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  Although a number of readers may already care deeply about the environment and the decline of tropical forests, it is highly likely that, to some extent, you still feel a certain safe estrangement from some of the world’s oldest land-based environments. Indeed, harsh working conditions, dense vegetation, and difficult navigation—as encountered by Victor and me on our Amazon expedition—have often hindered exploration of tropical forests and their histories. However, this book looks at how the latest scientific advancements, from laser scanning from the air to plant genetics in the lab, can take us through the canopy to show us how these habitats actually penetrate every inch of our lives, wherever we are. Our kitchen cupboards are filled with groceries that owe their beginnings to tropical forests. Our travel to work depends on latex originally tapped from tropical trees. Our consumer decisions about everything from furniture to beauty products continue to influence the nature and extent of tropical environments. Extinction, deforestation, and out-of-control fires in these environments can seem a world away from many of us as we hear about them in the news. But the loss of tropical forests and the colonial legacies that shape their plight today impact not just the lives and climates of people on the other side of the world, but also our own weather, politics, societies, and economies—from Manila to Munich, Colombo to Cardiff, and Nairobi to New York. This book is an attempt to convince you that the history of tropical forests is your history too. But before we get ahead of ourselves, it is time to look back beyond 500 million years ago. Back when plants had not yet graced the land surfaces of the Earth and things looked very, very different to how they do today.

  Chapter 1

  INTO THE LIGHT—THE BEGINNING OF THE WORLD AS WE KNOW IT

  School tours of natural history museums around the globe frequently rush past the fossilized remains of early plants on the way to the reconstructed dinosaur skeletons or stuffed blue whales that often take the plaudits as the heavyweight “stars” of evolution. Novels and films such as Jurassic Park have brought the work of paleontologists, picking away at long-extinct bones to determine the evolution of fascinating creatures, into the public arena. But no such dramatization has emerged for the explorers of the world’s ancient plants. This speaks to a more general apathy. As we go about our daily lives, we rarely spare a thought for the mosses growing on our pavements, the grasses covering our fields, the flowers occupying our gardens, and the trees that, if we’re lucky, line our streets. We often take for granted that plants have always been on our planet and always will be. There are also fewer well-

  known documentaries that explore the emergence, evolution, and conservation status of plants. Plants are traditionally considered less “exciting” than other life-forms. They are harder to empathize with than animals, with their emotional eyes and semblance of what we might recognize as families. Plants also do not make noises audible to the human ear, apart from perhaps some accidental creaking in the wind, making it unlikely that they will feature in a viral YouTube video “talking.” Yet, without plants, the world as we know it today would not exist.

  Indeed, recent research makes it seem like we might have been too quick to dismiss our photosynthesizing friends. For example, we now know that plants actually share some features that make us so able to relate to other animals. Time-lapse cameras have revealed the amazing, dynamic way in which plants constantly grow and move in different directions to access light. While they do not feel pain in a traditional sense, plants being eaten by caterpillars will release defense chemicals that make them less appealing to their attacker. Similarly, trees being eaten by giraffes in eastern Africa will release compounds into the air that warn other trees to protect their own leaves. Such “communication” has also been demonstrated in forests where neighboring trees will try to route nutrients to their damaged or ailing companions, making use of mass fungal “networks” that are some of the largest organisms in the world. In an attempt to maximize growth, trees will also sometimes gamble on the shedding of their leaves in temperate winter environments, with daring individuals paying the potentially fatal penalty of frost damage if they keep their leaves for too long. However, by far the most impressive part of these organisms is the way in which they engineer our world to make it hospitable to almost all other life. The sheer scale of this accomplishment is only truly apparent if we explore what the Earth was like without plants, which, thanks to recent scientific advances, we can now do.1

  Let’s travel back to the early Cambrian period (between 538.8 million and 509.0 million years ago). While it might seem an odd place to start in a book about tropical forests—its name derives from Cambria, the Latin word for Wales—this famous geological period witnessed a diversification of complex life, often termed the “Cambrian explosion,” that eventually produced the evolutionary lineages of all multicellular animals left on Earth today. Were we to dive into the significantly warmer oceans of the early Cambrian period, we would see many different species of trilobites (the characteristic wood-louse-like fossil arthropods many of you will be familiar with) and other invertebrates practicing predation, scavenging, and filter feeding as marine ecosystems began to take on a familiar form. However, if we left this marine cornucopia and stepped out onto the land, a seemingly deserted, and frankly apocalyptic, scene would greet us. The continents would be covered by dry, rocky landscapes, and the main type of life present would be in the form of patchy films of microbes smeared across the surface. Beyond that, the only visible signs of life on land would be the occasional slug-like mollusk precariously venturing out of the oceans to try and scratch a living on the slim pickings available. During the early Cambrian there was no variety of terrestrial life at all, let alone something we might associate with a tropical forest. However, the world was certainly “tropical” by common climate classification systems that define a tropical climate as one with monthly average temperatures of approximately 18°C or more. In the Cambrian, the global average temperature was around 19°C, a staggering 5°C higher than the present day, and it still remained at an average of approximately 18°C at the beginning of the Devonian (from 419.0 million years ago).2

