The Green World Dominated by Giant FernsLooking back at Earth's history, much of the fossil fuels we humans currently use were created by the activities of life from long ago. Coal in particular was formed in large quantities during a period called the Carboniferous Period of the Paleozoic Era, approximately 300 million years ago. This era, literally named for the massive generation of coal, plays an important role as an energy source for us today. However, the formation process of this coal is deeply connected to the existence of a protagonist called fern plants and the peculiar fate they followed.The Carboniferous Period lasted from approximately 359 million to 299 million years ago, and the Earth during this time looked completely different from today. The climate remained warm and humid, and much of the continents were covered with vast wetlands and swamps. This environment provided ideal conditions for plants, and fern plants in particular flourished explosively. The fern plants of that time were incomparably larger than the small ferns we see in parks and forests today, with some exceeding 30 meters in height. These giant fern plants formed dense forests, creating a green world across the Earth.There are several reasons why fern plants were able to flourish to such an extent. First, the atmospheric carbon dioxide concentration at that time was much higher than today, creating a very advantageous environment for plant photosynthesis. Additionally, the warm and humid climate spread over wide areas, providing optimal conditions for fern plants that required moisture. Furthermore, organisms that could efficiently decompose plants had not yet sufficiently evolved during this era, which also helped propel the prosperity of fern plants.The fern forests spread on an unimaginable scale. Giant tree ferns stretched toward the sky, and beneath them, various types of fern plants grew in layers. These plants absorbed carbon dioxide from the atmosphere through photosynthesis and released oxygen. As a result, the atmospheric oxygen concentration during the Carboniferous Period is believed to have reached 35 percent, far exceeding today's approximately 21 percent. This high oxygen concentration enabled the appearance of giant arthropods. There existed giant dragonflies with wingspans of 70 centimeters and centipede-like creatures exceeding 2 meters in body length.The Miraculous Conditions That Created CoalHowever, the prosperity of fern plants did not continue forever. When these plants died and fell, they should normally have been decomposed by microorganisms and returned to the atmosphere as carbon dioxide. But in the early Carboniferous Period, fungi and bacteria that could efficiently decompose lignin and cellulose, the main components of plants, had not yet evolved sufficiently. Therefore, the fallen giant fern plants accumulated in wetlands without being decomposed. These plant remains formed layers in oxygen-poor environments covered with water and were gradually compressed.Over time, the accumulated plant remains changed due to pressure from above and geothermal heat. First they became what is called peat, and with further pressure and heat, they transformed into lignite, bituminous coal, and finally anthracite. During this process, the carbon contained in the plants was concentrated, forming what we now call "coal," a fossil fuel. The coal seams formed during the Carboniferous Period exist as enormous deposits reaching thicknesses of several meters to several tens of meters in various parts of the world.This large-scale coal formation had a major impact on the Earth's environment. As plants absorbed carbon dioxide from the atmosphere and it became trapped underground as coal, the atmospheric carbon dioxide concentration gradually decreased. Since carbon dioxide is a greenhouse gas, as its concentration dropped, the Earth's temperature also began to decline. Ironically, this meant that by prospering, the fern plants themselves were destroying the environment suitable for them.The conditions that made coal formation possible were extremely special in Earth's history. Three conditions needed to be met simultaneously: the existence of large quantities of plants, their accumulation without decomposition, and compression over a long period of time. The Carboniferous Period was precisely the era when these conditions were perfectly aligned.Environmental Upheaval Brought About by ProsperityIn the latter half of the Carboniferous Period, climate change became pronounced. The warm and humid climate gradually became drier, and cooling also progressed. This environmental change was fatal for fern plants that preferred humid environments. The vast wetlands shrank, and places suitable for fern plant growth decreased. Meanwhile, during this period, a new type of plant called seed plants evolved and rose to prominence. Unlike fern plants, seed plants had the adaptability to grow even in dry environments.The appearance of seed plants was a revolution in the plant kingdom. Fern plants reproduced through spores, a process that required water. In other words, a moist environment was essential