How the Earth Turned Green: A Brief 3.8-Billion-Year History of Plants

By Joseph E. Armstrong

This “amazing and wonderful book” explores the evolutionary history of photosynthesis in a grand story of how the world became the verdant place we know (Choice).
 
On this blue planet, long before dinosaurs reigned, 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? With engaging prose and astonishing breadth, as well as informative diagrams and illustrations, How the Earth Turned Green demonstrates “how the Earth blossomed into such an incredible world that most of us simply take for granted” (San Francisco Book Review).

• Language: English
• Publisher: Open Road Integrated Media
• Release date: Oct 2, 2014
• ISBN: 9780226069807

Book preview

Joseph E. Armstrong is professor of botany, head curator of the Vasey Herbarium, and director of the Organismal Biology and Public Outreach Sequence for Biological Sciences Majors, all at Illinois State University.

The University of Chicago Press, Chicago 60637

The University of Chicago Press, Ltd., London

© 2014 by The University of Chicago

All rights reserved. Published 2014.

Printed in the United States of America

23 22 21 20 19 18 17 16 15 14      1 2 3 4 5

ISBN-13: 978-0-226-06963-0   (cloth)

ISBN-13: 978-0-226-06977-7   (paper)

ISBN-13: 978-0-226-06980-7   (e-book)

DOI: 10.7208/chicago/9780226069807.001.0001

Library of Congress Cataloging-in-Publication Data

Armstrong, Joseph E. (Joseph Everett), author.

How the Earth turned green : a brief 3.8-billion-year history of plants / Joseph E. Armstrong.

pages cm

Includes bibliographical references and index.

ISBN 978-0-226-06963-0 (cloth : alkaline paper) — ISBN 978-0-226-06977-7 (paperback : alkaline paper) — ISBN 978-0-226-06980-7 (e-book)

1. Botany.   2. Plants—History.   I. Title.

QK45.2.A76 2014

580—dc23

2013047074

This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper).

HOW THE EARTH TURNED GREEN

A Brief 3.8-Billion-Year History of Plants

Joseph E. Armstrong

The University of Chicago Press

Chicago and London

CONTENTS

Preface: A Botanist at Large

1. 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.

2. Small Green Beginnings

Wherein the discovery of microorganisms, their amazing numbers, and the places they live are explored; evidence of ancient life is examined; and the nature of metabolisms and the origin of their complexity is explained.

3. Cellular Collaborations

Wherein the diversity of unicellular organisms is explored, chloroplasts and mitochondria are obtained via symbiotic interactions between cells, and other features such as nuclei and sex are examined to determine what can be learned of their origins and functions.

4. A Big Blue Marble

Wherein algae are introduced, ocean ecology is explained, and phytoplankton diversity is explored.

5. Down by the Sea (-weeds)

Wherein coastal environments are contrasted to oceanic environments, and organisms adapt to the new challenges presented by living on coasts by becoming anchored, larger, and multicellular, which in the case of green organisms results in those algae called seaweeds.

6. The Great Invasion

Wherein the challenges and colonization of the terrestrial environment are examined so as to understand the adaptations of land plants, especially their life cycle.

7. The Pioneer Spirit

Wherein liverworts, hornworts, and mosses are examined to demonstrate their adaptations to terrestrial life and their relationships to each other and vascular plants.

8. Back to the Devonian

Wherein a field trip to the Devonian introduces early vascular plants and examines how, from such small beginnings, xylem and new ways of branching helped plants produce leaves and roots and grow into trees, Earth’s first forests.

9. Seeds to Success

Wherein the nature of seeds, their impact on the land plant life cycle, their history, and the diversity of seed plants is investigated.

10. A Cretaceous Takeover

Wherein the quite singular ecological dominance and species diversity of the flowering plants are examined in light of their novel features and their gymnospermous ancestry.

11. All Flesh Is Grass

Wherein the development of modern vegetation and recent interactions, like agriculture, and their impact on both plants and humans are examined.

Postscript

Appendix

Brown Algae and Tribophyceans

Clubmosses and Fossil Stem Groups

Conifers and Ginkgoes

Coniferophytes: Cordaitales and Voltziales

Cycads

Ferns

Gnetophytes

Green Algae

Green Bacteria

Hornworts

Horsetails

Liverworts

Mosses

Phytoplankton

Red Algae

Rhyniophytes and Trimerophytes

Seed Ferns

Whisk Ferns

Notes

Glossary

References

Index

PREFACE

A Botanist at Large

It must be said at once that this book is better than having a sausage stuck to the end of your nose.

