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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 great-
est 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.
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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 Road 3 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
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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 frac-
tured 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
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plants and animals, recovering the last remaining vestiges of energy. Plants are
the crucial green engines doing all the work to supply the energy that supports
life on land. After they convert the energy radiated from the Sun into energy-
rich organic matter, no other group of organisms adds energy into the food
chain, and everybody else extracts it. It is a straightforward rule of the natural world, placing plants indispensably at the base of the food chain. Without
plants, the herbivores starve; without herbivores, the carnivores starve. Without plants, the food chain collapses and there is nothing to sustain terrestrial life.
No mammals, no primates, no us. Herein lays the truth of that gnomic saying in
the Old Testament Book of Isaiah, ‘all flesh is grass’. Our improbable botanist-
astronaut Mark Watney understood the point well enough, as did John
Christopher in The Death of Grass. Christopher’s protagonists quickly realize that a virus wiping out wheat, oats, barley, and rye, then supplies of meat, dairy
food, and poultry, mean ‘all that’s left are the fish in the sea’. Satellite observations indicate that the photosynthetic productivity of phytoplankton in the
oceans that supports the fishes is roughly equal to that of plants on land. But the productivity of the land is squeezed and concentrated into one-third of the
planet’s surface, and this explains why most species on Earth (85–95% of all
organisms other than microbes) live on land.11 In forests and grasslands, for
example, the species diversity outnumbers that in the oceans by 25 to 1.
The establishment of plant life on land is, then, a prerequisite for sustaining a consumer society of land-dwelling animals. Insects were amongst the first animal
groups to stake out habitats on land, feeding on algal mats and early land plants right from the start.12 As the first-comers, they won the rights to digesting the cel-lulose that makes up plant remains by using bacteria in their guts, as insects do today, and presumably acquired those bugs by feeding on decaying vegetable matter. When the earliest limbed vertebrates (animals with backbones) crawled
ashore during the fin-to-limb transition, they struggled for millions of years to leave their aquatic ways behind. Early vertebrates, the ancestors of modern rep-tiles, birds, and mammals, inherited air-breathing lungs from their air-gulping
lobe-finned fishy relatives. They heaved themselves on to land around 370 million years ago, roughly a hundred million years after plants.13 Yet these lumbering rep-tilians were unable to eat plants directly. Encumbered with fish-like mouths, they fed instead by going into the seas to hunt animals. In the water, they could grab food items floating in front of them and use suction to draw it into their mouths
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and gulp it down. These primordial vertebrates took another 80 million years to
evolve jaws and teeth, adaptations that enabled them to properly exploit terres-
trial vegetation as a foodstuff.
‘ANIMALS ARE INFERIOR TO GREEN PLANTS’
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Professor Albert C. Seward (1863–1941), Master of Downing College and Vice-
Chancellor of Cambridge University, once wrote a no-nonsense book published
in 1932 and titled, Plants: What They Are and What They Do. Although written some time after the publication of his report on the fossil plants collected by Scott
of the Antarctic’s ill-fated 1910 Terra Nova expedition,14 this slim volume with its dull green cover failed to become a bestseller. Chapter Three, entitled ‘The
Superiority of Green Plants to Animals’, gives no quarter to the sensibilities of his zoological colleagues. In it, Seward forcibly argues the irrefutable point that plant life is the foundation stone of all living things. He emphasizes the obvious but often overlooked point that animals are unable to use carbon dioxide
directly from the atmosphere. They cannot transduce solar energy into
chemical energy. Animals obtain energy by burning carbohydrates during
respiration that plants manufactured from the sugars synthesized by photosyn-
thesis. No animal can manufacture its own carbohydrates directly from sun-
light, which is why Seward declares that ‘animals are inferior to green plants’.
Animals are inferior to plants in other ways too. Few animals live to reach
their hundredth birthday, for instance, while the lifetimes of trees usually exceed that milestone. Bristlecone pines enduring the inhospitable heat and dust of the
dry mountains of south-western North America live for thousands of years.
In the White Mountains of eastern California, the oldest specimen of all is an
astonishing tree nicknamed ‘Methuselah’, after the longest-lived person in the
Bible. Estimated to be over 4700 years old, Methuselah achieves its exceptional
longevity by repeatedly forming new structures and organs, such as needles and
roots. The continuous renewal of essential organs in this way constitutes the
tree’s winning strategy in the game of life, and plants adopted this modular
growth habit from their earliest days on land, half a billion years ago.
