The Carboniferous engine stalls
For roughly 60 million years during the Carboniferous Period, Earth’s coal-making machinery ran at full capacity. From about 359 to 299 million years ago, vast, swampy forests covered the equatorial regions of the supercontinent Pangea. Atmospheric oxygen levels soared to as high as 35%, compared to today's 21%, fueling the evolution of giant insects like the dragonfly-like Meganeura with a wingspan of almost three feet. The forests were dominated by plants like the lycopsid trees, such as Lepidodendron, which grew rapidly and tall. These plants had high lignin levels, a complex organic polymer giving wood its structural integrity.
When these trees died, they fell into oxygen-poor bogs. At the time, almost no organism could effectively decompose the tough, resilient lignin. As a result, the dead plant matter did not fully rot. Instead, it piled up in immense layers, eventually becoming compressed by heat and pressure into the massive coal seams we mine today. The Donets Basin, or Donbas, where these coordinates are located, formed during this time and contains some of the world's most significant Carboniferous coal deposits. Then, around the transition from the Carboniferous to the Permian Period, the coal factory nearly shut down.
A fungal breakthrough
The prime suspect for the coal gap is a microscopic innovator: white-rot fungi (Agaricomycetes). Through genomic analysis, scientists have traced the evolution of a critical adaptation in these fungi to roughly 300 million years ago, coinciding with the start of the coal gap. This adaptation was the ability to produce a suite of powerful enzymes, including lignin peroxidases. These enzymes gave fungi, for the first time, the ability to efficiently break down lignin.
This evolutionary leap changed the carbon cycle. Dead wood no longer lay nearly untouched for millennia. Fungi could now decompose it entirely, releasing its carbon back into the atmosphere as carbon dioxide. The raw material for coal simply vanished before it could be buried. This new fungal ability is considered a primary reason why coal formation dropped off so dramatically for millions of years.
The study of the microscopic fossil record, palynology, is important for understanding this event. By analyzing the preserved spores and pollen within coal seams, researchers can precisely date the layers and reconstruct the plant communities that formed them. The coal seams of the Donbas serve as a pristine record of the world before this fungal evolution, a time when dead trees were destined to become rock instead of rot. Some recent studies suggest tectonic and climate shifts also influenced the coal gap, but the fungal evolutionary event is a factor in the story of Earth's carbon cycle.