—Mark Golden, describing a book by S. Pomeroy in Classical Review

Laptop computers are a wonderful invention. My computer companion and I, both a little worse for wear, are sitting on a veranda overlooking some Central American rain forest. Far away from office, phone, and university (although the Internet lurks nearby), I am conducting field research on floral biology. Although it is still fairly early in the morning, my flowers opened at first light, so a couple of hours of field work and some breakfast are already out of the way. The stormy season is beginning and the weather here is changeable. For the last three days, hard, straight-down tropical deluges of warm rain have thoroughly soaked the forest. Today dawned pleasantly fair and breezy, rather unusual for the wet tropics where the air is often still and heavy with humidity. A front moved in off the Caribbean and I cannot tell what it will bring, but for the moment, this is tropical weather at its absolute finest. Perhaps this excellent tropical morning has produced the urge to be a bit reflective, an appropriate state of mind for writing a book preface, which is by no means the first thing written. Without all the distractions of modern life there is time to think about why I bothered writing a history of green organisms.

All evidences indicate we live on an ancient Earth, one with a long history. During this time green organisms arose early on from small, simple beginnings, developed, and diversified, culminating in something as interesting, as complex, and as diverse as a rain forest. But this history is a subject well known by only a fairly small cadre of biologists who call themselves botanists, and then only some of them.

So let me ask. Are you a botanist? If so, great! But now I must beg your indulgence. Please understand that this book was not written for you, and therefore is not like the books you usually read. This book was written for everyone else because the challenge I have set for myself is to relate the history of green organisms to nonbotanists in an interesting and understandable manner. As such, my primary purpose will be to function as both an interpreter and a filter. Science must be translated to be understandable to people who do not speak it, read it, or write it. Science is complex and detailed. If science is not winnowed of its many terms and details, people cannot see the forest for the trees. In fact, this is a common problem in teaching science. Translating real science into a more generally understandable narrative always risks allowing some errors in understanding to creep in. Science uses technical language, jargon, in order to be as precise as possible, so attempts to explain what we know in less technical language will mean some loss of precision; it is unavoidable. And of course this author’s understandings may not be perfect either, alas. So any botanist still reading this book will have to allow me some latitude, a degree of flexibility, and some understanding because this book is not written wholly in the language of botany. And this plays to my primary expertise, which is explaining science to nonscientists.

This history is about green organisms for several reasons. First, they are what I study. As a botanist I have come to appreciate green organisms’ subtlety, quiet resilience, sophistication, and diversity, and I enjoy telling people about them. Second, even though this book deals just with green organisms, so much knowledge exists that it poses a major challenge to assimilate, organize, and relate. My colleagues are industrious and the literature resulting from their scholarly activities accumulates more quickly than anyone can read. Fortunately, many others have helped by summarizing and explaining various parts of the whole story that they know well, and by necessity I must rely on their expertise and knowledge. On too many occasions, portions have been revised to take into account newly published studies, but you have to stop doing that or you never finish a book!

Another reason for writing this book is that we are surrounded by green organisms, but many people, and this includes more than a few nonbotanical biologists, are so plant blind that they perceive only a static green background, a passive scenery acting as a stage upon which the real actors, animals, move (Wandersee and Schussler 1999). Years ago, when still young, I asked a wise older colleague why so many nonbotanical biologists were so disparaging about plants, and he concluded that it takes a certain mental and emotional maturity to appreciate something as subtle as a plant, which goes a long way toward explaining their behavior.

In our familiar terrestrial environment, flowering plants dominate most landscapes, and they also feed us, clothe us, and make us happy. Thus flowering plants occupy our attentions and as a result they are the primary subjects of most botany and horticulture courses, most books on plants, and most botanical research. But of all the many groups of green organisms, flowering plants were the most recent to appear, although even this event was at least one hundred million years ago. Trying to imagine the Earth without flowering plants is very difficult, yet for the vast majority of life’s history on Earth, no flowering plants, not even land plants, existed. So suffice it to say at this point, far from being mere pond scum, blue-green algae (cyanobacteria) is arguably the most important, successful, and influential group of organisms in Earth history.

