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botany /’bot(ə)ni/ n. 1. the study of the physiology, structure, genetics, ecology, distribution, classification, and economic importance of plants. 2. The plant life of a particular area or time. botanic /bə’tanɪk/ adj. botanical /bə’tanɪk(ə)l/ adj. botanically /bə’tanɪk(ə)li/ adv. botanist n. [botanic via French botanique or Late Latin botanicus from Greek botanikos, from botaneˉ ‘plant’: botany is from botanic]
OED
PREFACE
My ancestors farmed the south-east corner of England for three centuries, tending to sheep on the lowland pastures of the Kent marshes, reclaimed from the clutches of the English Channel by the Romans and later the Saxons. Another branch of the family has been cultivating pasture and dairy cattle on the farmlands of Essex since before the last world war. The poet John Betjeman (1906–1984), a fine chronicler of life in England (whose statue, incidentally, can be found inside St Pancras Station), wrote of the vanished beauty of nearby Romney Marsh ‘where the roads wind like streams through pasture, and the sky is always three-quarters of the landscape’.
For the farming community, life was harder and less romantic than Betjeman’s take on things. Shepherds tended large flocks of sheep that grazed hundreds of acres of pasture. Shearing, lambing, rescuing animals from drainage ditches, building pens, and treating wounds were routine matters for them. They holed up at night in simple huts, each a basecamp for storing tools and medicines that offered few comforts and afforded minimal protection from the severe winds that raced in unchecked from the sea. Peripatetic shepherding—one man, his dog, a hut, and large flocks of sheep—was a hard way to make a living and produced a tough breed of men. Their lives depended on a collective wisdom, knowledge handed down from one generation to the next, that ensured stocking densities were adjusted and flocks moved to the best pastures available as the seasons turned.
The shepherd’s livelihood depended on getting things right for the landowner by understanding how pastures fattened the white woolly grazing herbivores. The payoff came when the fattened animals were sent to the London meat market and the wool sold to the Wealden cloth industry or smuggled to the European mainland. Wool was a valuable commodity in those days, and the marsh men were quite prepared to risk a jail sentence for the monetary rewards to be had from smuggling and selling fleeces abroad, especially to France. Romney Marsh
sheep keepers maintained the pastoral economy in this way for over 500 years and surviving huts can still be found dotted across the region, relicts of times past.
My grandfather’s tools hang up in the shed I can see from my office window and connect the past with the present. The handles worn smooth by over five decades of hard labour on the marshes remind me of his understanding between plants, stock, and the land. He understood that the grazing pastures of Kent fuel the rural economy of the county and have done so for centuries, and this regional situation is really a parable for how the whole planet operates. Plants are the food stock of the biosphere and everything else follows from there. Without plants, there would be no us.
Over 7 billion people (and rising) depend on plants for healthy, productive, secure lives, but few of us stop to consider the origin of the plant kingdom that turned the world green and made our lives possible. The origin of humans is frequently considered in books and TV documentaries, but not the origin of land plants. And as the human population continues to escalate, our survival depends on how we treat the plant kingdom and the soils that sustain it. The evolutionary history of our land floras, the story of how plant life conquered the continents to dominate the planet, is fundamental to our own existence.
This, then, is the subject of Making Eden. Building on the foundations established by generations of scientists, it reveals the hidden history of Earth’s sun-shot greenery and considers its future prospects as we farm the planet to feed the world. Our evolutionary journey stretches back over half a billion years with twists and turns decoded from clues encrypted in fossils, DNA molecules, and the ecology of the plant kingdom. It is a story of how plant life on land originated from freshwater algal ancestors, how it inhaled, diversified, and spread out to conquer the continents, slowly air-conditioning the planet. Finally we are glimpsing answers to the question of origins that have haunted botanists ever since Darwin.
In some respects, Making Eden can be regarded as the prequel to my previous book, The Emerald Planet (2007), which actually had rather little to say about how plant life on land got going and sustains the diversity of life there. Instead, it offered a closely argued case for recognizing plants as a ‘geological force of nature’, drivers of changes, sculpting continents, and changing the chemistry of the atmosphere and oceans, to set the agenda for life on Earth. It was a long overlooked story that went on to sow the seeds for the three-part BBC2 television series How to Grow a Planet (2012).
