The evolution of a lineage of mushrooms may have had a massive impact on the carbon cycle, bringing an end to the 60-million year period during which coal deposits were formed. Coal generated nearly half of the roughly four trillion kilowatt-hours of electricity consumed in the United States in 2010, according to the U.S. Energy Information Administration. This fuel is actually the fossilized remains of plants that lived from around 360 to 300 million years ago. An international team of scientists, including researchers at the U.S. Department of Energy Joint Genome Institute, has proposed a new factor that may have contributed to the end of the Carboniferous period — named after the large stores of what became coal deposits. evolution of fungi capable of breaking down the polymer lignin, which helps keep plant cell walls rigid, may have played a key role in ending the development of coal deposits. With the arrival of the new fungi, dead plant matter could be completely broken down into its basic chemical components. Instead of accumulating as peat, which eventually was transformed into coal, the great bulk of plant biomass decayed and was released into the atmosphere as carbon dioxide.
The researchers then used molecular clock analyses to track the evolution of the enzymes back through the fungal lineages. The idea is that just as the hands of a clock move at a defined rate around the dial, genes accumulate mutations at a roughly constant rate. This rate of change allows researchers to work backwards, estimating when two lineages last shared a common ancestor based on the amount of divergence.
The comparative analyses suggested that around 290 million years ago, right at the end of the Carboniferous period, a white rot fungal ancestor with the capacity to break down lignin appeared. Prior to that ancestor, fungi did not have that ability and thus the lignin in plant matter was not degraded, allowing these lignin-rich residues to build up in soil over time. Because molecular clock analyses have substantial error, fungal “fossils” are needed for calibration.