How the Earth Turned Green

A Brief 3.8-Billion-Year History of Plants

Joseph E. Armstrong

• Éditeur: University of Chicago Press
• Année: 2014
• ISBN:
  9780226069807
• Sujet: Sciences biologiques
• Sous-sujet: Science générale

On this blue planet, long before pterodactyls took to the skies and tyrannosaurs prowled the continents, tiny green organisms populated the ancient oceans. Fossil and phylogenetic evidence suggests that chlorophyll, the green pigment responsible for coloring these organisms, has been in existence for some 85% of Earth's long history—that is, for roughly 3.5 billion years. In How the Earth Turned Green, Joseph E. Armstrong traces the history of these verdant organisms, which many would call plants, from their ancient beginnings to the diversity of green life that inhabits the Earth today.Using an evolutionary framework, How the Earth Turned Green addresses questions such as: Should all green organisms be considered plants? Why do these organisms look the way they do? How are they related to one another and to other chlorophyll-free organisms? How do they reproduce? How have they changed and diversified over time? And how has the presence of green organisms changed the Earth's ecosystems? More engaging than a traditional textbook and displaying an astonishing breadth, How the Earth Turned Green will both delight and enlighten embryonic botanists and any student interested in the evolutionary history of plants.

ONE

A Green World

Wherein a discussion of “plant” and “plant kingdom” introduces the science of taxonomy and classification, and the nature of science is illustrated by explaining how we know the age of the Earth and why biologists care about elements from stars and molecules from space.

In tropical forests, when quietly walking along the shady pathways, and admiring each successive view, I wished to find language to express my ideas. Epithet after epithet was found too weak to convey to those who have not visited the intertropical regions the sensation of delight which the mind experiences.

—Charles Darwin, 1839

IN THE RAIN FOREST

Biologists are fascinated by rain forest, and that is where this book about green organisms begins and ends. When you thought “green organism” did you envision a tree—or a tree frog? You may be disappointed, but the only green organisms considered are those that are green with chlorophyll, the pigment of photosynthesis. As you will see, the chlorophyll green of plants explains why tree frogs are green too, but that is as far as this book goes with animals. The lush and diverse vegetation of the wet tropics is as green as it gets, but these forests haven’t always existed. And the same goes for other forests and grasslands, tundra and deserts too. All these types of communities and the organisms in them were different in the past, illustrating a fundamental principle of biology: things change. Organisms alter their environment, and then they must change to adapt to the new conditions they produced; organisms¹ must evolve or go extinct. Evolution is a necessary aspect of life. This back-and-forth interaction between changing environments and adaptation means that life has a history of change resulting in organisms that possess a myriad of ways to succeed at making a living in the natural world. Biologists make a living explaining both.

Fig. 1–1 Flower of Anaxagorea crassipetala showing three perianth whorls, the outermost sepals, and two whorls of petals, the outermost of which are very thick and fleshy (approx. life size). (Image source: The Author)

The time is not quite 9 in the morning, the locale is tropical northeastern Costa Rica, and I have been up for over four hours because the flowers I am studying open at first light, just after 5 a.m. The flowers belong to a small, understory, rain-forest tree in the custard apple family, Anaxagorea crassipetala, and like many tropical organisms it lacks a common name. Rather than having the broad, flat, thin, and colorful petals of familiar flowers, this tree’s outer whorl of three petals are a dull creamy color and nearly circular in cross section, looking like three little peeled bananas (Fig. 1–1). I am attempting to determine why this tree invests so much energy, 64% of the flower’s dry weight, in making thick petals (crassipetala = with thick petals) that provide no visual display to speak of. Such research is evolutionary and yet another test of Darwin’s hypothesis of natural selection, which allows us to predict that such a big energetic investment in petals should function to enhance the plant’s production of offspring, so I need to figure out how.

Charles Darwin was not the first person to propose evolutionary change. His primary contribution was to propose a mechanism by which species of organisms could change, and that was natural selection. Within any species, organisms display heritable variations. Darwin’s idea was that under any particular set of environmental circumstances some variants would be more successful in producing offspring than others. This would result in those variants becoming more common in the next generation. Simply put, nature “selects” certain variants as measured by the number of offspring they produce (the number of times those genes are passed on to the next generation). Natural selection is about differential reproduction, the exact opposite of random reproduction. And of course while “selects” sounds like a conscious decision, nothing of the sort takes place. Although lots of species of Anaxagorea and other custard apples have fairly thick, somewhat fleshy petals, none are as thick as those of A. crassipetala. So here I presume that the ancestors of this particular species were those trees that produced the fleshiest petals, and as a result, produced the most offspring by setting the most seed. How that might be the case is a question of interest and a worthy intellectual project for a botanical scientist. And before you get too excited, I have yet to find the answer although I have learned many other interesting things along the way.