But now the stage is set for Making Eden. My hope in writing this book is that it might at least give readers pause for thought before dismissing botany as boring and irrelevant. I also hope that it might persuade readers to think of plants, and the scientists who study them, in an entertaining new light. Steve Jones, a leading geneticist at University College London, and one time regular science columnist for the Daily Telegraph, wrote an article a few years ago entitled ‘Where have all the botanists gone, just when we need them?’ He rightly pointed out the central role of plants in the challenges facing humanity, and the challenges facing the subject of botany, writing ‘why do students find the vegetable world so boring when without it we would perish?’ Plants and botanists are in need of greater advocacy. Ultimately, I hope readers may come to appreciate that botany is an astonishing and deeply engaging field of scientific enquiry, with immediacy to all life on Earth.
d.b. Sheffield, 2018
ACKNOWLEDGEMENTS
Iam fortunate to be located in an outstanding academic department at the University of Sheffield, with many talented and generous colleagues. I offer sincere thanks to Jonathan Leake, Charles Wellman, Sir David Read, Colin Osborne, Ben Hatchwell, David Edwards, Jon Slate, and Pascal Antoine Christin, who all read and commented on various drafts, as well as shared and discussed ideas with me. The department houses a ‘writer in residence’ scheme funded by the Royal Society of Literature: conversations with successive incumbents, Fiona Shaw and Frances Byrnes, about writing and readers proved enlightening.
I offer warm thanks to the following colleagues from other institutes who kindly took the time to read and comment on early and late-breaking drafts of chapters, or have discussed issues that arose: Phil Donoghue, Burkhard Becker, Yves Van de Peer, Doug Soltis, David Hibbett, Steve Banwart, Chuck Delwiche, Jane Langdale, Nick Harberd, Jill Harrison, Lawren Sack, Peter Franks, Alistair Hetherington, Dominique Bergmann, Joe Berry, Ralf Reski, John Bowman, Stefan Rensing, Kevin Newsham, Chris Berry, Bill Stein, Linda VanAller Hernick, Frank Mannolini, Martin Bidartondo, Jeff Duckett, Dana Royer, Christine StrulluDerrien, and Jim Hansen. All of these individuals helped sharpen my thinking.
John Bowman and Stefan Rensing kindly shared copies of their unpublished manuscripts on the genomes of Marchantia and Chara, respectively. David Malloch helpfully discussed his mycological thinking with me, and both he and David Hawksworth provided the back-story on Kris Pirozynski and a picture or two. Peter Raven generously agreed to be interviewed and treated me to a memorable lunch in St Louis made unforgettable by his compelling narrative on the state of the planet, and plant life in particular. Paul Kenrick at the Natural History Museum, London, kindly provided a valuable critique on a late, nearly complete draft of the manuscript and caught some crucial errors. I thank all of these people whose work has helped shape and improve the text. Of course, any errors are my own responsibility.
We all need great mentors, no matter which walk of life we choose. In having the late William (Bill) Chaloner FRS (22 November 1928–13 October 2016) as a mentor for nearly two decades, I was fortunate to have had one of the best. Bill died before I completed this book, but had already offered me his encouraging, critical comments on several chapters. This book is dedicated to Bill’s memory; a man who really did ‘leave an afterglow of smiles when life is done’. In 1970, Bill published an influential academic paper entitled ‘The rise of the first land plants’ (Biological Reviews, 45, 353–77) in which he dealt with ‘the facts from which a hypothesis of evolutionary progression may be constructed’. Nearly 50 years on, I realize that I have unconsciously written my own update on that same thesis.
Financial support for my research group over the past decade has been provided by the Royal Society, the Leverhulme Trust, and the Natural Environment Research Council, UK, for which I am most grateful.
Sincere thanks must go to my patient and enthusiastic editor, Latha Menon, whose editorial comments and wise suggestions improved the text. My gratitude too, to Jenny Nugée, and the rest of the Oxford University Press team, who efficiently took the book through the publication process.
Writing this book has taken quite some time, which in one way has been helpful because the suspicion is that a long manuscript improves with a long gestation time. But the down side is the steeply rising forbearance required of my wife, Juliette, who additionally provided title inspiration. So there comes a point, after so many years, when you have to get on with it. Joshua’s imminent arrival helped daddy get a move on, so that we can all spend more time together growing apples, walking, bird watching, and learning how to fish. I thank Juliette and Joshua for their patience, love, and support, and look forward to our fun times together now that this is done.
Plate 1. Colonization of terrestrial habitats by streptophyte green algae, the group that includes all land plants. ZCC abbreviates the higher branching grades (Zygnematophyceae, Coleochaetophyceae, and Charophyceae) and KCM abbreviates the lower branching or basal grades (Klebsormidiophyceae, Chlorokybophyceae, and Mesostigmatophyceae).