  It was into this “tropical” world that the first land plants arrived to perform their greatest trick, with an alchemical skill we can only dream of. All of you will have had a high school introduction to photosynthesis, the process by which most plants and certain bacteria can use the energy of the Sun to convert carbon dioxide (CO2) in the air or oceans into sugars and oxygen (O2). However, the fundamental nature of this process to life is easily forgotten. In the absence of land plants, even with some bacteria and aquatic algae performing photosynthesis, early Cambrian atmospheric CO2 concentration was around 4,500 parts per million by volume. That’s ten times higher than it is today. Meanwhile, oxygen made up only about 7 percent of the Earth’s atmosphere, around three times less than today. As a result, on our tour of the early Cambrian land surface, we would require oxygen tanks to survive. Furthermore, in the absence of plants, there were no root systems to efficiently break down bedrock, and nutrients that are crucial to most food chains today remained locked away in the Earth’s tough crust. We will now see how the broadly “tropical” state of our planet between the Cambrian and the Devonian represented the perfect “greenhouse” for the emergence of not only the first land-based plants but also the first trees and forests. While these pioneering organisms actually put an end to widespread warm conditions, causing significant drops in CO2, global cooling, and even the formation of polar glaciers, they also formed the building blocks for the first complex, land-based ecosystems that were to emerge around the remaining warm, wet regions of the equatorial tropics in the Carboniferous period (359.3 to 298.9 million years ago). These forests fixed soils, released nutrients, and stabilized the climate and the composition of the atmosphere. They also provided ideal homes for a growing variety of interacting animals that would eventually lead to the amphibian, reptilian, and mammalian life that surrounds us today. It is no exaggeration to say tha
t they changed the face of the world forever.3

  THE PROCESS OF photosynthesis and the extension of roots into the very surface of the Earth meant that, from their first arrival, plants had the potential to be significant movers and shakers in the context of our planet’s atmosphere, climate, and geology. Professor Timothy “Tim” Lenton, director of the Global Systems Institute at the University of Exeter, is interested in how the first plants on land might have influenced the various “systems” (e.g., atmosphere, geosphere, hydrosphere, and biosphere) that interact to shape the Earth’s climate. As he puts it, “There is a complex interplay between our oceans, the proportions of different gases in the air around us, and, crucially, what covers the land that affects how the Sun’s energy is stored and moved around the world.” Earth systems scientists such as Tim rely on powerful computers to “model” how changes to the amounts of different gases in our atmosphere, to the speed and direction of ocean currents, and to the distribution of different types of vegetation will alter the overall condition of the planet and whether such changes will impact different parts of the Earth in different ways. The outputs from what might be best described as an earth scientist’s version of Minecraft can then ultimately be tested against available records of CO2 and O2 concentration and temperature from ancient sedimentary or ice-core records to see whether they “work” and can effectively explain the state of the Earth at a given point in time.

  One of the main uses of this climate “sandbox” has been to make predictions about how twenty-first-century greenhouse gas emissions will impact global temperatures in the decades to come. However, it has also been used to explore how the arrival of the first plant life would have altered the state of the planet. Yet, in order to do this properly, these computer whizzes must turn to the more “classic” approaches of exploring ancient plants, or “paleobotany,” to answer the key question of when the first true land plants (or Embryophyta) appeared. It is generally accepted that the first land plants were nonvascular, or “lower plants,” meaning that they lacked complex structures, such as the transport network of xylem cells that carry water and phloem cells that convey nutrients from a plant’s roots to the rest of its body. These plants evolved from aquatic algae and have long been thought to have resembled the modern liverworts, hornworts, and mosses (or bryophytes) you will have likely seen covering large patches of ground, rocks, and trees in your backyard. These spore-producing plants are, today, well known for their ability to retain water and can also act as important sources of food for animals. However, suggestions that liverworts, in particular, were the earliest diverging “sister” lineage of land plants meant that climate models often neglected the impacts of the earliest land plants on the Earth’s surface and atmosphere. This is because liverworts lack a number of physical features, such as stomata—the tiny holes in leaves that open and close to help plants control their uptake of CO2 and loss of water during photosynthesis—that are found in more complex, “vascular” land plants. As a result, they seemed to represent a perfect halfway house between “primitive” aquatic algae and complex, terrestrial vascular plants. Nevertheless, as you can imagine, directly testing this assumption by actually finding and studying the earliest of these plants has posed a major challenge to even the most enthusiastic of paleobotanists.4

