Coal was first formed by tectonic activity and a wet, humid climate, say researchers from Stanford University. They found that Earth's first coal formations must have come from the Carboniferous Period, around 350 million years ago; the same time that the supercontinent Pangea was emerging.
Kevin Boyce, researcher on the study published in the Proceedings of the National Academy of Sciences, said: "There's only a narrow band in time in the Earth's history where you had both a wet tropics and widespread holes to fill in the tropics, and that's the Carboniferous. That plant matter was eventually transformed into the coal that powered the Industrial Revolution and helped usher in the modern age."
Boyce said: "If you want to generate coal, you need a productive environment where you're making lots of plant matter and you also need some way to prevent that plant matter from decaying. Where that happens is in wet environments."
Boyce says that this wet environment is linked to the Carboniferous Period, approximately 350 million years ago. He also says that coal forms in a big hole, where organic matter builds up over a long period of time. These would have been created during the same Carboniferous Period, as the formation of the supercontinent Pangea helped in making these holes.
Some of the tectonic plates would push away from each other as Pangea came together, causing volcanic magma to push up and create huge mountain ranges. These mountains would subsequently have deep basins on either side of them, which could be the 'holes' required to collect organic matter. Without bacteria to break it down the organic matter would keep accumulating.
This research disputes the previously accepted time frame of coal formation. The authors came to their conclusion by analysing the lignin content of North American plants. The organic polymer lignin makes up the cell wall of plants and in high concentrations prevents bacteria from breaking it down, allowing coal to form over time.
It was previously believed that carboniferous coal was formed in the 60 million year gap between the Earth's first forests (400 million years ago) and the earliest bacteria that could break down the lignin-heavy organic matter (around 350 million years ago).
However, the researchers found that not all of the plants existing during that time had high concentrations of lignin, so the bacteria that did not specialise in breaking down lignin would have been able to prevent the formation of coal.
"Our analysis demonstrates that an evolutionary lag explanation for the creation of ancient coal is inconsistent with geochemistry, sedimentology, paleontology, and biology," said Matthew Nelson, first author on the paper.
He added: "Central to the evolutionary lag model is the assumption that lignin is the dominant biochemical constituent of coal. However, much of the plant matter that went into forming these coals contained low amounts of lignin."