An Earth buried in wood
Between 359 and 299 million years ago, during the Carboniferous Period, Earth’s climate was different. Atmospheric oxygen levels were exceptionally high, peaking at around 35 percent, compared to 21 percent today. This oxygen-rich environment fueled the growth of immense, swampy forests that covered vast areas of the planet's tropical landmasses.
The dominant plants in these forests were not trees as we know them. They were giant lycophytes, primitive vascular plants, with genera like Lepidodendron and Sigillaria. These "scale trees" could grow to heights of 40 or even 50 meters. Their trunks were supported by a thick bark and contained lignin, an organic polymer that provides rigidity to wood.
When these colossal plants died, they fell into the oxygen-poor water of the swamps. An element was missing from these ancient ecosystems: organisms capable of efficiently breaking down lignin. Without effective decomposers, the dead plant matter did not fully rot. Instead, it accumulated in thick layers. Over millions of years, the immense pressure and heat from geological processes compressed this organic debris, transforming it into the massive coal seams that would later fuel human industry. This period was so marked by this process that its name, Carboniferous, means "coal-bearing".
The great rot begins
The era of immense coal formation did not last forever. Near the boundary of the Carboniferous and the subsequent Permian Period, around 300 million years ago, the rate of coal deposition declined significantly. This begins the "coal gap." For years, the primary explanation for this shift centered on a major evolutionary innovation. The hypothesis points to the appearance of a group of fungi called Basidiomycetes, known today as white-rot fungi. Genomic analysis and "molecular clock" studies, which estimate the timing of evolutionary divergences, suggest that these fungi evolved the necessary enzymes to break down lignin around this time. These enzymes, known as peroxidases, gave the fungi the ability to decompose the resistant wood that had previously been inedible.
Once these fungi appeared, the carbon cycle experienced a change. Dead wood on the forest floor could now be efficiently decomposed, returning its carbon to the atmosphere rather than being buried and sequestered. This new biological process effectively shut down the planet’s primary coal-making engine. While the fungal evolution hypothesis is a strong explanation, some researchers argue that other factors, such as a changing climate and tectonic shifts that reduced swamp environments, also contributed to the end of this geological period.