Introduction

The next time you walk outside, take a good look around you. You might see some animals, you might not. If you're like most people living in the developed world, you probably won't see more animals than you can count on your fingers.

Next, count the plants. Too many to count? You're not alone. Unless you live in a sandy desert (or a snowy desert such as Antarctica) you'll see an abundance of plants.

Plants first colonized land over 400 million years ago, though those earliest pioneers were wispy, weird little things. We don't often think about it, but plants evolved, too, including goodies like seeds, flowers and wood. But long before our own ancestors started walking upright, plants dominated land, and flowering plants in particular remade the world.

Because they make their own food, and because they can't get up and walk away when things go bad, plants are a direct reflection of the environment, including temperature, precipitation and seasonality.

Fossils aren't always petrified bones or shells. Leaves, flowers, seeds and wood can fossilize too. In fact, plant fossils — like living plants — are far more abundant than animals. As a result, they provide copious information about past environments.

Plants can even give us hints about our own future.

As a groupie of both Charles Darwin and his grandpa Erasmus Darwin (poet, evolutionist and plant enthusiast), I decided to learn more about plants in 2017.

To learn more about what plants say about our past and future, I contacted Scott Wing, Curator of Fossil Plants at the Smithsonian Institution. As one who studies fossil plants, he's a paleobotanist.

Back in 2000, I had traveled to the Smithsonian with some fellow volunteers from the Denver Museum of Nature and Science. Under Dr. Wing's guidance, we prepared fossils and organized specimens in the National Museum of Natural History's cleared leaf collection (more on that later). Wing is one of the nicest people I know, and when I asked for his help 17 years later, he said yes.

Washington, DC, has the Smithsonian Natural History Museum, the Smithsonian Zoo, and a Dean and Deluca's fancy food store in Georgetown. So off I went, telling myself I was somehow needed in Washington.

A Visit with Scott Wing (September 12)

After greeting me in the museum lobby, Wing led me through a maze of dusty, box-filled back hallways to the Smithsonian's Paleobiology Department. Museum storage segued seamlessly into his office, where worn sofas surround an old paleobotany exhibit case converted into a coffee table.

Scott Wing and his leaf-exhibit coffee table
Scott Wing smiles next to his leaf-exhibit coffee table in the Paleobiology Department at the Smithsonian National Museum of Natural History.

Before vising in person in September 2017, I watched a talk Wing gave at the World Economic Forum where he showed a group photo from his first field expedition. I'm always curious about scientists' beginnings, so I asked how he wound up on that first trip. In fact, Wing took a circuitous path to paleobotany.

Wing grew up liking the smiling green dinosaurs at Sinclair gas stations. (Many of us did.) In high school, he had some free time, and signed up for a college-level course at Duke University. The subject was human evolution, and the professor was recently transplanted from Yale. Wing did well in the class, and his professor suggested he try some paleontology fieldwork, writing a cover letter of introduction. Even with that cover letter, though, Wing didn't have much luck; few paleontologists wanted an inexperienced 17-year-old digger tagging along. But Elwyn Simons said yes, and Wing spent several weeks digging Wyoming fossils in 1972 on Simons's team. Wing was hooked.

Scott Wing at the World Economic Forum
Wing stands next to his 17-year-old self in his talk at the World Economic Forum. Screen capture from YouTube.

"I got interested in Mesozoic and Cenozoic plants because I was interested in climate change around that time. I got interested in climate change because I was interested in mammals. I wondered why they were evolving and changing shape," Wing told me. "A good way to figure out climate is to study plants."

Since fossil plants span hundreds of millions of years, a paleobotanist can choose from a variety of topics and times in Earth's history. For instance, a rich field of research sprouted up years ago around "coal balls": clumps of fossil plant matter preserving exquisite internal detail. Coal balls date from the Mississippian and Pennsylvanian Periods, between roughly 360 million and 290 million years ago. The plants Wing studies are much younger, from the late Cretaceous Period and early Eocene Epoch, roughly 70 million to 50 million years ago.

