In 2012, scientists were puzzled by evidence of ice sheets advancing in the Ordovician period between 488 and 444 million years ago. At that time, the continents were all clustered over what is now the South Pole and stretched as far as the Equator. Some scientists now believe that plants were the culprits: As the plants spread across and took root over dry land, they extracted minerals from the rocks they lived on. The scientists further offered several scenarios involving what happens to these minerals—all of which could eventually cause carbon levels to lower, along with the temperatures (in other words, almost the opposite of what we call “global warming”). The researchers suggest that the spread of terrestrial plants could have brought about a series of ice ages.

Most of the coal mined today was formed from prehistoric remains of primitive land plants, particularly those of the Carboniferous period (approximately 300 million years ago). Five main groups of plants contributed to the formation of coal: three groups were all seedless, vascular plants (ferns, club mosses, and horsetails) and two groups which are now extinct (seed ferns and the primitive gymnosperms). Forests of these plants were located in low-lying, swampy areas that periodically flooded. When these plants died, they decomposed, but as they were covered by flood water, they did not decompose completely. As this material accumulated, layers of sediment covered the decaying plants, with the resulting heat and pressure of the layers eventually converting the plant material to coal. Various types of coal (lignite, bituminous, and anthracite) were formed, depending on the layers’ exposure to varying temperatures and pressures.