Flowers and fruits present displays, either colorful (visual) or fragrant (olfactory) or both, and certain animals react to these displays to obtain a reward, which is often, but not always, food. Flowering plants do not provide such rewards to be nice to animals. These payoffs attract animals for the plant’s purposes, the dispersal of pollen and seeds. Almost all of the many different rain forest plants in the surrounding community engage in such “cooperative” plant-animal interactions; a couple of ferns provide exceptions. Such cooperative interactions between animals and plants are one hallmark of flowering plants, the angiosperms. Prior to the appearance of flowering plants, the interaction between animals and plants could hardly be called “cooperative”; animals fed upon plants, a rather one-sided interaction. Of course, animals, including ourselves, still feed upon plants, but flowering plants found a way to benefit from some of this animal feeding by using animals as agents of dispersal. These cooperative interactions are one of the reasons for the extraordinary evolutionary success of flowering plants.

Can you envision how natural selection does this? Any individual plant whose reproductive structures were even slightly more attractive and more rewarding got better pollination and better seed dispersal, which resulted in more offspring carrying those genes that resulted in more attractive and more rewarding flowers and fruits. Animals that responded most efficiently to these displays got the most reward, which resulted in them having more offspring, who had similar behaviors. And pretty soon it’s hard to tell who invited whom to the dance. But every study of any biological interaction is in one way or another fundamentally about evolution.²

Another dimension exists to such studies; they can be extended into time and space. Anaxagorea is found in the tropical forests of both Southeast Asia and the New World tropics of Central and South America, different species separated by thousands of miles of ocean. How do we account for such a distribution in plants lacking any means of long-distance dispersal? If you examine a world map you cannot help but notice that South America and Africa look like they could fit together like pieces of a jigsaw puzzle. Antarctica, Australia, New Zealand, New Guinea, and India also fit together with South America and Africa to form the former supercontinent Gondwana (see Fig. 9–5 for a map). Africa and South America rifted apart forming the South Atlantic. More rifting and rafting fragmented the rest of Gondwana, a process that continues to this day. Each land mass “rafted” to its present position with a complement of organisms. The genus Anaxagorea is estimated to be 44 million years old (Scharaschkin and Doyle 2006), but even this great age is not old enough for it to have been on opposite sides of the Atlantic Ocean before there was an Atlantic Ocean. The custard apple family is estimated to be over twice as old, so some 80 to 90 million years ago ancestors of this tree were flowering and fruiting while dinosaurs were still the dominant land animals. This still does not explain the distribution of Anaxagorea, which remains an unanswered question. Flowering plants appeared well over 100 million years ago, but of all the major groups of green organisms, flowering plants appeared most recently. Does that boggle your mind? Most of us are not used to dealing with such time frames. Humans tend to talk of ancient history in the thousands of years, but biologists toss off millions of years like dollar bills at a cake raffle. If recent events in the history of green organisms took place during the age of dinosaurs, then early events in this history are going to be really, really old, and in fact, the history of green organisms begins when the Earth itself was still quite young.

In our familiar terrestrial habitats, flowering plants dominate most areas. Flowering plants also occupy our human attentions because they are of utmost importance to us. We depend on flowering plants for most of our material needs, and because of this importance and their ecological dominance of Earth’s terrestrial landscapes, flowering plants are the primary focus of botany, plant science, horticulture, and agriculture. But rather than just take flowering plants for granted, some of us ask questions. Where did flowers and fruits, and the plants that bear them, come from? How do flowering plants differ from their ancestors, and which differences account for their extraordinary success? What plants dominated the Earth before flowering plants, and how did they reproduce and disperse? To be curious is human, and lucky ones get to be botanists and satisfy some of that curiosity.

The fossil record tells us that other groups of plants appeared and flourished long before the flowering plants appeared. Other seed plants flourished, diversified, and dominated terrestrial landscapes, and while some, like the conifers, are still common and important, many other groups of seed plants have become extinct. Even earlier, over 360 million years ago, clubmosses, horsetails, and ferns were common and diverse components of ancient coal-forming forests. Today only a few descendants of clubmosses and horsetails remain as mere relicts of their former glory. A few descendants of these ancient ferns still exist, but new groups of ferns have appeared and become common. While small and often overlooked, mosses, liverworts, and hornworts appear to be still older. All of these afore-mentioned green organisms share a life cycle that produces an embryo, an indication that they all share a common ancestry. All are quite correctly called plants, or even more specifically, land plants or embryophytes (EM-bree-oh-fights),³ embryo-producing plants.

This brings up Charles Darwin again because he explained evolution as descent with modification. The characteristics of a species change through time because of natural selection (and other mechanisms that have been discovered since), but life is connected through time via common ancestry. The evidence of this is shared characters inherited from ancestors, the basis of classification, although at its inception the science of taxonomy included no evolutionary concept. Darwin fundamentally changed that. Now classification is about common ancestries and that in turn tells us about the histories of organisms and the characters they inherited from those common ancestors. The two, evolution and classification, are interwoven such that biologists talk about lineages and phylogenies (fye-LAH-jen-eez) more than groups. Still, classification provides useful labels.

