Making Eden

by David Beerling

  lichens, tiny invertebrate animals, and bacteria. This extraordinary site was an

  ancient hot spring environment, analogous to that found in Yellowstone National

  Park, and its unusual setting explains the exceptional preservation quality of

  the fossils. As hot, silica-rich hydrothermal fluids bubbled up from below, they

  inundated the cells and tissues of the primitive flora and fauna surrounding the

  warm pools, preserving astounding microscopic detail. The resulting fossils are

  contained in a hard, siliceous fine-grained form of rock known as chert, and spe-

  cimens must be prepared by cementing pieces of the rock matrix to microscope

  slides and then grinding and polishing them to produce near-translucent ‘thin sections’. Laborious though such efforts may be, they can deliver spectacular results.

  Land plants at that time reproduced by the fertilization of female sex organs with sperm, and fossils prepared in this way record the young sperm developing inside

  the male sex organs. On release, the motile sperm would have swum through the

  Ancestr Al AlliAnces a 123

  watery environment and fer tilized the egg cells, just as happens today in the

  bryophytes, lycophytes, and ferns.

  The Scottish doctor and proficient part-time geologist William Mackie (1856–

  1932) made the sensational discovery of the Rhynie lagerstätte in 1912, quite by

  chance.1 ‘Lagerstätte’ is the name given to an exceptionally preserved fossil assemblage, derived from the German lager and stätte; literally meaning ‘place of storage’.2 One version of the story is that, after a strenuous morning of geological

  mapping, Mackie sat down on a dry stone wall to eat his lunch and reached for a

  nearby loose chunk of chert. On inspecting the rock in his grasp, possibly between mouthfuls of sandwich, Mackie was stunned to discover it was packed with fossilized plant stems. Alternatively, it may be that it was only after he returned to the lab and made thin sections of the blocks of chert found in walls, and littered

  around in fields, that the spectacular nature of his discovery started to become

  clear. The fossil collector, and member of the Geological Survey, Mr Tait was

  enlisted to undertake later exploration of the site, funded by the Royal Society, London, and the British Association.3 This involved digging a trench through the

  top soil (Figure 17) and excavating blocks of chert suitable for cutting petrographic thin sections to reveal the spectacular riches of fossilized early Devonian life on land.4

  The formidable and accomplished palaeobotanical duo of Robert Kidston

  (1852–1924) and his younger friend and colleague William Lang (1874–1960) exam-

  ined the thin sections. In the resulting series of papers, published in the Transactions of the Royal Society of Edinburgh, Kidston and Lang documented in extraordinary detail the external appearance and anatomy of the small-stature early land plants, and other organisms, found in the silicified deposits.5 These classic works included beautifully illustrated accounts of primitive vascular plants with upright leafless stems less than 20 cm tall and horizontal rhizomes adorned with rhizoids

  (Figure 18) and are now regarded as ‘the most important contributions ever made

  on the knowledge of the plants of the Devonian’.6 Discussions initiated by Kidston and Lang concerning the evolution of roots, shoots, and leaves, and the nature of plant life cycles, later proved foundational to the field of ‘evo-devo’ from the world of molecular biology, as we have seen.

  In the final paper of the series, Kidston and Lang dealt extensively with the fungi associated with the plant remains. There they wrote about how fungi appeared

  localized inside the tissues of the rhizomes and lower stems of the fossil plant

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  Figure 17 The village of Rhynie, Aberdeenshire, Scotland. Photograph below shows exposure of the fossiliferous chert beds beneath the mantle of soil.

