Making Eden

by David Beerling

  The dispersal of ferns occurs largely through the release of millions of tiny spores from packets located on the underside of arching fronds. Those spores, landing

  in a suitably moist environment, germinate into a small, filmy, heart-shaped sex-

  ual plant called a prothallus. The prothallus is a small package of cells that looks superficially like a liverwort, and which produces male and female sex organs.

  This is the gametophyte generation. Cells on its underside release male sperm

  into a film of water that allows them to wriggle towards the egg cell and fertilize

  36 a FiFt y shades oF green

  it. After fertilization, an embryonic fern develops by producing a little leaf above the gametophyte, and a little root below. Initially the young plant is supported

  nutritionally by the gametophyte but it quickly becomes independent. After the

  development of a few more leaves and roots, it grows into an adult plant, com-

  plete with graceful fronds for spore production. In ferns and clubmosses, this

  adult plant is, of course, the sporophyte generation. It has become a large, freeliving entity, able to shed millions of spores into passing air currents. But that freedom comes at a price. The prothallus required for sexual reproduction

  remains small and confined to damp soil, where it is vulnerable to being eaten,

  drying out, or starvation if neighbouring plants interpose and cut out life-giving sunlight.

  Seed plants—the gymnosperms and flowering plants—solved this and the other

  problems facing the ferns and their kin by moving the locus of sexual generation up into the canopies of the forest. This opened up a new range of possibilities for ensuring the sex cells could meet each other without the need for water-based

  transportation. Take the conifers, for example: all have the same basic life cycle, involving the production of distinctive male and female cones. Male cones manufacture masses of wind-blown pollen grains and the female cones hold the eggs.

  When a pollen grain lands on a female egg-bearing cone a remarkable feat of

  endurance ensues, the individual grain slowly growing a pollen tube down towards

  the egg, sometimes taking a whole year to reach its final destination. Once it

  reaches the egg, the tube releases its sperm cell, and fertilization occurs without the need for water. The fertilized egg remains on the cone for a further year. During that time, rich food supplies are laid down around it, and it is further wrapped in a protective coat to form a seed. More than two years after fertilization, when the cone dries, the segments slowly creak open, releasing the seeds, which await conditions suitable for the germination of a next generation. Conifers had at last eliminated the need for water to transport sperm for fertilization.

  These points were not lost on the American anthropologist and gifted natural

  science writer, Loren Eiseley (1907–1977), who wrote with an almost poetic sensi-

  bility on plant evolution in his collection of essays entitled The Immense Journey. 73

  Selling over a million copies at the time of publication, the book become a runaway bestseller to an adoring public on both sides of the Atlantic. In a single paragraph, he provided a neat summary of the immense journey of the title:

  FiFty shades oF green a 37

  Slowly, toward the dawn of the Age of Reptiles, something over two hundred and

  fifty million years ago, the little naked sperm cells wriggling their way through dew and raindrops had given way to a kind of pollen carried by wind. Our present-day

  pine forests represent plants of a pollen disseminating variety. Once fertilization no long required exterior water, the march over drier regions could be extended.

  Instead of spores, simple primitive seeds carrying some nourishment for the young plant had developed.

  Successful though the conifers undeniably are, they rely on wind-blown pollen

  for sexual reproduction, and this is a haphazard and biologically expensive busi-

  ness at the best of times: the majority of the pollen that trees release to the winds is wasted, failing to land on female egg-bearing cones. Prodigious quantities of

  pollen must be made, each cone manufacturing millions of grains; tap a male pine

  cone in spring and great clouds of golden dusty pollen diffuse into the air with

  nothing to fertilize. The solution to this problem, evolved by the angiosperms,

  was both elegant in its effectiveness and beautiful in appearance. That innovation was, of course, the flower.74

  Rather than producing masses of pollen and releasing it into the wind to take

  its chances, flowers provide receptacles containing male pollen-producing organs

  and female egg-containing organs for receiving the pollen of other flowers. Unlike modern flowers, those of the earliest flowering plants were probably small and

  inconspicuous, and pollinated by the wind and insects, though it is difficult to be sure about this.75 Later lineages evolved flowers that shifted the balance of power towards plants. They attracted insects as animal couriers that would make precise simultaneous collections and deliveries of pollen. By producing scent and brightly coloured petals to advertise the rewards on offer, flora gained a certain mastery over fauna. Fertilization of a flower is independent of water. It leads to the development of an embryo encased in a seed supplied with its own energy reserves.76

  What could provide a better start in life?

  Notice that, by this stage of plant evolution, the gametophyte generation in

  flowering plants has dwindled to virtually the smallest possible biological

  denominations: the male version is simply the pollen tube with sperm cells; the

  female version is a handful of cells containing the egg. They are so small that

  botanists invented a new term for them—microgametophytes. So an ‘immense’

  evolutionary journey that started with gametophytes as the earliest dominant

  multicellular green life forms on land reached a point where this phase of the

  38 a FiFt y shades oF green

  life cycle is subsumed into the sporophyte generation. The sporophyte gener-

  ation now accounts for most of the amazing diversity of photosynthetic life

  forms on land.