As hard as it is to imagine, the fossil record tells us that the land was not always green with plants. Even the appearance of land plants and the greening of the land are relatively recent events occurring in the last one-eighth of Earth history. Prior to this, green organisms were found only in aquatic habitats, and most such organisms are called algae. Some algae are quite large, seaweeds that live anchored in coastal regions. Many more are microscopic organisms that drift along in the oceans. Still other green organisms exist for which even a general label, like algae, does not seem appropriate. Some bacteria are green, and one of these groups, the cyanobacteria or blue-green algae, is among the oldest, most common, most successful, and most influential groups of green organisms in Earth history. You may wonder how that can be. Whether any or all of these algae and green bacteria are correctly called plants remains a matter for further discussion. Yet even if technically not plants, all are part of this story, and as a botanist I make my living by learning and teaching about all these green organisms.

WHAT ARE THE CONSEQUENCES OF BEING GREEN?

Some important consequences of being green must be understood for this history to make any sense. Green organisms are green because they possess a pigment called chlorophyll, which captures solar energy. But green organisms cannot use light energy directly. The captured energy is used to synthesize molecules of sugar, so those crystals in your sugar bowl represent sunlight captured, concentrated, and transformed by a plant, probably sugar cane, into a molecular form, which is what photosynthesis refers to. Sugar is crystallized sunlight. No wonder a little candy can brighten your day! Sugars and their polymers (starches and celluloses) are part of a class of organic molecules called carbohydrates, an appropriate name for molecules made from very simple raw materials like carbon dioxide and water. From carbohydrates and their metabolic intermediates, green organisms synthesize every other molecule they need to grow and reproduce. So when I say green, I mean green with chlorophyll. Other green organisms, like some lizards, frogs, and katydids, are green because they have non-photosynthetic pigments that provide camouflage among foliage green with chlorophyll.

Green photosynthetic organisms are autotrophs (“self feeders”) and in their abundance they provide for us all, so we refer to them as producers. Organisms that require pre-made organic molecules for energy and raw materials are called heterotrophs (“other-feeders” or consumers). As such we and other consumers must “eat” other organisms (either wholly or in part), their secretions, or their metabolic waste products as food. Eating and being eaten is a fact of life, a process by which the light energy captured by green organisms is passed through a series of consumers, a food chain, before eventually being lost as heat, which dissipates.⁴ Everything else is recycled with the able assistance of decomposers, primarily fungi and microorganisms, heterotrophs who obtain their food from dead organisms or their metabolic wastes. A large part of ecology concerns such trophic (TROW-fic) or feeding interactions, the energy transfers that result, and the cycling of biogeochemical, the elements of life.

Corner (1964) described a plant as “a living thing that absorbs in microscopic amounts over its surface what it needs for growth.” Of course, this definition is so broad it would include fungi and bacteria too. But his point was that in one way or another, all the raw materials that green organisms need (light, carbon dioxide, water, and mineral nutrients) are dilute or diffuse, and thus plants must spread a tremendous surface area into their environment. As any solar engineer can explain, the problem with solar energy is that it takes a tremendous surface area to absorb any significant amount. Our familiar plants generate tremendous surface area in both the air and soil. Their roots and stems branch again and again, ramifying until both leafy stems and roots are a network filling the space around a plant, making the plant an environmental obstruction for capturing diffuse and dilute resources. Branches end in leaves, flat arrays of tissue for absorbing sunlight and carbon dioxide. As a consequence of hanging lots of broad, thin leaves in the air, plants constantly lose water, which needs to be replaced. So a network of roots is needed to absorb water and the dilute mineral nutrients dissolved in it, and a conducting tissue is needed to carry water from the network of roots to the crown of leaves. A weighty crown of leaves and branches also requires considerable structural support. In most familiar plants of forest and field, both support and conduction are performed by a vascular tissue called xylem (ZEYE-lem). Xylem cells are dead at maturity, but their thick cell walls form tubes, which are both strong and a convenient shape for conducting water. Whether support or conduction was the initial function will be discussed later. Trees and shrubs produce a new layer of xylem in their stems and roots each year, and the accumulated layers of xylem are called wood.

Plants rooted in the soil, stiff and massive with thick-walled xylem cells, are not motile (MOH-til) and free to move about seeking needed resources and mates, but yet they must acquire both. The form needed to obtain diffuse resources results in immobility, which explains why flowering plants use rewards to entice animals to act as pollen and seed dispersers. Other plants must disperse too, but they largely rely upon movement by wind or water. No costly rewards are needed for wind or water dispersal, but such abiotic dispersal agents generate another cost in the production of vast numbers of dispersal units needed to compensate for the randomness of the physical elements. The physical and biological constraints for acquiring the basic necessities of plant life and the costs of reproduction and dispersal very much shape most of the recent chapters in the history of the green organi...