Many people fail to understand how we can know anything at all about such ancient historical events. Such events are not subject to an experimental approach, but nonetheless we can construct hypotheses,¹ which can be evaluated. Although no one has ever observed a bacterial-type cell becoming a nucleated cell (better labels will be forthcoming), everything we know about the biology of cells and the diversity of microorganisms suggests such an event took place and many more events like it as well. Just as in much of theory [hypothesis] formation, the scientist starts with a conjecture and thoroughly tests it for its validity, so in evolutionary biology the scientist constructs a historical narrative, which is then tested for its explanatory value (Mayr 2004). Such narratives should be plausible biologically, compatible with observations and other data, and internally consistent, and they should have an explanatory power that makes sense out of previously unexplained or puzzling phenomena. Several such narratives will be presented and evaluated during the course of this history. Such hypotheses can be tested to see how well they account for things known. Often these scientific explanations attempt to determine patterns of descent with modification, one of Darwin’s two great ideas.² To some people these narratives sound like just-so stories, and while they may start that way, ultimately these explanations get shaped by what is known and changed by new findings. So while not experimental, this is science nonetheless.

The task I have undertaken is to write a brief, understandable history of green organisms from their earliest beginnings to present day. Many biology students have found this subject interesting, but many more are interested in pursuing medical careers and avoiding anything sounding even remotely botanical. Still I operate on the premise that lots of people (okay, maybe not lots, but at least some really bright, curious people) will be interested in reading about green organisms, in finding out how many different ones there are, how we have come to have the ones we have, and why they look, grow, and reproduce the way they do. Along the way some basics of biology will be explained, fundamental stuff really, but seldom well explained in textbooks, if explained at all, which points out a major problem in science education.

An hour has passed since I began writing this. Beyond the veranda the lush, dense greenery of a well-watered rain forest is warming up. The day is noticeably hotter now, suggesting that a typical hot, muggy, tropical afternoon will follow. The history of green organisms, like other scientific histories, is composed of a series of events, and like players on a stage, the cast of organisms changes through time; groups of organisms appear and disappear or give rise to modified descendants. What we know of ancient organisms comes in part from the fossil record and in part from the patterns of relatedness observed among living organisms. Most often plant diversity is presented and taught as a series of groups, classification categories, thus putting the cart directly in front of the horse. But the real science is found in the explanations, the hypotheses, both supported by and accounting for the data that resulted in these classifications, and these are my primary subjects. Information about each specific group of organisms is relegated to appendices.³ Thus in my educational approach I greatly deviate from the approach of botany textbooks by concentrating on what most books omit, and relegating the usual textbook material to appendices. So let me say how thankful I am that a publisher allowed me the latitude to use this approach.

Lastly, although far from perfect or complete, a coherent history of life and its diversity is one of the greatest of human intellectual achievements constructed by logically combining facts and inferences, and sadly, too few people know much about it. Many scientists have spent their careers accumulating these data and producing these explanations, so credit is given where credit is due. If you are so inclined, the references provided will connect you to many scientists and to their work, but no attempt has been made to present an exhaustive or comprehensive review of the literature. Such a list of references alone would be as big as this book. Further, no matter what, some of this book’s contents will be out of date because science is an ongoing process; my colleagues continue to collect data, test hypotheses, and generate new explanations. By the time this book is published, some explanations included will have changed and new studies will have been published, but this rapid outdating is a measure of research activity and scientific progress.

In several instances I purposely digress from the main narrative to explain and demonstrate how science operates. This is, or should be, a motive for all teachers of science. The idea is to illustrate not just what we know but also how we know it and why we have confidence in it. Science is more than a body of knowledge; science is the process by which this knowledge was gained. At the present time, far too few people understand this. This lack of understanding allows many people to treat scientific explanations as if they were merely opinions to which, in fairness, their undocumented, unsupported opinions can be equated. This special form of ignorance is growing in influence, a type of know-nothingism that demonstrates the darker side of relying too much on faith and not enough on evidence and reason. Although the nitty-gritty details are omitted in this book, whenever feasible the observations that must be explained are presented to illustrate the tyranny of data. When you do science you cannot ignore the data. Anyone can pose an alternative explanation by ignoring what is known; thinking of a plausible alternative explanation that accounts for everything known is a real intellectual challenge not to be taken lightly. Critics of science routinely and universally fail this challenge.

I hope this approach will improve your understanding and knowledge of green organisms and, along the way, improve your appreciation for the scientific process that figured out all these things. After all, is it possible for anyone to study plants without an elevation of thought, a refinement of taste, and an increased love of nature (B. S. Williams, 1868)? I think not, so let us get started.

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 organisms. As major obstructions, plants also greatly influence and change their environment, a lesson many people have not yet learned. Plants are the food and habitats for many other organisms, and they all exert influences on each other. As things change, all these organisms must adapt, reacting to both the environment and the influences of other organisms, so all of this pushing and pulling gets played out in the nonrandom reproductive success of many individuals, with the result that the entire evolutionary system generates biological diversity.⁵ To provide you with an understanding of what is and what is not needed, and what was and was not possible, these biological basics must be explored, along with a very ancient history.

