico? The origin of monarch butterflies that make it to Mexico has been hotly debated because it has profound consequences for how we approach monarch conservation.

A new study is remarkable in its use of historical collections over the past 40 years and modern isotopic analysis. The scientists address the most important regions in the U.S. for producing monarch butterflies that actually make it to Mexico. This sort of data has been very difficult to come by and there has been a lot of speculation. As outlined in my new book from Princeton, the midwest has dominated discussions as being the most important region in the U.S. for monarchs. In the study, the authors find that the Midwest contributes a whopping 38% of the butterflies that make it to Mexico.


The regions studied by Flockhart et al. separated to highlight their relative areas

I would add two points for discussion. The first is that the areas of land that the authors designated as Midwest, Northeast, etc., seemed totally reasonable, but also somewhat arbitrary. In particular, an issue arises when you consider that, as designated in the paper, the Midwest is about 2.5 times as big as the Northeast. It is therefore not surprising that the Midwest produces about 2.5 times as many butterflies that make it to Mexico (38% vs 15%). In other words, the butterflies that make it to Mexico have about an equal probability of coming from the Midwest and the Northeast when land area is considered. Yet another way to think about this is that two states that are about equal sizes in the two regions (for example, Indiana and Maine) will on average produce about the same number of butterflies that make it to Mexico.


The annual migratory cycle of the monarch butterfly from Monarchs and Milkweed. In my past research, we have opted for a three simple regions defined by the butterfly generations.

Quite interestingly, the North Central area (including my home in the Finger Lakes region of NY) is slightly more important for butterfly production given its size. When you factor out the area of the Great Lakes (where there are no monarch caterpillars), the area of North Central is small (36% of the size of the Midwest). Thus, about 20% more butterflies per square mile come out of the North Central than the Midwest or Northeast. Where does this leave us?  The agricultural Midwest is certainly important, but perhaps not as important as previously thought.

The other point worth thinking about is that the Southwest (read: Texas) comes out as big in terms of area (equal to the Midwest) and relatively less important in terms of contributing butterflies (11% of the total).  The critical importance of the Gulf States including Texas, however, is not in the last generation of butterflies produced in fall that migrate south, but rather in the first generation of butterflies that are produced in spring and that migrate north to the Midwest and Northeast.  In other words, the Gulf States are absolutely critical for the annual migratory cycle, even if that is not where fall migrants are produced.  Without a spring generation there, the Midwest and Northeast would be empty!  In chapter 9 of the book, I summarize the critical importance of Gulf States not only for the spring, but also in providing floral resources for fall migrating butterflies.

I hope we see more studies like this in the future, as it provides new important information and was inspiring to read.

AgrawalAnurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He is the author of Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.

Anurag Agrawal: Monarch overwintering

by Anurag Agrawal

The estimates of the monarch butterfly overwintering population were announced February 9th by WWF Mexico. The butterflies are so dense at their dozen or so mountain-top clustering sites that overwintering butterflies cannot be individually counted. Instead, the area of forest that is densely coated with butterflies (at about 5,000 butterflies per square meter looking up into the canopy) is estimated as a measure of monarch abundance. Butterflies arrive in Mexico around early November and stay until March.


This winter season (2016-2017), there were approximately 2.9 hectares of forest occupied with dense monarchs (somewhere in the neighborhood of 300,000 million overwintering butterflies). This estimate is down 27% compared to last year. Nonetheless, the previous two years were a 600% increase over the all-time low recorded in the winter of 2013-2014.


Where does this leave us? This year’s population was higher than predicted by many. The season started with a late spring storm that killed an estimated 5-10% of monarchs in March 2016, and many reported low numbers of adults last summer. Nonetheless, the lower numbers this season compared to last are within the range of year-to-year variation, and overall, the population seems to be relatively stable over the past decade. With these 24 years of data, there are various ways to plot and assess the trends. Below I have plotted the four year averages for six periods beginning in 1992. Any way you slice it, the trend has been negative, and the population is not nearly what it once was. Nonetheless, the downward trend seems to have lessened this last period. Is this the new norm? How dangerously low are these numbers? And what can we do to reverse the trend?


AgrawalAnurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He is the author of Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.