Specifically, Wing studies the radiation — rapid spread and diversification — of flowering plants, or angiosperms.

Seed-bearing plants fall into two groups. Angiosperms enclose their seeds in plant versions of ovaries, frequently fruits, while gymnosperm seeds go naked, often nestling in cones. If you're thinking pinecones, you're right. Gymnosperms include pine trees, and have a distinguished pedigree extending past the Age of Reptiles, well into the preceding Age of Fishes.

Angiosperms came later, but they changed the world like nothing else. "In terms of making the world we live in, angiosperms are the biggest deal there is," Wing explained. "The Arctic and the high mountains are dominated by pine trees. Everywhere else you have plants you have mostly flowering plants."

It wasn't that gymnosperms were subpar or poorly adapted. It's just that angiosperms had better reproductive success, including faster growth and generation rates. Angiosperms were better at coaxing pollinators such as honeybees to assist with pollen delivery to other angiosperms of the same species. Angiosperms also lured animals, including ourselves, with fruit, tricking us into spreading their seeds when we answer nature's call. If you want something to poop your potential progeny, you don't want that to happen too far from the tree, which is why pitted fruits are prized (or reviled) for their laxative qualities.

Angiosperm examples
Angiosperms not only encompass ornamental plants, but also cacti and grass.

Angiosperms are flowering plants, but when it comes to identifying fossils, flowers are all too scarce, and when they do occur, they are rarely attached to the far more common fossils: leaves. Given what's rare and what's abundant, paleobotanists have worked for years to refine the techniques of identifying fossil leaves. In the 1990s, Wing played a part in a pivotal project.

Classifying fossil leaves means applying the appropriate categories, but while descriptive terms have been around a long time, their use hasn't always been consistent. Spearheaded by Denver Museum of Nature and Science volunteer Beth Ellis, a 1990s effort involved Wing and several other researchers. The result was the Manual of Leaf Architecture, which standardized terms, and walked the reader through the steps to classify fossil leaves, initially by its toothy or smooth margin, and eventually by the patterns of veins inside the leaf. This is where the Smithsonian's cleared leaf collection is so valuable.

Cleared leaves at the Smithsonian are modern leaves that have been chemically treated and dyed red to give their venation patterns prominence. Many of the images in the Manual of Leaf Architecture are from the cleared leaf collection.

The Manual of Leaf Architecture offers two advantages over the old leaf-classification system. One it that it uses unambiguous terminology that's been agreed upon by researchers in the field. Another is that it's largely a standalone tool. "You could use the old terminology," Wing explained, "but it was something of a cottage industry. People had to apprentice with others to learn how to use it. Now you can hand someone the Manual of Leaf Architecture and he or she can more or less figure it out." Initially released in 1999, the manual was republished a decade later.

Venation patterns from the MLA
Venation patterns can help paleobotanists identify fossil plants. From the Manual of Leaf Architecture by Ash, Ellis, Hickey, Johnson, Wilf and Wing.

The next step in this process might be using computers to identify venation patterns in angiosperm leaves. In 2016, Wing coauthored a paper with Peter Wilf of Pennsylvania State University titled, "Computer Vision Cracks the Leaf Code."

Computer vision codebook
The computer's learned codebook. From "Computer Vision Cracks the Leaf Code" by Wilf, Zhang, Chikkerur, Little, Wing and Serre in PNAS.

Wing is encouraged by the success of computer vision, but he also has some reservations. "Computer vision has had remarkable success at classifying, given about 20 families. On average, the computer does about 13 times better than chance. If you give the computer training images and tell it whether it's right or wrong, it can learn to categorize leaves into higher families with a lot of success," he said. "But it's very hard to back out exactly what the computer is doing. If you wanted to reverse engineer what the computer does, it's not easy. I'm not sure what features it's using to be so successful."

Since they are literally rooted in place, plants must adapt to their surroundings. Leaf features can therefore tell scientists quite a bit about the environment. Features such as drip tips — long, narrow runners off the ends of leaves — serve to drain away excess water, preventing fungi from taking hold on the leaves and hampering photosynthesis. So drip tips on fossil leaves are a good clue to wet environments.