  Ancestr Al AlliAnces a 125

  Rhynia

  gwynne-vaughani

  Rhynia

  major

  5 mm

  10 mm

  Figure 18 Original reconstructions of

  early Devonian fossil vascular plants

  by Kidston and Lang from the Rhynie

  Asteroxylon

  chert.

  mackiei

  Horneophyton lignieri

  5 mm

  10 mm

  126 a Ancestr Al AlliAnces

  Asteroxylon, writing: ‘the localization of the fungus to the inner cortex, leaving both the outer cortex and the phloem free, is in support of the view it was mycorrhizal’. They went on to cautiously draw the conclusion that, ‘although the evi-

  dence of a mycorrhizal relation is considerably stronger [than for Rhynia], it does not amount to proof’. Their reluctance to draw definitive conclusions hinged on

  the difficulty of distinguishing between saprotrophic fungi that invaded the plant material after its death, and symbiotic fungi that interacted beneficially with living plant tissues. Indeed, the majority of the fungi described based on fossil

  spores/cysts and filaments were saprotrophs.

  Clinching evidence followed a century later with the discovery that fossilized

  stems of another primitive plant from Rhynie, called Aglaophyton majus,7 contained beautifully preserved fungal arbuscule structures (from the Latin arbusculum, or ‘small tree’) (Plate 6).8 Fossils of these highly branched fungal structures were present within the cells of Aglaophyton rhizomes and prostrate stems, which the authors described as being ‘morphologically identical to those of living arbuscular mycorrhiza in consisting of a basal trunk and repeatedly branched

  bush-like tuft within the plant cell’.9 Later research discovered arbuscules in the gametophyte phase of the plant’s life cycle as well.10 Arbuscules are the characteristic structures of symbiotic arbuscular mycorrhizal fungi formed within the liv-

  ing cells of modern vascular plant roots by, whose importance for the symbiosis

  will become clear shortly. Other studies of Rhynie fossils have since discovered fungal spores with characteristic shapes and distinctive cell-wall structures indicative of several different groups of these fungi, leaving no doubt that arbuscular mycorrhizal fungi had evolved by this time.11

  Mycological discoveries from the Rhynie chert set the stage for the later publi-

  cation of one of evolutionary biology’s most provocative ideas, put forward by

  Polish-born Kris Pirozynski12 and his colleague, the Canadian David Malloch

  (Figure 19). In their classic 1975 paper13 entitled ‘The origin of land plants: a matter of mycotropism’, Pirozynski and Malloch argued that a beneficial partnership

  between mycorrhizal fungi and land plants catalysed the origin of our terrestrial floras. If you met either of our two mycological mavericks today, neither would

  strike you as a revolutionary. Since his retirement, Pirozynski has turned his back on mycological matters and become preoccupied with stamp collecting. Malloch,

  on the other hand, continues to teach mycology courses to enthusiasts at week-

  ends and vacations at the University of Toronto, where he is an Emeritus Professor.

  Ancestr Al AlliAnces a 127

  Figure 19 Mycological revolutionaries, Kris Pirozynski (left) and David Malloch (right).

  Forty years ago, they proposed that soil fungi facilitated plants’ transition to land.

  In 1975, they put the argument like this: ‘Terrestrial plants are the product of an ancient and continuing symbiosis of a semi-aquatic ancestral green alga and an

  aquatic fungus’. According to their theory, ‘the Silurian–Devonian “explosive”

  colonization of land, and indeed the very evolution of plants, was possible only

  through mutualistic partnerships—partnerships th
at were equipped to cope

  with the problems of desiccation and starvation associated with a terrestrial

  existence’. The following statement staked out their position: ‘Our claim is that land plants never had any independence [from fungi], for if they had, they could

  never have colonized the land’. And they only blotted their copybook by backing

  the wrong horse in claiming that a fungal group called oomycetes was involved.

  Oomycetes were classified as fungi at that time, but we now know they form a

  distinct lineage of fungus-like filamentous eukaryotic microorganisms, not sym-

  biotic fungi. Nevertheless, as we shall see, the notion of fungal symbioses with

  early land plants has gathered more and more supporting evidence ever since

  their pioneering paper four decades ago.

