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Silvia’s work has also added a further dimension to this emerging picture. You might have read about how plants and fungi combine to produce a close symbiotic, or “mycorrhizal,” relationship. The fungi, interlocking with the roots of the plant, provide it with more water and soil nutrients, helping it to break down bedrock and soils and even protecting it from pathogens. Meanwhile, the plants provide the fungi with sugars for sustenance from photosynthesis. We now know that most land plants make use of this win-win partnership. About 90 percent of plant species today are supported by fungal helpers, and the mapping of species of fungi and bacteria between the roots of trees across the world has come to be known as the “wood wide web,” enabling flows of nutrients across entire ecosystems. This relationship was already thought to be ancient because of the presence of fungal structures, including little “treelike” structures referred to as arbuscules, in the cells of 400-million-year-old fossil plants. However, some bryophytes, namely mosses, lack these interactions, leaving the question open as to whether the earliest plants on land had also already established fungal partnerships. By studying the diversity and function of fungi across the bryophyte groups, and by sequencing the modern genomes of mycorrhizal fungi, researchers like Silvia have confirmed that this “subterranean cooperation between plant and fungus extends back to the roots of plant evolution on land,” as she puts it, and likely involved a greater diversity of fungi than previously thought. It is becoming increasingly clear then that the first land plants emerged earlier and, together with their fungal aides, had more significant impacts on the planet than often considered.8
Back in 2012, Tim’s team had used their simulation-based approach to make two significant arguments about the first land plants. They performed modern experiments showing that modern mosses, even with limited root systems, significantly boost the weathering of rocks to nearly the same degree as more complex, “higher” vascular plants. They do so by releasing acids from their tissues onto the surrounding land surface. The weathering of rocks traps CO2 from the atmosphere in acidic solutions with ions (bicarbonate, calcium), while at the same time CO2 is absorbed from the atmosphere by photosynthesis. Applying these new weathering estimates—which would be significantly increased by the findings of the 2018 study and the discovery that, unlike mosses, the earliest land plants would have already established a joint venture with fungi—enables an estimation of the planet’s CO2 concentrations and temperatures during the Ordovician. Although these computer models are inevitably imprecise to some degree, assuming that nonvascular land plants had managed to colonize just 15 percent of the current vegetated parts of the Earth’s surface between 475 million and 460 million years ago (plants make up around 32 percent of the Earth’s surface today), they suggested a staggering halving of atmospheric CO2 and a global cooling of at least 4°C to 5°C. The modern aim to limit global warming to 1.5°C in order to avoid a series of devastating tipping points for human societies around the world should put the scale of this planetary change into some perspective. The earlier dating of the first land plants to c. 500 million years ago could mean that these changes may eventually trace their origins to the late Cambrian period c. 490 million years ago. Nevertheless, it is in the Ordovician period that land plants become more widespread across the surface of the Earth. It is also in the Ordovician period that their predicted modeled impacts on earth systems become clearly visible in the available climate records.9
First, the reconstructed cooling was enough to trigger an “ice age” by the time of the Ordovician-Silurian geological transition 443.1 million years ago. The Silurian is yet another name derived from Wales. Although the chilly conditions at the start of this period may be more in keeping with popular ideas of a trip to the Brecon Beacons, the real reason so many of these periods have names deriving from this country is that a lot of formative work in the discipline of geology was done by British people in Britain, with the names of classic blocks of time first identified on these isles frequently being imposed upon the rest of the world. The drop in temperatures at the Ordovician-Silurian boundary was so extreme that it was one of the coldest periods in the last half billion years of the Earth’s history (over 1,500 times longer than we have existed on the planet!). Second, enhanced weathering would have released significant nutrients, and specifically the element phosphorous, from the previously barren rocky land cover. Phosphorus is critical to the growth of organisms. However, too much phosphorus can be a bad thing, particularly in aquatic settings, and today phosphorous pollution can stimulate a rapid growth of toxin-producing algal blooms. Back in the Ordovician, the washing of newly weathered phosphorus into the oceans would have had a similar impact. Although these “blooms” would have supported more and more marine life, this would also have gradually sucked the limited oxygen supply from the water. Ultimately, suffocation and a mass extinction of marine life characterized the end of the Ordovician. The latest botanical and earth sciences research is therefore clear in showing that even the most “primitive” of land plants could, and did, dramatically alter the climate, atmosphere, and surface of the Earth. These plants survived their self-induced ice age, huddling in the warmer equatorial areas of the planet. There they waited, ready to grow and change the world to an even greater extent. While the oceans initially suffered, the land was bracing itself for an unprecedented expansion of life.10