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Appreciating the essential role plants play in sustaining Earth’s astonishingly
rich biological diversity begins to make clear why the origin and diversification of plant life on land was such a pivotal chapter in the history of our planet. What
could be more important? It paved the way for the evolution of terrestrial ani-
mals, and ultimately led to the appearance of human beings, the most complex
organisms in the known Universe. Yet the inconvenient truth is that many of us
fail to appreciate the essential role plants play in our lives. We are, it seems, suffering from the affliction of ‘plant blindness’ (PB).15 Defined as an ‘inability to see or notice plants, leading to the inability to recognize the importance of plants in the biosphere and in human affairs’, PB is an ‘anthropocentric ranking of plants as
inferior to animals’.16
Although it sounds like something Seward might have come up with, PB was
actually put forward by North American academics in the 1990s. At one level the
explanation may come down to a question of timescales. The lives of plants
unfold on a different timescale to our own. Compared to our lives the actions of
plants often seem imperceptibly slow, but it may also be more deep-seated than
this.17 Our ancestral brains form part of a visual monitoring system wired by evolution over millions of years to detect animals rather than plants.18 Streams of
images containing 10 million bits of information are received and transmitted by
the retinas of our eyes every second and require visual processing by the optic
lobe. From this data onslaught, the brain extracts a mere 40 bits, fully processing only 16 of these to reach our conscious attention. How does the brain decide
which crucial 16 bits of information to focus on? The answer, shaped by evolu-
tion, is a matter of priority for survival. It searches for movement, colours, patterns, and objects that are potential threats. Our brains are hardwired to be more vigilant at noticing animals than plants and for good reason. You could imagine a plausible evolutionary scenario in which human survival rests on identifying
fellow humans as possible mates or foes, and recognizing animals not plants as a
deadly threat or a potential meal. Few plants are as immediately life-threatening and urgently demand our attention in the same way as snarling, slavering predatory
animals. Even John Wyndham’s fictional triffids are not fast, agile, life-threatening green aliens. Instead, he invents them as lumbering opportunists, farmed and
domesticated for their valuable vegetable oil, with stings docked, until their
time comes. Wyndham’s novel may lack psychological depth but is, in part, a
Darwinian parable in which
an environmental catastrophe threatens human
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survival, as a strange meteorite shower blinds all those who observe it. In this literal manifestation of PB, plants exact the ultimate revenge; incapacitated, sightless humans preyed upon by deadly, superbly adapted, ambulatory, carrion-eating
triffids.
Michael Crichton, through the eyes of palaeobotanist Ellie Sattler in Jurassic Park (1993), persuasively sums up the issue like this:
people who imagine that life on earth consisted of animals moving against a green background seriously misunderstood what they were seeing. That green background was busily alive. Plants grew, moved, twisted, and turned, fighting for the sun; and they interacted continuously with animals – discouraging some with bark
and thorns, poisoning others; and feeding still others to advance their own reproduction, to spread their pollen and seeds. It was a complex, dynamic process which she never ceased to find fascinating. And which we she knew most people would
never understand.
No wonder it is not easy seeing green and shaking off our deep-rooted zoo-chau-
vinism. Our brains are wired to notice animals, not plants, and our innate PB casts a new light of forgiveness on editors running on zoo-centric software. In 2009,
the international science journal Nature reported ‘15 evolutionary gems’ as part of the celebration of the bicentennial anniversary of Charles Darwin’s birth since the publication of the Origin of Species, intended to give readers ‘a resource . . . for those wishing to spread awareness of evidence for evolution by natural selection’.19 In an extraordinary oversight, plant life is conspicuously absent from the list that features, amongst other organisms, snakes, guppies, fruit flies, whales, lizards, birds, and water fleas. The top three gems from the fossil record were: #1. the
land-living ancestor of whales, #2. from water to land, and #3. the origin of
feathers. The explanatory text for #2 immediately captures our attention. Here we are told that ‘the first transition from water to land took place more than 360 million years ago’, and that ‘it was one of the most demanding such moves
ever made in the history of life. How did fins become legs? And how did the tran-
sitional creatures cope with the formidable demands of land life, from a desiccating environment to the crushing burden of gravity?’ Plants made land, and evolved remarkable adaptations to solve the tricky challenges of making a living out of
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