  With the evolution of seeds and flowers, reproduction changed forever, and the

  need for water to achieve fertilization finally became superfluous. As biological inventions, both flowers and seeds are fascinating. This is not the place to become side-tracked by the marvellous array of specialized devices flowering plants

  evolved to trick insects, birds, and mammals into pollinating them. Or, come to

  that, to prevent self-pollination. Instead, here is Loren Eiseley again, this time providing a charming vision of the rise of flowering plants:

  The ramifications of this biological invention were endless. Plants travelled as they had never travelled before. They got into strange environments heretofore never

  entered by the old spore plants or stiff pine cone seed plants. The well-fed, carefully cherished little embryos raised their heads everywhere. Many of the older plants

  with more primitive reproductive mechanisms began to fade away under this

  unequal contest.

  The explosive diversification and global ecological takeover of flowering plants

  constitutes one of the most extraordinary and far-reaching bursts of plant

  diversification ever witnessed on Earth, and it happened within an astonishingly brief window of a few million years early in the Cretaceous.77 In the tropics, forests of angiosperm trees sprang up whose dense, complex canopies filled with

  animals such as butterflies and beetles exploiting the opportunities on offer high above the forest floor. The corpulent bees in Updike’s description of the Cretaceous diversified simultaneously with the evolution of specialized pollination m
odes by flowering plants.78

  By around 40–30 million years ago, another group of plants began to stir: the

  remorselessly successful grasses.79 Grasses and grassy habitats are now everywhere you look—think of the vast temperate grasslands such as the Great Plains of

  North America and the tropical savannas. Think also of the grasses that feed the

  world—rice, wheat, corn, barley, rye, and oats. Grasses owe their success, in

  part, to a remarkable feature of their biology: an ability to sprout new leafy

  shoots from underground corms and rhizomes. It ensures they can survive and

  regenerate following wildfire and makes them resistant to grazing. The spectacu-

  lar rise of grasslands across the continents was driven by climatic deterior-

  ation throughout the Cenozoic (the past 65 million years) as the great ice sheets

  FiFty shades oF green a 39

  of the world formed in the polar regions, locking in water and increasing global

  aridity and the incidence of fire. Humans have increased fire and grazing over

  the past 10 000 years of the Holocene epoch, and accelerated grassland expan-

  sion by clearing land to make it available for growing crops and the grazing of

  domestic animals.

  In the lower latitudes, grasses with a specialized photosynthetic pathway

  originated as their ancestors moved out of the shady tropical forests and into the bright, open, sunlit world of tropical woodlands and savannas.80 These are the C 4

  grasses, a name that derives from details of the photosynthetic biochemistry

  which turbocharged their productivity.81 Indeed, the most productive plant spe-

  cies in the world is a tropical C forage grass called Echinochloa polystachya. 82 Modern 4

  C grasslands account for nearly a quarter of the terrestrial primary productivity 4

  on the planet. The oldest C grass lineage is the Chloridoideae, which may have

  4

  originated nearly 30 million years ago.83 By far the majority of C grass lineages 4

  originated more recently than this, smouldering on a long fuse before diversi-

  fying to dominate the ecological stage within a few million years during the

  Miocene, around 7–8 million years ago.84 As fertile grassy savannas and plains

  around the world filled with C grasses, they began forcing the evolutionary hand 4

  of herbivorous mammals; in came herds of grazers, out went the browsers and

  the nibblers.

  Elsewhere, in the deserts of the globe, at around the same time as the grasses

  were conducting their grassy revolution, other plant forms like cacti and ice

  plants—the succulents—began seizing the initiative. Once upon a time, the his-

  tory of desert vegetation was a closed book. The potential for fossilization of plant life in such inhospitable regions is almost non-existent; conditions are too dry to favour organic preservation in the rock record. Cacti, like the epiphytic ferns, have left little behind in the way of fossils to tell us of their evolutionary history. Once again, though, analyses of the DNA of living cacti have uncovered how old these

  succulents are, and when they began populating the deserts of the world. Before

  DNA evidence, we knew little about when they evolved and estimates of the time

  of their emergence ranged from 90 million years ago to 14 million years ago. DNA

  sequencing of cacti now reveals the group to be about 35 million years old. They

  underwent a dramatic burst of diversification synchronously throughout the deserts of the New World late in the Miocene.85 Old World succulents, ice plants and their kin, diversified around the same time too. These radiations are tied to geological

  40 a FiFt y shades oF green

  events, such as the uplift of mountain ranges, and the opening and closing of seaways, triggering the expansion and intensification of arid climates.