One of the strongest indicators of whether plants lived in temperate or tropical zones are the leaf margins. Smooth, or entire, margins are more common in tropical environments; toothed margins are more common in temperate environments.

A deciduous plant loses an entire set of leaves each autumn, and must sprout a replacement set each spring. In contrast, a leaf on a tropical plant might persist for years. Because temperate trees must rapidly grow a whole new set of leaves each year, there might be something easier about making toothed leaves. It made sense to me, but I had heard the argument that this reasoning is a bit circular, so I asked Wing.

"Toothed margins have a couple functional advantages," he explained. "Teeth are 'outies' — they stick out. It turns out, that tissue matures faster." He told me that studies of modern leaves indicate that the teeth might begin to photosynthesize sooner each spring. "If you have a short growing season, then the plant gets a payback on its investment. It can use that part of the leaf faster, when the leaves are just starting to emerge."

Teeth are also a site of rapid water-vapor loss. "This can be good or bad," Wing said. "It can be good to rev up the exchange of gases and get carbon dioxide into the leaf, so teeth might allow the leaf to photosynthesize sooner, but the downside is that they can make the plant vulnerable to drought."

Wing went on to explain that, in multiple families of plant species, there can be a mixture of entire or toothed margins, with tendencies in one direction of another. "But teeth are not free to come and go. It's not super-flexible."

Looking at what fossil plants and other ancient life forms say about the organisms' environment, paleontologists have used fossils to assemble pictures of ancient climates. The results have often been surprising, to the paleontologists themselves, and to climate scientists.

I didn't realize it when I helped sort cleared-leaf specimens for him in 2000, but Wing's interests had started to focus intently on climate a decade earlier. It had to do with a study by L. Cirbus Sloan and Eric Barron, "'Equable' Climates during Earth History?" The study modeled climates of the Cretaceous Period, which ended 65 million years ago, and the Eocene Epoch, which lasted from about 55 million to 34 million years ago. The model predicted wintertime freezing conditions across large stretches of North America and Eurasia, sort of like today.

Wing knew the model had to be wrong.

Today we live in an interglacial — a relatively warm period in between ice ages. Though the Earth is warmer than it was when the Laurentide Ice Sheet sprawled to its maximum extent over North America some 20,000 years ago, we're still relatively cool compared to many periods in Earth's more ancient past. About 55 million years ago, there were no ice sheets. Instead, crocodiles ambled and palm trees grew above the Arctic Circle. We know this because paleontologists have found the fossils. The Eocene Epoch wasn't just warm, it was one of the warmest periods in Earth history, at least for the vast stretches of geologic time that left evidence. The transition from the Paleocene Epoch to the Eocene Epoch, around 55 million years ago, is known to paleontologists and geologists as the Paleocene-Eocene Thermal Maximum (PETM).

Temperatures since the Mesozoic
Multiple lines of evidence indicate a spike in global temperatures around the time of the Paleocene-Eocene Thermal Maximum. Graph from NOAA Climate.gov.

In 1972, the 17-year-old Wing knew that fossils could be a lot of fun. By 1990, he had found that fossils could illuminate ancient climate like few other tools we have. He wrote a refutation of the Sloan and Barron study. That set the record straight, but it also set him up for an awkward moment years later.

The L in L. Cirbus Sloan stood for Lisa, and when Scott Wing met her afterwards, he learned that the modeler he had set straight was just a grad student at the time. To be a grad student facing a refutation from an established researcher can be pretty rough. Sloan confessed to Wing that she set up a dartboard and threw darts at his refutation paper. It was an inauspicious start to a friendship, but Wing and Sloan did become friends, and taught a course together at UC Santa Cruz.

Sloan no doubt learned from Wing's refutation, but Wing also learned from Sloane. "In 1990, I didn't realize how important models were. If you're interested in climate, you have to care about how it changes," Wing told me in September 2017. "If you don't look at that, you're not really doing science."