ALTHOUGH THE FIRST plants certainly affected the Earth’s systems, these changes only began to stick with the arrival of the most complex of vascular plants, the trees. The earliest “higher” vascular (or “tracheophyte”) fossil body plant form, with xylem, phloem, and complex, invasive rooting systems that fit with our more traditional school-based definitions of “plants,” dates to around 420 million years ago in Canada, nearly 80 million years after the first plants colonized the land, and represents the beginning of the plethora of complex green plant forms we are familiar with today. Shortly afterward, during the Devonian (419.0 million to 359.3 million years ago), the first trees appeared on the planet. Trees had certain evolutionary benefits over other plants that enabled their rapid success. Unsurprisingly, their sturdy structure enabled them to grow taller and gain better access to light for photosynthesis. More photosynthesis meant more body mass, and their root systems could search more deeply and more widely for nutrients, with weathering from roots and their fungal collaborators increasing the rate of soil formation and greatly expanding the area in which subsequent plants could take root. More photosynthesis also increased the amount of CO2 absorbed from the atmosphere. As much of this CO2 became stored in robust lignin-based and, later, woody trunks resistant to decomposition after death, it also became locked away from the atmosphere, increasing the relative proportion of O2 in the air that remained. The true giants of vegetation cover, and thereby its impacts on the Earth’s systems, had arrived.11
Until recently, the story of our planet’s trees was thought to begin at a stone works near Albany, New York. Here, at Riverside Quarry in Gilboa, workers extracting stone needed for a nearby dam project in the 1920s discovered hundreds of log-like shapes, one after the other, sticking out upright from the rock they were chipping away. New York State’s first female paleontologist, Professor Winifred Goldring (1888–1971), was called to the scene, and her research on these amazing finds shaped discussions of ancient forests for a nearly a century. The trunks were identified as Eospermatopteris, a plant very similar to modern tree ferns. While it was considered to have a broadly “treelike” (what paleobotanists call “arborescent”) structure, Eospermatopteris lacked proper leaves. Instead, photosynthesis occurred on fronds (like those of palms) inserted into almost vertical branches. The trunks of Eospermatopteris, dating to around 380 million years ago, were also not composed of wood but rather consisted of a tough, hollow lignin structure. Later work in the 2000s identified a woody “aneurophytalean” vine at Gilboa that, with mosses, apparently climbed the Eospermatopteris tree forms. While Eospermatopteris has subsequently often been heral
ded as the first anatomical “tree,” the aneurophytalean vines were actually considered a better biological example of a “progymnosperm”—an early ancestor of the woody, seed-bearing trees of today—that is, until 2019, when a new fierce competitor for the title of the oldest “tree” emerged.12
The findings at Riverside had stimulated further paleobotanical investigation of the rock formations on the East Coast of North America that eventually led to the discovery of another major ancient fossil in the region. Again in New York, and again in a quarry (albeit this time abandoned), scientists uncovered an incredible record of a different type of ancient arboreal life-form. Dating to 2 million to 3 million years before Riverside, the new site at Cairo produced the same hollow Eospermatopteris and same woody aneurophytalean vines as Gilboa. In addition, the scientists found so-called lycopsids (distant relatives of the still living Lycopodium genus of club mosses, commonly known as “ground pines”) and liverworts. However, most amazingly, these Devonian sediments also preserved the remarkable root and body structure of a member of the Archaeopteris genus. Although Archaeopteris was known from previous studies, the Cairo record clearly demonstrated that, unlike Eospermatopteris, it also already had a clear set of seed plant (“spermatophyte”) features much earlier than previously thought. This included a vast, deep root system indistinguishable from those of modern seed plants; a large, upright form; thick, woody trunk; varied spore types; and flat green leaves for the efficient capture of sunlight and absorbance of CO2. As a result, it is considered to represent an ancestral lineage from which all seed-producing plants, which dominate the Earth’s forests today, evolved. Furthermore, while Eospermatopteris was largely limited to flooded, lowland swamp habitats, the root systems of Archaeopteris, and the ability of its sophisticated leaves to function in dry systems, enabled these trees to expand much more widely across the Earth’s surface, with the potential for wider-scale planetary impacts. By approximately 388 million to 359 million years ago, fossil records suggest that Archaeopteris was a dominant member of many forests in not only North America but also Morocco and China, among other places.13
Whichever group of organisms best represents the first trees, the Gilboa and Cairo fossil sites together provide a remarkable snapshot of the world’s first forest ecosystems. These new leafy kids on the block began to further reconfigure the Earth’s surface and atmosphere, and not necessarily to their own advantage. These novel Herculean plants were the world’s first “geo-engineers.” True, deep, wide root systems converted significant amounts of rock into plentiful, supportive soils. As the roots reached outward, they continued to form ever larger bonds with fungi and microbes that characterize the dynamic floors and soils of modern forests. Calcium, magnesium, sodium, iron, aluminum, and potassium were mobilized from the Earth’s crust into new, thick, nutrient- and even clay-rich soil ecosystems and mineral carbonates. Once again, however, not all of these minerals remained on land. As at the end of the Ordovician, they made their way into the Earth’s seas, driving an expansion of life. More rapid and wide-scale weathering trapped more CO2 in solution, further reducing CO2 in the atmosphere, alongside more efficient photosynthesis by these plants’ extensive leaf systems. This new plant-based shock to the planetary system once more sent the world into a drastic phase of cooling, even more significant than before. Polar glaciers pushed right up to the lower latitudes. Numerous extinctions occurred both in the oceans and on land at the end of the Devonian. There was also widespread decline of many of the first forests that had dared to stake their claim on our planet.14