  We can see now that a comparatively narrow window of Earth history wit-

  nessed the contemporaneous spread of specialized grasses and succulents across

  the global landscape, driven by a relentless exploitation of new ecological opportunities created by expanding arid climate regimes. By around 10 million years

  ago, Earth’s spectacular transition to a grassy planet, with deserts populated by cacti, marked the final stages of the evolutionary assembly of modern-day floras.

  Land floras recognizable as those resembling their modern forms had finally

  arrived. What started half a billion years ago, when the descendants of freshwater green algae moved on to land, triggered an unstoppable succession of ferns, forests, and flowering plants that transformed the continents. Of course the crypto-

  biotic crusts where it all began are still with us, and doing nicely thank you, living in extreme environments. They remind us that one form of plant life is not simply replaced by another. The modern plant kingdom draping the continents in colour

  is an astounding evolutionary legacy stretching back to green algae, and further

  back still, to the momentous capture of a cyanobacterium nearly two billion years ago. That cellular hostage-taking event handed the distant ancestors of all plant life the ability to photosynthesize. Nothing like it has ever happened since, and plants never looked back.

  In the end, the conquest of the land by green plants is really a story about an

  extraordinary developmental innovation that produced an extra step in their

  life cycle. This is the multicellular embryonic plan that we call the sporophyte

  generation. It provided the platform for all that followed, building bigger, more complex, and more productive plants, and ultimately emancipating them from

  the vagaries of water supplied by the weather gods for reproductive success. We

  would probably all concede that ‘modern parents hover at about A-level standard

  on the prehistoric fauna’ while ‘children are all PhDs’.86 Yet when it comes to

  appreciating prehistoric floras, their origins, and how our modern floras came

  to be, adults and children alike are firmly rooted at kindergarten level. The green tree of life constructed from fossils, DNA molecules, and a close reading of the

  biology of living plants, reveals the vanished ecology of the green world’s evolutionary history. It helps redress the plant–dinosaur balance. Instead of simply

  seeing plant life around us, we begin to grasp a sense of the complex and extraordinary evolutionary trajectory that brought it to this point, and to appreciate the

  FiFty shades oF green a 41

  importance of these developments for our own evolution and survival, and that

  of all other animal life on the planet.

  In Chapter Three, we discover how the genomics revolution of the past decade

  is revealing unprecedented insights into the major transitions in the evolution-

  ary history of our land floras, as a riot of green diversity exploded across the

  continents.

  3

  GENOMES DECODED

  ‘The genome revolution is only just beginning.’

  J. Craig Venter, 2010, Nature, 464, 676–7

  In 2008, scientists announced they had obtained the partial genome sequence

  of long-extinct woolly mammoths using DNA extracted from the hairs of a

  mummified mammoth buried in Siberian permafrost for at least 20 000 years.1

  A year later, they trumped this remarkable achievement by reporting the genome

  sequence of Neanderthals, the closest relatives of modern humans, who once

  lived in Europe and western Asia before disappearing 30 000 years ago.2 Inevitably, the world’s press seized on the Jurassic Park-like possibility of resurrecting mammoths and Neanderthals by mining these DNA strands and filling the gaps with

  gene
tic material from the next best thing, elephants and humans. Plant biologists cannot yet make these claims for such iconic forms of past life on Earth, but smile at the trivial distances back in time the studies reach. Genome sequencing of

  photosynthetic life forms at the base of the green tree of life takes us back nearly two billion years to the origins of photosynthesis itself.

  The genome of an organism is simply the entire DNA content packed inside its

  cells. Sequencing means determining the exact order of the four chemical build-

  ing blocks (‘letters’ or, more technically, bases) from which the DNA molecule is constructed. The DNA molecule is a double-stranded affair, with specific bases on each strand pairing together opposite each other in a manner strictly determined

  by chemical rules of attraction and repulsion. These letters code for small molecules known as amino acids. Strung together into long-chain molecules to make proteins, amino acids are the building blocks of life. Each sequence of amino acids

  yields a different protein. The staggering truth is that variations in a genetic alpha-bet of just four letters generate the enormously rich diversity of life on Earth.3

  Genomes decoded a 43

  It is worth pointing out early on that it would be a mistake to think of the

  genome as a series of genes linked together, immediate neighbours residing next

  door to each other. The reality is that genes are just a small fraction of the total DNA of a genome. Often genes are separated by vast tracts of repeated DNA

  sequence, whose function (if any) is not yet clear, and genes themselves can be

  split into multiple parts. In fact, genes, and those snippets of DNA that regulate gene expression, typically account for only a small fraction of total genome size.

  The rest of it is composed of stretches of repeated sequences that have accumu-

  lated over millions of years. These sequences, known as mobile DNA elements,

  can move around their host genome and multiply via ‘copy and paste’ or move

  via ‘cut and paste’ mechanisms. Over time, their activities generate huge vari-

  ations in genome size without apparent correlation with organismal complexity

  (Figure 4).4 This helps explain Steve Jones’ remarks in his book Darwin’s Island that