WHAT IS A PLANT? A TAXONOMIC PRIMER

You may have grasped from the preceding paragraphs that not all green organisms are correctly called plants. So what is a plant? If all plants are green photosynthetic autotrophs, why is it incorrect to call all green autotrophic organisms plants? The ecological role of a plant as a green photosynthetic autotroph tells us nothing about how the diversity of such organisms should be organized. As a term, plant is used as a label for one discrete group of organisms. Although presently no consensus exists of this group’s membership, one thing is clear: it does not and cannot include all green photosynthetic organisms. In day-to-day usage plant and green photosynthetic producer are one and the same because of the prevalence of flowering plants, but obtaining a more sophisticated perspective is what education is all about.

Among all green autotrophic organisms you find only three different kinds of photosynthesis,⁶ and if you organize all green organisms on the basis of their type of photosynthesis it produces an interesting result. Each unique type of photosynthesis is found in a different group of bacteria: the green sulfur bacteria, the green nonsulfur bacteria, and the cyanobacteria. The metabolic details of the photosynthesis do not matter to illustrate this idea. The type of photosynthesis illustrated in all the textbooks, the kind found in the chloroplasts of all other green organisms including plants, is the photosynthesis possessed by the cyanobacteria (blue-green algae). But this does not make them plants. To further compound the problem, some organisms that are clearly flowering plants on the basis of their structure and reproduction have lost their green color and become parasites on other plants (e.g., beech drops and witch weeds). Without question they are heterotrophs ecologically, but yet they clearly have green ancestors. So being green and photosynthetic cannot be used to group these organisms. As Darwin (1859) understood so clearly, common ancestry provides the key concept for organizing biological diversity and making sense of all of this. Organizing groups on the basis of common ancestry actually produces classifications that can be treated as hypotheses that yield testable predictions. And of course, testable predictions are both necessary and sufficient for doing science, both as a process by which we learn about the natural world, and for the explanations, the knowledge, this process generates.

As an organizing principle common ancestry reflects a very unique pattern in the natural world. Plants that share the features of flowers and fruits are the flowering plants. Flowering plants share the feature of seeds with other seed-producing plants, thus forming a broader inclusive group of seed plants. Seed plants are part of several successively broader and more inclusive groups, until finally they are included in a group of all land plants, which share a life cycle producing an embryo. Diverse evidence indicates that land plants have a common ancestry with green algae. And so it continues. The green algae and land plants are part of a very broad group of organisms with nucleated cells, which includes all large, familiar organisms and a good many microscopic ones as well. Organisms with nucleated cells share even more-basic features—for example, DNA, a genetic code, amino acids, and ribosomes—with bacteria whose cells lack nuclei. Thus all living organisms demonstrate an overall unity, leading biologists to conclude there was a universal common ancestor (a hypothesis). All of this diversity forms a pattern of nested sets based on common ancestry, groups nested within groups nested within groups, and so on. The nested sets outline a sequence of common ancestries and a sequence in which novel shared features appear. This is the fodder upon which many biologists feed.

The traditional classification hierarchy groups species in genera, genera in families, families in orders, orders in classes, classes in phyla, and phyla in kingdoms, producing nested sets. This is how humans organize everything from food to furniture to mailing addresses.⁷ Our classifications of human artifacts are totally arbitrary, but to be useful scientifically our classification of life must accurately reflect groupings that resulted from real historical events, common ancestries. Classification as a science predates the theory of evolution by over one hundred years, so biological classification began with no preconceived notions about relatedness. Pre-Darwinian classifiers, called Linnaeans (after Carl von Linné, also known as Linnaeus, the father of taxonomy), made observations of similarities and then organized groups accordingly, and this resulted in nested sets because organisms possess more specific and more general similarities. Darwin grasped that only descent with modification would result in a classification that formed nested sets. Darwin’s insights led to a paradigm shift, a change in how the taxonomic data were explained. After Darwin, shared features were interpreted to be the result of common ancestry, unless the similarities prove to be from convergent evolution, where unlike and unrelated ancestors came to have similar-looking descendants because of adaptations to similar environmental conditions.⁸

With Darwinian principles guiding the way, biological classification has continued to grow as a science; there are new classification hypotheses almost daily. Science continues to learn things about living organisms, and new data result in changes in our understandings and hypotheses. Indeed, in the past 20 to 25 years the growth of our knowledge and understanding of common ancestries has changed faster than ever, which results in lots of new hypotheses. Anyone who thinks classification is a dead, do-nothing, going-nowhere science could not be more wrong. All of this research has now caused biologists to question the continued usefulness of traditional classification categories.