  We can begin to understand quite what a brave step it was to introduce the con-

  cept of symbiosis as a means of explaining the early appearance of land plants by considering how evolution was understood at the time. Darwin’s mechanism of

  evolution works through the competitive process of what the English philoso-

  pher Herbert Spencer (1820–1903) dubbed ‘the survival of the fittest’, but said

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  nothing about the ‘arrival of the fittest’. Symbiosis, the co-operation between distantly related organisms, rather than competition within or among closely related species, is a concept that requires a radical shift in thinking, and weighs in on the arrival-of-the- fittest question. Frank Ryan’s book popularizing symbiosis, Darwin’s Blind Spot, documents the gloves-off heated battle that took place in scientific circles before Darwinians accepted symbiosis as a valid mechan ism of evolution.14

  The point is that it challenges the idea of natural selection by competition as the only mechanism of evolution, and sees both partners in the symbiosis increase

  their fitness, rather than the expected outcome with one winner and one loser. This central argument of a zero-sum game also helped explain the speed of evolutionary change. In the traditional view, natural selection operating on a mutation-by-mutation basis was simply too slow to account for the ‘sudden’ appearance and

  diversification of plant life on land. The conquest of a radically different and hostile terrestrial environment from the watery milieu of freshwater algae was possible

  only (in their eyes) through a major evolutionary breakthrough, on a ‘scale equal to or surpassing that responsible for the Cambrian “explosion” of animal diversity’.

  Arbuscular mycorrhizal fungi, whose remains sit entombed within the fossil

  plants of Rhynie, now form one of the most widespread symbioses on Earth.

  Found in the roots of around 80% of all plant species, these fungi forge a symbi-

  otic alliance beneficial to both partners.15 In modern floras, host plants receive inorganic mineral nutrients scavenged by the fungi from the soil. In return, these plants provide the fungi with sugars and lipids (fats) produced by photosynthesis to enable them to grow, forage for nutrients, colonize other roots, and undergo

  asexual reproduction by spores. The exchange of materials—sugars in one dir-

  ection and inorganic nutrients in the other—occurs across the arbuscules inside

  root cells. The tree-like shapes of these sophisticated symbiotic interfaces generate a large surface area for the efficient two-way exchange of materials to take

  place. Note that arbuscular mycorrhizal fungi are obligate symbionts, meaning

  they are dependent on plants for providing them with energy to proliferate and

  reproduce. Unlike their saprotrophic fungal cousins, they lack the capacity to

  obtain energy by decomposing organic matter.16 Not all plants, on the other hand, are dependent on mycorrhizal fungi.

  High-precision radiometric measurements on the rocks and minerals date the

  Rhynie site at 407 million years old.17 This date broadly accords with the estimated age range for the probable origin of arbuscular mycorrhizal fungi based on

  Ancestr Al AlliAnces a 129

  OVERTHROWING SCIENTIFIC

  ORTHODOXY

  -

  The hypothesis that plants might form beneficial partnerships with soil-

  borne fungi reaches back to the nineteenth century, and had a culinary

  impetus. The German botanist Albert Bernhard Frank (1839–1900) deduced

  the essential details in the late 1880s when His Excellency, the Minister of

  Agriculture, Domains, and Forestry for the Kingdom of Prussia understand-

  ably commissioned him to investigate the possibility of cultivating truffles.

  Truffles are really the subterranean reproductive structures of fungi that

  grow attached to the roots of oaks and other tree species. The invitation

  changed the course of Frank’s scientific career and led him on a decade-long

  quest to understand plant–fungal collaborations. With simple but elegant

  and pioneering experiments, combined with careful observations of the

  roots of trees, he challenged the then current orthodox opinion that all fungi

  were harmful (i.e., pathogenic) to plants. In early experiments, he collected

  seed from pine forests and sowed one set in soil in its natural state and

  another in sterilized soil. Those seedlings germinating in natural soil formed

  fungal partnerships and developed into stronger healthier saplings than

  those that grew on the sterile soil lacking fungi. Tolkien, who was ‘much in

  love with plants, and above all trees’, made detailed descriptions of them in his writing and appeared to understand the benefits of their symbiotic partnerships with soil fungi. He saw the need to add ‘ a grain of precious dust from Galadriel in the soil with the root of each [tree]’, noting how it stimulated sapling growth.