  The most direct evidence for the appearance of land plants comes from fossilized impressions of their bodies or the spores they used to reproduce, which were left behind in ancient sediments. Following a series of high-pressure geological processes, these traces were eventually converted into rock to be preserved to this day. The earliest embryophyte-like spores come from sediments in Argentina dating to around 470 million years ago, during another geological period with a Welsh connection, the Ordovician (486.9 million to 443.1 million years ago), which was named after an ancient group (the Ordovicii) subjugated by the Romans in what is now North Wales. More recently, five bryophyte clades (a clade is a group of organisms considered to all be the descendants of a single common ancestor) have been argued to be represented by fossil imprints from a sinkhole that was excavated during the creation of the Douglas Dam in Tennessee, which date to c. 460 million years ago. This would be significantly earlier than one of the most widely accepted earliest fossils with a land-plant body structure, from Ireland, with a minimum age of approximately 427 million years. Nevertheless, if you have ever touched a bryophyte while working in your garden or taking a walk in the woods, you will have noticed its fragility, and it will often simply crumble away in your hands. As a consequence, existing fossil discoveries are almost certainly underestimates of the true age of the first land plants. Comparisons of modern and fossil plants can be used to estimate how long it would have taken for different “morphologies,” or body shapes, to have emerged in the past. However, this is imprecise, as evolution can occur at markedly different rates. Furthermore, the incredible patchiness of the fossil record of plants, particularly during this early time frame, means that there is frustratingly little to go on when attempting to understand when and how land-plant lineages emerged and proliferated, as well as what they looked like and what implications they might have had for the planet.5

  Figure 1.1. An image of the liverwort Marchantia polymorpha. The earliest land plants are thought to have resembled living “Bryophytes” such as liverworts, hornworts, and mosses. Liverworts, such as Marchantia polymorpha, have often been considered the oldest lineage of land plants, acting as a model system for a transition between aquatic algae and complex vascular land plants. However, the exact nature and physiological features of the earliest plants to colonize the land have remained unknown until recently. Silvia Pressel

  Some scientists have therefore adopted different approaches to exploring the evolutionary history of land plants. For example, they have sampled the modern genetic data of different families of liverworts, mosses, and other land plants to try and produce a kind of “molecular clock.” These “clocks” work on the principle that mutations within the genetic or protein sequences of plants will have occurred at a constant, calculable rate. With this in mind, the modern genetic sequences of different plant groups can then be compared to each other, with the amount of difference providing some kind of idea as to how long ago the different lineages formed and separated from each other. This work has allowed scientists to determine that vascular plants, or “tracheophytes,” which include the trees, shrubs, flowers, ferns, and fern allies that characterize the green plant life most visible to us today, indeed emerged after the liverworts and mosses. More significantly, it has also been demonstrated that liverworts are not actually the earliest land-plant lineage, calling into question the relevance of their physical features for characterizing the earliest plants to colonize the land. Nevertheless, the earliest evolutionary history of plants remains hazy, and estimating a robust date for the first land plants has been incredibly difficult. Given the complexity of genetic mutations and differences in the rate of change between populations, species, and even plant families, dates based on modern genetic data alone are notoriously unreliable, providing a rough ballpark figure at best. If neither the date for the first plants nor the nature of their biological features can be determined, then working out when and if they had impacts on the Earth’s climate obviously becomes a major challenge.6

  Fortunately, in 2018, scientists decided to combine the strengths and weaknesses of the different fossil and genetic approaches to the origins of land plants to develop a more accurate and comprehensive model for their appearance. This team, headed by scholars at the University of Bristol, combined the genetic and morphological estimates of rates of plant evolution with the available “fixed points” provided by actual remains in the fossil record. According to Dr. Silvia Pressel, head of the Algae, Fungi and Plants Division at the Natural History Museum of London and coauthor of the resulting study, this work allowed “initially rough evolutionary models to be tested and refined using the evidence that we can hold in our hands.” Moreover, by repeat
ing these models thousands and thousands of times, the researchers could work out just how likely it was that they represented reality. All of their analyses converged to indicate that the first land plants actually appeared about 500 million years ago, slap bang in the middle of the Cambrian period, just after the evolution of multicellular animal species had started to take off underwater. Even more significantly, biomolecular analysis of different plant lineages undertaken by a separate multidisciplinary team of paleobotanists indicated that the ancestor of land plants was probably more complex than assumed so far, already possessing rootlike filaments and stomata on its surfaces. These features would have allowed it to process more soil and more carbon dioxide than previously thought.7