HOW CAN KINGDOMS CHANGE?

A plant kingdom consisting of green algae and land plants was proposed by Copeland (1956), but a broader plant kingdom concept prevailed, one that included all algal organisms, as well as fungi, basically putting all the subjects of botanical study into the plant kingdom. This was neat, efficient, and very wrong. This was pretty much the state of affairs when I began studying biology over 40 years ago. My freshman biology textbook, Life (Simpson and Beck 1965), used three kingdoms. Members of the plant kingdom included land plants, all the algae, fungi,⁹ and fungal-like organisms such as the slime molds and water molds. Of course animals were a kingdom.¹⁰ The third kingdom consisted of protists, diverse unicellular organisms such as amoebae and paramecia. Bacteria were a phylum¹¹ within the kingdom of protists, but cyanobacteria were placed in a different phylum from the rest of bacteria. Unicellular organisms like euglenozoans and dinoflagellates formed still other phyla. Both euglenozoans and dinoflagellates have green and nongreen species—that is, some possess chloroplasts and some do not. So even when the plant kingdom was defined very broadly, it did not include all green organisms; some were protists.

By the time I was done with my college education, which took nine years from BA to PhD, the classification I had learned as a freshman had been replaced by a five-kingdom classification: animals, fungi, plants, protists, and monerans (Whittaker 1969, Margulis 1974, Margulis and Schwartz 1988) (Fig. 1–2). In this classification all bacteria and bacteria-like organisms were placed in Kingdom Monera. Fungi, long recognized as a distinct group of organisms, were removed from the plant kingdom and treated as a separate kingdom. A number of animal-like, fungal-like, and plant-like organisms, mostly motile and unicellular, remained classified as protists. Four of these kingdoms (animals, fungi, plants, and monerans) were well but narrowly defined, and any organism thus excluded ended up classified as a protist. Perhaps the most surprising change was the removal of all algae from the plant kingdom, leaving it restricted to just land plants!

Fig. 1–2 Five kingdoms of living organisms. Monerans (bacteria and all similar organisms) occupy the basal position as lower or primitive life. Protists (composed of largely unicellular and small, simple multicellular organisms) occupies a position intermediate between the monerans and the kingdoms of higher, multicellular organisms: plants, fungi, and animals.

Traditionally the term protist refers to unicellular and small, simple multicellular organisms—for example, filaments and colonies. While some algae are motile and unicellular or of simple multicellular organization, like other protists, other algae are large, complex, multicellular seaweeds like the giant kelps. Classifying seaweeds as protists did not just enlarge the concept of a protist a little, it bent it completely out of shape. One creative solution was to not change the composition of this catch-all kingdom, but rather to change the name of the kingdom from Protista¹² to Protoctista, meaning first to establish (Margulis and Schwartz 1988), thus eliminating the conceptual problem of calling great big seaweeds protists via creative relabeling.

Not much came of this proposal. Scientists actually vote on such proposals, not in a formal ballot or in any organized manner, but by using or not using the proposed term or concept in their teaching and publications. If protoctista had been used by enough biologists, then this term would have found its way into mainstream biology. Sometimes the decision to use a new term or classification is partly a matter of opinion or preference; however, if a proposal is accompanied by some new and compelling data, then the likelihood it will be adopted improves greatly because it has the backing of evidence, the scientific trump card. For example, biologists readily accepted a new classification that placed brown algae within a diverse assemblage of algal and fungal-like organisms because they share a novel cellular organization, and for a while this grouping formed its own kingdom, but more on this later.

While this restrictive classification simplified the plant kingdom, and greatly shortened the syllabi for plant diversity courses, this hypothesis produced some huge problems. Even at that time no one ever doubted that green algae and land plants shared a common ancestry even though better evidence would be forthcoming. In this classification, two groups of organisms with a common ancestry are in different kingdoms (protists and plants), and similarly other protists were considered to have a common ancestry with either animals or fungi. This classification only makes logical sense if you view the relationships among these five kingdoms in terms of increasing complexity, as opposed to common ancestry (Fig. 1–2). The three kingdoms (animals, fungi, and plants) that contained all the large, complex multicellular organisms (except for the seaweeds) were viewed as advanced or higher. The protists were unicellular or simple multicellular organisms, so they were viewed as intermediate between the primitive or lower organisms (the monerans) and the three higher kingdoms. The fundamental flaw in this type of thinking would be exposed later, but it resulted in goofy statements like there is no such thing as a unicellular plant (Kaveski et al. 1983).