  In proposing the revolutionary theory of tree nutrition involving a sym-

  biosis between roots and fungi bound into a single organ—a mycorrhiza—

  Frank faced formidable opposition from incredulous colleagues. It took 50

  years to settle the matter decisively but he never did crack the puzzle of culti-

  vating truffles. Cultivation of Périgord or black truffle ( Tuber melanosporum) and summer or burgundy truffle ( Tuber aestivum) is now possible in Europe mainly through methods that mirror Tolkien’s idea—spore inoculation of

  the soil into which the host tree is planted.18

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  molecular clocks.19 Rocks and molecular clocks, then, point to the great antiquity of associations between early land plants and arbuscular mycorrhizal fungi.

  Yet the fossils from Rhynie, those compelling messengers of past events, remain

  silent when it comes to telling us about the nature of the interactions between

  these two groups of organisms,20 leaving the presumed status of the symbiosis

  uncertain.

  Not surprisingly, Pirozynski and Malloch’s paper met with a sceptical reac-

  tion. Reflecting on it years later, Malloch remarked, ‘most of my botany depart-

  ment colleagues thought it was pure fantasy’.21 You can only suspect that, at the time, they were not alone. Nonetheless, Lynn Margulis, the prominent evolutionary biologist and great champion of symbiosis as a powerful creative force

  driving the evolution of life on Earth, promoted their extraordinary suggestion

  in her book Symbiotic Planet.22 There she asserts that ‘the great expansion on land up and out of both sea and freshwater was grounded in the intimacy of plant

  and fungus, and still is’. The evidence for this was little more than that which

  Pirozynski and Malloch relied upon in 1975—mainly fossils from Rhynie.

  Contrast this to the more cautious reading of the same fossil evidence many


  years earlier by Kidston and Lang, who wrote ‘The appearances [of fungi] in the

  Rhynie plants do not afford such evidence [of symbiosis], although they leave

  open the possibility that a mycorrhiza may have been present in the actively

  living plant’.

  So, with what might be regarded as a combination of brilliant insight and good

  luck, the duo offered the scientific community a major hypothesis that has hard-

  ened over the years into the current paradigm: mycorrhizal associations catalysed the origin and evolution of our land floras. Implicit in the argument is the idea that such ‘mycorrhizal associations’ were mutualistic, with both partners benefiting, like their counterparts in modern plants. Yet, surprisingly, this critical assumption has remained firmly in the realms of speculation for decades. Without definitive experimental evidence of how the partnership concealed in the fossil record might have functioned, it is difficult to say what, if any, benefits arose from the alliance.

  Scientific attention had instead focused exclusively on the details of the cellular interface between fungi and the cells of host plants such as liverworts, lycophytes, and ferns,23 without knowing whether the exchange of materials between organisms essential to understanding potential symbiotic interactions actually occurred.

  Ancestr Al AlliAnces a 131

  A few years ago, we teamed up with my mycological colleague, Jonathan

  Leake, and other collaborators, in an attempt to discover the nature of the alliance between early-evolving land-plant lineages and their fungal partners. The idea

  was to undertake experiments that blended mycology, ecology, and palaeon-

  tology. In doing so, we hoped to open a window for the first time on how this

  ancient partnership might have functioned during the greening of the Earth

  hundreds of millions of years ago. We focused on liverworts, as one of the oldest surviving lineages of land plants, and representatives of early colonists forming associations with mycorrhizal fungi.24 The rhizoids of liverworts provide the

  primary pathway of mycorrhizal fungal colonization, and this also seems to

  have been the case for some of the early land plants at Rhynie.25 The investiga-