Since his interaction with Sloan, Wing has coauthored a plethora of papers about ancient and current climate. He has spent years trying to better understand this PETM. Evidence so far uncovered points to a spike in global temperatures prompted by a release of greenhouse gases (carbon dioxide, methane or both) but the exact source and location of that release, and just how long it lasted, remain areas of active research and debate.

Some researchers have argued that climate change at the PETM was instantaneous (in geologic terms, that means happening in a matter of years), but Wing disagrees. "Imagine you added red dye to the atmosphere," he said. "The atmosphere would turn pink pretty quickly, but there would be a delayed effect on the oceans. The mixing of the deep ocean would take longer — centuries to millennia to complete."

Fossil palm
Sub-tropical conditions dominated much of the continental United States round the time of the Paleocene-Eocene Thermal Maximum, and palm trees thrived in what is now Fossil Butte National Monument, Wyoming. Image from U.S. National Park Service.

The red dye in Wing's analogy is the isotope 12C. In the process of photosynthesis, plants pull carbon from the atmosphere. They can choose from 12C and 13C, and 12C is generally easier for plants to use. By comparing ancient ratios of 12 and 13C, and seeing how they changed over time, scientists can develop at least a fuzzy picture of ancient plant activity and carbon exchanges.

Wing explained that, if the greenhouse gas release had been instantaneous, the isotopic composition would have taken a few decades in surface ocean, and a few millennia in the deep ocean. Likewise, the surface ocean's isotopic composition would have changed much more quickly than its temperature, due to the ocean's thermal inertia. The rock record shows neither of these things. Instead, the evidence suggests that the carbon release at the onset of the PETM took several millennia.

By understanding how big the Carbon Isotopic Excursion (CIE) was around the time the PETM started, it might be possible to identify the source of the carbon. Sediments from Wyoming's Bighorn Basin has been proven to be an especially useful site in Wing's studies.

"Was the greenhouse gas release at the PETM caused by carbon dioxide, methane, or both? Buried in that is another question: Did the carbon all come from the same place? Or were there vicious cycles causing the release of carbon from one reservoir after another?" When we talked, Wing was of the opinion that at least some of the PETM's carbon dioxide was from volcanoes, and at least some was from organic carbon. He returned to the red dye analogy. "There are two things to ask: How big was the package? And How red was the dye?" In other words, there might have been a big release of weak red dye, or a smaller release of strong red dye. Both things could look similar tens of millions of years later. One way to discern the difference is to look at curves of carbon isotopes in ancient sediments. A single, big pulse would produce a spike followed by a rapid decline followed by a slower decline. Multiple pulses would result is a slower decline from the carbon peak.

Regardless of the cause of the PETM, fossils tell what the world looked like. Wing described the changes in his talk at the World Economic Forum: a roughly 50-percent extinction of deep-sea small marine organisms, movement of high-latitude plants and animals between North America and Eurasia, and mid-latitude die-offs and replacements of plant populations. These changes are documented in a large part by fossil plants.

Leaf fossils not only show what the leaves looked like, they show how much leaf-eating bugs were munching. At the PETM, there's evidence of increased insect damage. This could result from more heat and more bugs, since roaches materialize out of nowhere in hot, humid climates. But it could also result from the fact that, while more carbon dioxide in the atmosphere might prompt plants to grow, the plants will be less nutritious. Trying to answer that question, Wing is overseeing an experiment led by Richard Barclay, also at the Smithsonian. They two have set up plots of ginkgoes with different levels of greenhouse gases pumped onto each tree. They are collecting leaves that may be used for later analysis of insect damage. More immediately, they are looking at leaf stomata (pores facilitating gas exchange) to see whether ginkgoes respond anatomically to an increase in carbon dioxide. If this experiment shows a change, paleobotanists might use fossil ginkgo anatomy to infer past levels of carbon dioxide.

These 55-million-year-old changes are relevant today because our species likely releasing greenhouse gases at a higher rate than anything did during the PETM. In his talk at the World Economic Forum, Wing remarked, "This is like the PETM on steroids." And he pointed out something perhaps even more sobering: The changes will last for millennia. Even if we stop all greenhouse-gas emissions tomorrow (we won't) higher carbon dioxide levels are inevitable. But choosing a more sustainable path over business as usual will sharply reduce the amount of carbon dioxide we release.