In spite of its flaws, the five-kingdom classification held sway for almost 25 years. By this time, several new lineages of organisms had been identified, leading to a total of seven, eight, or more kingdoms. Clearly the whole question of kingdoms, and the traditional classification hierarchy, had changed dramatically. Many biologists were no longer trying to decide how many kingdoms there were, but rather trying to decide whether the concept of a kingdom made sense any longer at all.

Why does it matter? To those of us who use classifications to explain things, it matters very much. Chlorophyll is found in only two places: in certain bacteria and in chloroplasts, the photosynthetic organelle of all other green organisms. As it turns out, the two places are exactly the same, but for the longest time we did not know that. Evidence strongly supports the hypothesis that chloroplasts were free-living photosynthetic bacteria that became cellular slaves within a host cell. While this hypothesis explains many unusual features of chloroplasts, biologists wonder how many times this evolutionary event has happened. If red algae, green algae, and land plants share a common ancestor, then chloroplasts need to have been acquired only once. Otherwise chloroplasts must have had two or more independent origins. Evidence suggests the chloroplasts of red algae and green algae were passed around quite a bit, and the actual number of times chloroplasts have been borrowed or stolen all depends upon common ancestries.

Historically, this discussion about kingdoms is all backwards. Classification did not begin by deciding on kingdoms, the most general of classification groupings, and working down, it began with individual species and worked its way up. Several species concepts exist, but for our purposes a species is a group of similar organisms. Some definitions emphasize that the members of a species have the potential to interbreed—that is, members of a species share a common pool of genes, and thus species are real biological entities in a reproductive sense. But taxonomists seldom apply the interbreeding rule to decide upon species, and sometimes keep species separate even if they readily hybridize with others or reproduce only asexually. In practice the limits of species are determined by observing patterns of variation among a sampling of individuals that have been collected. When a significant discontinuity in the pattern of variation is encountered, someone decides whether these two groups constitute different species, and then each gets its own species name. The underlying assumption is that the discontinuity in variation results from a genetic isolation, and thus the two species concepts are connected in theory if not in practice.

Of course, classification greatly predates science, and many traditional and useful classifications of various portions of the natural world exist and persist. We all refer to weeds and wildflowers, toadstools and mushrooms, trees and shrubs, and so on, collective terms for certain sets of organisms. Such classifications are very arbitrary, and they can vary from time to time, from place to place, and from person to person. Some aspects of scientific classification are also somewhat arbitrary—for example, the amount of specific variation that constitutes different forms, different varieties, and different species can vary from classifier to classifier. Classification criteria are not universal, even in biology—for example, in comparison with botany, the genera and families in ornithology (avian biology) seem extremely narrow and based upon the slightest of differences. There are only about 9,000 species of birds in total, but they are organized into more than 200 families, which make Class Aves about the same size as a big flowering plant family (e.g., the grass family has about 8,000 species). In contrast, the 220,000 or so species of flowering plants are organized into not quite 400 families. Clearly botanists and avian biologists have different family concepts. Scientific classification differs from the many folk classifications in its scope and its intent because it attempts to be universal, classifying all living organisms, and in the process, biologists try to document how the natural world is organized, a necessary prelude to seeking underlying mechanisms.

The science of classification, taxonomy, began with the purpose of naming each and every species. In 1753 Linnaeus proposed using only binomial (= two names) species names consisting of a genus and a specific epithet to replace long descriptive names. Most of the 5,900 plant species names coined by Linnaeus still exist and they can be recognized by the initial L. following the species name—for example, Liriodendron tulipifera L., tulip tree (Table 1–1). Linnaeus gave catnip, one species of the cat mints, the species name Nepeta cataria L. Previously catmint was called Nepeta floribus interrupte spicatus pedunculatis (cat mint with stalked flowers in an interrupted spike). Clearly the latter name conveys more information, but such names quickly become a challenge to the memory. Remember Liriodendron tulipifera is the binomial, the species name. One of the most common of biological mistakes is to call tulipifera the species name; this is the specific epithet, which functions as an adjective, so it cannot stand alone. Specific epithets can be used more than once by combining them with different genera—for example, Phaseolus vulgaris, common bean, and Thymus vulgaris, common thyme. Each genus must be a unique name.