Plate 2. Present-day species of charophyte algae, the group which includes the freshwater ancestors of land plants. (A) Klebsormidium nitens (Klebsormidiales), (B) the stonewort Nitella hyalina (Charales), (C) Coleochaete pulvinata (Coleochaetales), and (D) Spirogyra (spiral chloroplast) and Mougeotia (flat chloroplast) (Zygnematales). Note basal branch in Mougeotia (white arrow) and holdfasts (black arrows).
Plate 3. The diversity of plants. (A) An assemblage of Phaeoceros (a hornwort; white arrow), Fossombronia (a leafy liverwort; red arrow), and interspersed mosses. The arrows point to the sporophytes. (B) Lycopodium digitatum, a lycopod, showing spore-bearing cones. (C) A tree fern (Cyathea horrida), (D) the cycad Cycas revoluta, a widely cultivated cycad sometimes called ‘Sago Palm’, (E) Nymphaea hybrid, a water lily, a representative of one of the basal branches of flowering plants, and (F) Ampelopsis sp., grape family (Vitaceae) being pollinated by a wasp. Ampelopsis represents the eudicots.
Plate 4. Coastal redwoods, the tallest trees on Earth. This titan lives in Prairie Creek Redwoods State Park, California, USA and is probably over 1500 years old. Photo composed of a mosaic of 84 images.
Wild type “nor mal” moss
Moss line without the SMF1 gene (no stomata develop)
Moss line without the SMF2 gene (stomata develop)
Moss line without the SCRM gene (no stomata develop)
Plate 5. Normal moss develops stomata on its sporophytes, whereas lines lacking SMF1 and SCRM genes develop sporophytes lacking stomata. In contrast, those with the SMF2 gene develop normal stomata. The top line of images were taken with an epifluorescence microscope which causes the stomata to glow. The lower set are photos taken using a scanning electron microscope. Scale bar in all images = 50 µm.
Plate 6. Reconstruction of the early vascular land plant Aglaophyton with images of a cross section of a rhizome and fossilized fungal structures resembling arbuscules. Top right photo (cross section): ×15 magnification; bottom photo: × 600 magnification.
Stomata
Plate 7. Exhibit of sedimentary tree stumps outside the Gilboa Museum.
Plate 8. Reconstruction of the 385-million-year old complex forest at Gilboa, one of the first forests on Earth.
Plate 9. Mobile truck-mounted rig for drilling rock cores at a quarry near Cairo, New York State.
Plate 10. Rock cores of fossil soils drilled from a 385-million-year-old forest floor. Smaller image shows small tree (archaeopteridalean-type) root, termed a rhizolith, preserved as clay cast with central carbonaceous strand, surrounded by a ‘drab-halo’. Scale bar = 1 cm.
ALL FLESH IS GRASS
‘Going up that river was like travelling back to the earliest beginnings of the world, when vegetation rioted on the earth and the big trees were kings.’
Joseph Conrad, Heart of Darkness, 1899
The spectacular rise and diversification of plant life on land reshaped the global environment, and the possibilities of our lives. It was one of the greatest revolutions in the history of life on Earth. But there’s no need to take my word for it. Here is palaeontologist Richard Fortey’s view on the matter, writing in his book Life: An Unauthorised Biography, ‘There cannot be a more important event than the greening of the world, for it prepared the way for everything that happened on land thereafter in the evolutionary theatre’. This book tells the story of how it happened. Or, at least, the story of how we think it may have happened. A celebration of discovery and scientific enquiry, this book lifts the lid on the evolutionary story of how plants won the land. How the Earth went from being a dull, rocky, naked planet to today’s world cloaked in a wonderful diversity of plant life on which we all depend for our very existence. It looks also to the future as the sixth great extinction in the history of life on Earth looms unwantedly and alarmingly on the horizon.
First, though, let us take a step back. Imagine, for a moment, a strange alternative world without plants: a naked Earth shorn of its greenery. This alien planet is a favourite haunt of fiction writers and directors of post-apocalyptic movies, and for good reason. In a world where plant life never evolved to cloak the continents in green, never changed the fabric of the landscape or the cycling of elements through the biosphere, Earth’s planetary prospects for life support look bleak. Its barren, windswept landscapes are coloured in drab mackerel greys and monkey browns; leafless, treeless, grassless, and useless for supporting a diversity of animal life.