for reproduction. However, seed plants evolved a drought-resistant structure called seeds, enabling them to reproduce even in environments without water. This difference became a decisive advantage in the increasingly arid Earth environment.Furthermore, in the latter half of the Carboniferous Period, fungi capable of decomposing lignin finally evolved. With the appearance of these fungi, dead plants no longer accumulated and became coal as before, but were decomposed and returned to the soil. While this meant the establishment of a natural cycle for plants, from the perspective of large-scale coal formation, it signaled the end of the Carboniferous Period. Indeed, after the Carboniferous Period, an era of large-scale coal formation never occurred again.When the Carboniferous Period ended and the next Permian Period began, the decline of fern plants accelerated further. Climate drying and cooling continued, and the giant fern forests that once covered the Earth rapidly disappeared. Instead, seed plants expanded their power and seized the leading role in the plant kingdom. Fern plants did not become completely extinct, but their former glory was lost, and they became limited to moist environments.The Lesson Left by Fern PlantsLooking back at this history, one senses deep irony in the fate of fern plants. They achieved unprecedented prosperity during the Carboniferous Period and dominated vast areas of the Earth. However, that very prosperity became the cause of their own decline. As fern plants absorbed large amounts of carbon dioxide and it became trapped underground as coal, the atmospheric carbon dioxide concentration decreased. As a result, the Earth's climate changed and the environment in which fern plants could grow was lost. In other words, fern plants collapsed their own survival foundation through their own success.We in the modern era have used coal, the legacy left by Carboniferous fern plants, as an energy source. Since the Industrial Revolution, humanity has developed civilization by burning large amounts of coal. Burning coal means releasing back into the atmosphere the carbon dioxide that fern plants absorbed from the atmosphere 300 million years ago and trapped underground. Ironically, humanity is returning to the atmosphere in just a few hundred years the carbon dioxide that fern plants removed from the atmosphere over a long period of time.This fact provides important implications for considering modern climate change issues. Carboniferous fern plants unintentionally changed the Earth's environment and invited their own decline. Modern humanity is also rapidly increasing atmospheric carbon dioxide concentration through the burning of fossil fuels, changing the Earth's environment. The difference between fern plants and humanity is that humanity has the ability to understand the consequences of its own actions and to deal with them.The tragedy of fern plants teaches us the difficulty of balancing success and sustainability in ecosystems. Short-term prosperity does not guarantee long-term survival; rather, there is the possibility of threatening one's own survival by greatly changing the environment. Fern plants dominated the Earth for approximately 60 million years, but were ultimately replaced by seed plants with greater adaptability. This history demonstrates the importance of the ability to adapt to changing environments.The carbon of fern plants trapped in coal has become, after 300 million years, an energy source for modern civilization. The story of how fern plants prospered and then declined is not merely a past event but contains an important message for those of us living in the modern age. That is the importance of understanding our own role in the ecosystem and maintaining harmony with the environment. Fern plants had the power to change the environment, but their ability to adapt to that change was limited. On the other hand, humanity has not only the power to change the environment but also the wisdom to predict its effects and implement countermeasures.The story of coal's prosperity and fern plants' tragedy is one example of the cycles of rise and fall repeated throughout Earth's history. However, it is not merely a lesson from the past but also a problem in progress. We are now using up the legacy left by fern plants while simultaneously changing the Earth's environment through it. Whether we can learn from this history and build a sustainable future depends on the choices of us humans.ReferencesRichard Fortey, translated by Masataka Watanabe, "Life: A Natural History of the First Four Billion Years of Life on Earth," SoshishaPeter Ward and Joseph Kirschvink, "Why Did Life Emerge?" Kawade Shobo ShinshaJapanese specialized books: "Paleontology," Asakura Publishing; "History of the Earth," Iwanami ShotenEnglish specialized books: Thomas N. Taylor et al., "Paleobotany: The Biology and Evolution of Fossil Plants"Academic papers: Robert A. Berner, "GEOCARBSULF: A combined model for Phanerozoic atmospheric O2 and CO2"; Floudas et al. (2012), "The Paleozoic origin of enzymatic lignin decomposition reconstructed from 31 fungal genomes," Science