The genus was the first collective classification category grouping two or more similar species together based on more general shared features. The genus Magnolia has about 80 species including Magnolia virginiana L., Magnolia grandiflora L., Magnolia tripetala L., and Magnolia acuminata L. Observations of shared features throughout the biological world made it clear that bigger and bigger sets of organisms shared more and more general features, and thus higher classification categories were constructed (Table 1–1). Magnolia and Liriodendron share numerous general features and so these genera were placed in the magnolia family (Magnoliaceae) along with several other genera. The magnolia family in turn shares features with other families like the nutmegs and custard apples, and so this set of families was placed in the order Magnoliales. This order shares with a great many other orders the feature of embryos bearing two leaves (i.e., two cotyledons) and so formed a class of dicotyledons¹³ (Magnoliopsida). Dicotyledonous plants (dicots) together with the monocotyledonous (monocots = embryos with one leaf) plants (grasses, palms, orchids, and gingers) constituted the flowering plants, the angiosperms. In traditional classification, angiosperms are usually treated as a phylum, the Magnoliophyta.¹⁴ Angiosperms, along with all the other phyla of land plants, either alone or with one or more algal groups, constitute the plant kingdom. In a similar manner all biological diversity was organized into a classification hierarchy from species to kingdoms.

Table 1–1. Classification of tulip tree

Genera and binomial species names are written in italics because they are Latin in form. Genera are capitalized; the specific epithet is not. A name, abbreviated name, or initial(s) following a species indicates who named the species, which in this example is L. for Linnaeus. Boldface suffixes are standardized endings denoting that taxonomic rank when added to a generic root. Irregular taxonomic names not based on generic roots can be found in older literature, e.g., Angiospermae, Dicotyledonae. Botanical taxonomy formally uses Division instead of Phylum, but I opted to use the well-known term phylum to eliminate some unneeded jargon.

Discovering and giving a scientific name to all organisms was a large task, but in the 1770s it seemed an attainable goal, especially from the perspective of European biologists. This is because only a small fraction of the Earth’s biological diversity resides in Europe, and little did they know what exploration of the world beyond Europe, especially the tropics, would yield. Today the quest to identify and name all species continues with no end in sight, but modern taxonomists are faced with the rather gloomy prospect that many species will become extinct before they are known to science. This situation is so chronic some of my colleagues are even proposing that we give up trying to name newly discovered organisms, which takes too much time and study, and just try to find out how many species exist. But the number of species gets hard to count, and new species get hard to identify if previously discovered species have not been accurately described and named.

Another problem is that fewer and fewer people are studying taxonomy at the species level. In fact it is getting hard to find experts who can identify organisms for those of us who are not experts. One such taxonomic expert is the youngish fellow who occupies the office next door. He is without question the world taxonomic expert on an order of insects, the Psocoptera. Having accumulated some experience and knowledge, he is at the peak of his game at the age of nearly 80. Who will replace him? It is not that there is no need, but such biological study is no longer fashionable or fundable, and without some major research groups at botanical gardens and museums, this level of taxonomy would just about have ceased to exist. Taxonomic experts are themselves becoming endangered species.

Nonetheless this shift in emphasis away from taxonomy and toward higher-level phylogenetic studies during the past thirty years has changed our understanding of evolutionary relationships more than at any time since Darwin. Our knowledge has grown primarily from molecular studies that use shared characters in the genetic material itself, DNA. DNA is a long linear molecule composed of two strands of alternating phosphates and sugars.¹⁵ Each sugar carries one of four different nucleic acids, ATGC,¹⁶ each of which pairs with a nucleic acid on the other strand, which runs in the opposite direction. Because each nucleic acid always pairs with the same counterpart (A-T, G-C), the two strands have complementary sequences of nucleic acids (CATTAGG—GTAATCC). The nucleic acids are the letters of the genetic alphabet and they form three-letter words, the genetic code. Four letters taken three at a time yield 64 possible codes that with some redundancy stand for one of 20 amino acids plus a code for start and stop. For example, GGG, GGA, GGC, GGT all code for the amino acid glycine. So a particular sequence of nucleic acids can be read like an amino acid book to construct a particular protein with either an enzymatic or structural function.

DNA is copied with great fidelity, but not perfectly, so mistakes, mutations, occur. Mutations can vary from substituting one nucleic acid for another, which happens on the order of one mutation per 100,000 copies, to major rearrangements of genetic material like gene duplications or inverted nucleic acid sequences, which occur far less frequently. Shared sets of changes in the nucleotide base sequences are more likely to have been acquired via a common ancestor than via numerous independent chance events. Confidence increases as the size of the data set and the number of shared features increases because the odds of the same changes happening two or more times independently by chance alone go down as the number of similar changes goes up. Mutations are the ultimate source of genetic variation, although they are often thought of only as harmful changes. Some mutations cause lethal changes—that is, they render a gene nonfunctional; some alter a gene function. The new version might work better or worse, and that may depend upon the environment too. Many mutations are known to be neutral, not altering gene function—for example, a mutation in the third position of the glycine code (above) has no effect on the protein even though there was a mutation. Phylogenetic studies find and compare shared changes in DNA sequences, mutations that have been successfully incorporated into a gene. The more changes two organisms share, the more likely it is that they were acquired via a common ancestry.