When science fiction writers describe a vision of humanity’s dystopian future, it is no accident that the destruction of plant life is a key motif. In John Christopher’s haunting sci-fi classic, The Death of Grass, 1 a fictional virus wipes out the crops that feed humanity, and the grasses that feed the cattle that feed the people. Mass starvation decimates Asia and when the deadly virus hits Britain, beleaguered survivors quickly discover that, when there is nothing to eat, society degenerates with alarming speed. In J.G. Ballard’s vividly imagined The Drought, 2 industrial waste has produced a mantle of artificial polymers over the oceans, destroying the hydrological cycle and transforming the planet into a wilderness of dust and fire. Ballard’s survivors ‘follow the road upwards, winding past burnt-out orchards and groves of brittle trees like the remnants of a petrified forest’. Half a century later, Cormac McCarthy’s The Road3 depicts a ravaged landscape with charred dead trees, rooted in scorched earth and coated in silver by drifts of ash. McCarthy introduces his survivors into this post-apocalyptic landscape where there is nothing to eat, no plants and no cattle; civilization has crumbled and, like the protagonists of Christopher’s novel, they face the terrifying dangers of a degenerate society. Grasses, incidentally, have form for bending animals to their collective wills and John Christopher also succumbed subconsciously to their charms. He settled in the pleasant medieval coastal town of Rye in East Sussex, the only town in England named after a grass.
Fictional works by Christopher, Ballard, McCarthy, and other practitioners of the post-apocalyptic genre succeed partly because they recognize that a world without plants spells disaster for humanity. This theme resonates with our own deeply rooted concerns about food and survival. Movie directors too have tapped into these anxieties. In Christopher Nolan’s 2014 blockbuster Interstellar, repeated crop failures slowly render Earth uninhabitable, prompting the need to evacuate the population to a new planetary home via a wormhole. The following year, Ridley Scott’s box office hit The Martian saw Matt Damon play astronaut Mark Watney marooned on Mars. Watney’s immediate concern is with the need to grow food. Being probably the only astronaut who ever trained as a botanist, he naturally harnesses his skills and expertise to improvise a potato garden, utilizing Martian soil fertilized with human waste.
The Romans recognized the central importance of plants to our world too. Any aspect of life deemed important enough had its own dedicated god or goddess, and Ceres was their goddess of agriculture, depicted on Roman coins with a wheat
crown standing on a chariot drawn by winged serpents. They worshipped Ceres because without her blessings, harvests might fail and starve the Empire. Our modern tribute to Ceres has been to name the largest asteroid in the inner Solar System after her. Ceres sits in an asteroid belt between Mars and Jupiter.4 Remote sensing surveys5 suggest Ceres (and Mars) contain deposits of the same sea-floor minerals that perhaps sparked life on a young Earth, billions of years before plants made land. Current ideas for the origin of life favour deep-water hydrothermal vent settings, with the microbes involved drawing their energy from seawater chemistry rather than from the Sun.6 ‘White smoker’ hydrothermal systems are currently prime candidate locations. Forming when minerals deep in the fractured oceanic crust react with seawater, white smokers are distinct from the hot, acidic ‘black smokers’ that give birth to new sea-floors as the continents shift apart.7 The warm (ca. 70ºC), alkaline hydrothermal fluids of white smokers move up through the splintered crust to emerge at the sea floor rich in dissolved hydrogen.8 Towers of calcium carbonate develop, reaching upwards 60 metres or more from the sea floor, each riddled with networks of tiny pores and adorned with feathery fans of minerals. These porous spires, bathed in volcanic hydrogen-rich effluent, may have offered the ‘goldilocks’ environment for booting up life—not too hot, not too cold, and not too acidic.9
Today’s green continents are an evolutionary legacy of those distant events marking the dawn of life. Instead of drawing energy from seawater chemistry like their microbial progenitors, plants harvest the solar energy showering our planet. That free source of energy has travelled an astonishing 93 million miles in just eight minutes. Two-thirds of it hits the world’s oceans, where it drives photosynthesis by marine plants, mainly free-living phytoplankton. These microscopic plants form the base of the oceans’ food chains.10 One-third of it hits the land surface, where the leaves of forests, grasslands, and crops capture it to power photosynthesis and synthesize biomass—organic matter—from carbon dioxide and water. By converting solar energy into chemical energy stored in the organic carbon compounds that make up their bodies—the tissues of leaves, roots, shoots, flowers, and grains—plants act as nature’s wonderful green energy transducers. Herbivores eat plants, and carnivores eat herbivores, with each group of organisms extracting energy as they inexorably convert plants into flesh. Finally, fungi and bacteria, the microbial heroes of decay, employ a remarkable repertoire of metabolic tricks to feast on the decaying