Scott Wing at the World Economic Forum
Wing discusses human impacts in geologic time. Screen capture from YouTube.

Plants, being literally rooted in place, won't likely be able to transition to more favorable ranges soon enough. A workaround is for humans to plant them in promising new habitats. As an example, he mentioned Torreya, a conifer genus that may soon struggle to survive along the Gulf Coast, and could gain a foothold in Southern Appalachia. Some might object since Torreya isn't native to Southern Appalachia.

"There are two kinds of mistakes people make," Wing told me. "One is to say, 'We've already ruined the world.' That isn't true. It wasn't true 100 years ago, and it wasn't true 5,000 years ago." Agriculture, animal husbandry, and megafauna extinctions go back thousands of years, and the world has still been quite livable. "People let their ideas of pristine nature get in the way of solving problems. We shouldn't feel like we fail if we can't put the world back exactly the way it was." The other mistake people make is to assume we're not changing climate. "That's denial. Because we are." Wing smiled and said, "It's like being middle aged. You're too old to not bear any responsibility for the world, but too young to decide you'll die soon and it won't be your problem. Sorry, but you're important in this, and you have to learn to be responsible for a thing you care about."

Our conversation happened at an oddly appropriate time. Weeks earlier, Hurricane Harvey inundated Houston and surrounding areas with unprecedented levels of rain. Soon after that, Hurricane Irma devastated Florida. Eight days after our chat, Hurricane Maria would tear across Puerto Rico. Compared to heatwaves and droughts, severe convective storms are less clearly linked to climate change because there are so many other factors that might be involved. But climate is clearly part of the story. Meanwhile, I had watched Denver's typically bright blue skies dull as the jet stream carried smoke from wildfires burning across the parched Pacific Northwest.

Wing is one of a growing group of geologists and paleontologists who favor a new term for the geologic timescale that reflects human influence: Anthropocene. Given that human impacts extent thousands of years back in time, I had guessed he might favor a wholesale replacement of Holocene (the last 10,000 years) with Anthropocene, but he doesn't. He explained that geologists like to be able to identify a geologic age in the sedimentary record, and gave the iridium layer at the K-P (previously known as K-T) boundary between the end of the Mesozoic and the start of the Cenozoic, when dinosaurs went extinct. For the start of the Anthropocene, Wing favors the mid-20th century when hydrogen bomb tests led to fallout strong enough to appear in the rock record.

After I finished pelting him with questions, Wing walked me back through the crowded corridors. Though overflowing, Smithsonian storage space is ample; when I visited in 2000, I realized that each aisle of cabinets merits its own light switch. Exiting to the public space, Wing pointed out a museum map to me. Much of it was under construction, and he told me that's the space to watch. It was common knowledge when I visited that the Smithsonian was revamping its dinosaur hall. But it won't just be dinosaurs. Wing and all his colleagues were hard at work brining all of prehistory to life in the upcoming exhibition.

Real and Confectionary Animals (September 13)

With an extra day in DC, I took in other sites. First stop was the Smithsonian Zoo, where I saw one-year-old Redd and his mother Batang (though I didn't take a very good picture of them through the glass). Without realizing she was a celebrity, I noticed her walking on a rope above the path, and only because a conscientious volunteer suggested I let Batang pass, "In case she wants to use the bathroom up there." I enjoyed some New Deal Art in the Reptile House, a fennec fox asleep in a box, and of course meerkats.

DC sites
Dinosaur cookies
DC sites, including Georgetown University, inhabitants and New Deal artwork at the Smithsonian Zoo, and dinosaur sugar cookies at Dean and Deluca.

A walk across Key Bridge from my hotel took me to Georgetown, where I ambled into Healy Hall, and with more purpose, strode into Dean and Deluca to discover they sell dinosaur cookies. Life doesn't get much better than dinosaur cookies.

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