In particular, molecular studies have shed considerable light upon the phylogenetic relationships among microorganisms, which in turn has led to considerable discussion of kingdom-level classifications among all living organisms (Blackwell 2004). New kingdoms have been proposed to accommodate newly defined groups with two consequences. First, the five-kingdom classification has been falsified. Second, sooner or later a decision will have to be made about whether each distinct lineage defined by common ancestry should be called a kingdom, or whether the concept of a kingdom remains useful at all. This problem will be examined in a couple of chapters. Even if everyone could agree, a taxonomic problem remains because the current phylogenetic pattern of life requires many more nested sets than provided by the traditional classification hierarchy.

The classification of tulip tree presented above does not show the problem (Table 1–1). But we now know that flowering plants are nested among seed plants, which are nested among woody plants, which are nested among megaphyllous plants, which are nested among vascular plants, which are nested among land plants, which are nested among a certain group of green algae, which are nested among the rest of green algae. And this is not everything! Quite a few new taxonomic levels would have to be added between phylum and kingdom to accommodate what is known. So this is why traditional taxonomy will be largely ignored from this point on, both in this book and in biology.

No consensus exists about what to do. A lot of new nonhierarchical names have been proposed for newly identified or redefined groupings, and some are both informative and useful. However, as much as possible familiar and common place names for groups of organisms will be employed because using all these new names becomes counterproductive when trying to communicate with the more general reader.¹⁷ However, some familiar names refer to groups of organisms that have been found to be artificial assemblages—that is, those classification hypotheses have been falsified, and it would be incorrect or misleading to continue using those names. In some instances, these names can be redefined and used with care; in others, the name must be changed.

This common portrayal of the evolutionary relationships among the traditional five kingdoms of organisms (Fig. 1–2) demonstrates a popular misconception that requires a phylogenetic perspective to correct. The concept implied is simple: monerans are the smallest, simplest forms of life so they gave rise to more complex unicellular and colonial protists, which in turn gave rise to multicellular organisms. While in some general sense this is true historically, it fails to correctly represent relationships among living organisms. Present-day protists are not ancestral to fungi, plants, or animals. And present-day bacteria are in no way ancestors to living protists. More correctly, plants, fungi, and animals had a common ancestry with organisms that if alive today would be classified as protists, but modern protists have the same ancestors. Similarly, if bacteria-like organisms were ancestors of protists, then modern monerans have the same ancient organisms as ancestors. To correct this diagram, it should be redrawn as a tree where the end of each branch represents living organisms and the joining of the branches to the trunk represents the common ancestries (Fig. 1–3).

Fig. 1–3 Major branches (lineages) of the tree of life whose branching pattern shows the hypothesized ancestral relationships. Generally members of the Protista are considered intermediate between the monera and the higher or multicellular organisms: plants, fungi, and animals. However, this diagram depicts a very different relationship than Figure 1–2. Here all five kingdoms occupy the ends of branches. Xs indicate places where divergent lineages above had a common ancestry.

The branching pattern of this tree of life only generally hints at hypothesized common ancestries as presently understood. Its shape implies several relationships that have not been explored yet. Fungi were placed closer to animals purposely even though traditionally fungi have been considered botanical organisms. Plants were placed closer to the protists. The animal-fungi branch and the plant-protist branch join further down, suggesting a more ancient common ancestry of all organisms with nucleated cells, and finally at a much lower branch, the monerans have a common ancestry with all the other

Reviews for How the Earth Turned Green

[subarcticmike · Rating: 4 out of 5 stars]

Engaging and humorous "brief" history that backstops a whole lot of science at a time when paleobiology, molecular clockwork and fossil record realignment are shaking the tree of life from base to top. Armstrong is a dedicated, life-long botanist who has thoroughly prepared much material and enlightenment for the non-botanist. But it's still quite a workout with lots of highlighting and go-fetch in chapter-sized bites. A bigger glossary yet and/or some direction to online resources would help persons with college-level biology shortfalls who want to challenge themselves to a bit more (or much more) learning. At stake is one's essential self-interest, courtesy of immersion in a green world. Wonderful first edition! and timely book.