Nevertheless, the pioneering biological and geological success of these tree-based configurations meant that the planet was now forced to contend with something of an “inevitability of forests.”15 By the time of the Carboniferous period, 359.3 million to 298.9 million years ago—a time when the supercontinent of Pangaea was forming, glaciers were contracting, and much of the continental landmasses of present-day Europe and America existed around the warm, wet equatorial regions—forests were well and truly back. Woody trees with thick bark, alongside their decking of vines and climbers, expanded in moist, swamp-like habitats across these latitudes. Indeed, the name of this geological period comes from all of the carbon that became trapped in the ground following the deaths of these new, more resistant trees. The thick layers of peat that formed from these extensive forests, in the absence of microbes and fungi that could digest tough wood fibers, were buried deep under the surface. There, millions of years of heat and intense pressure turned them into the Carboniferous coalfields, which geologists around the world could use to visibly recognize a distinct period. In an ironic twist of fate, these same coalfields, formed from forest habitats that had begun to turn the Earth’s surface into a lusher and more comfortable setting, were rediscovered as a key fuel source for the Industrial Revolution of the eighteenth and nineteenth centuries. The human-induced release of CO2 back into the atmosphere has reversed a process that began hundreds of millions of years ago, threatening not only the forest descendants of these Carboniferous ecosystems but also the habitability of various parts of the Earth’s surface for plants and animals (including ourselves).16
Various ancient trees (known as “lycophytes”) lived in these first Carboniferous forest environments, extending between forty and fifty meters high and spanning up to two meters in diameter. Sweeping vines climbed these gigantic arborescent versions of modern-day club mosses, while horsetails added density to the extensive, humid, tropical-swamp “coal forests” that, by approximately 300 million years ago, made up the majority of the now plentiful forest cover visible in fossil records dating to this period. The remaining third consisted of “progymnosperms” and even seed-producing trees, such as the now extinct giant “cordaites” that may have been ancestral to later conifers, ginkgos, and cycads, which broke free of the wetlands and adventurously struck out into well-drained, drier upland conditions. True seed-producing plants (“spermatophytes”) emerged in the Devonian, but they expanded across latitudinal zones within the warm conditions of the early Carboniferous. The fossil evidence for vast mats of roots from 305 million years ago shows the “earth-shattering” capabilities of Carboniferous forests—producing ample soils they could anchor into both horizontally and vertically. The resulting active, nutrient-rich soils were teeming with fungi, bacteria, and also now invertebrates (mainly insects), which were highly effective at breaking down organic matter and would not have been out of place on the ground-level of twenty-first-century tropical forests. Photosynthesis continued to absorb and trap significant amounts of CO2, leading to atmospheric levels less than five times preindustrial levels. At certain times, O2 was even more abundant in the atmosphere than it is today. The shift in the atmosphere during the Devonian and into the Carboniferous was so significant that even the plants themselves likely had to adapt. While we have already seen that stomata have inferred origins as far back as the Cambrian with the first land plants, there appears to be an increase in the frequency of, and surface area covered by, these pores on plants as the Devonian period wore on. It has been suggested that the proliferation of stomata and a growing function in gas regulation was an adaptation to declining CO2 and environmental conditions at this time, though some researchers dispute this. Regardless, overall mild global climatic and atmospheric conditions were now ripe for the expansion of life and, as we will see, a proliferation of familiar land-based organisms. Forests growing in the tropical latitudes of the time were the key catalyst for this process, which human hands are only now beginning to undo.17
IN CONTRAST TO our tour of the desolate landscapes of the Cambrian, if we strode through the new forests of the Carboniferous, we would feel significantly more at home. Walking across the Earth’s surface, we would now see organisms and ecosystems that looked like the plants and forests we know today. Compared to the chatter and squawking of diverse Amazonian rainforests or even the sounds of British woodlands, Devonian and Carboniferous forests would have been eerily silen
t. No herbivores yet existed that could process the new cellulose- and lignin-rich plants that had begun to populate the planet, leaving them largely unmolested to grow upward and outward. Nevertheless, the rapid formation of soils, thanks to the lofty trees, now provided moist, warm homes for millipedes and centipedes, consuming rotting plant matter or hunting other tiny organisms trying to make their way in this very new world. The earliest insects appeared in Devonian and Carboniferous forests, some flying through the humid swamp forests on wingspans as wide as two feet by the end of the latter period, thanks to soaring atmospheric oxygen. Predatory spiders and scorpions had likewise begun to call these lush forests home, while worms and snails likely slithered along the ground, although their soft body tissues have not been preserved well enough as fossils for us to be certain. Ultimately, if we were to dig away at the ground of the Carboniferous forests, the communities of creepy crawlies we uncovered would not differ significantly from those seen around the world today.18