For the first time, in the Carboniferous period 360 – 300 million years ago, the primordial plants of the Earth grew to reach the canopy heights we now find in our modern forests. That was the era when the trees evolved the capacity to strengthen their trunks using lignin as a cross-linking agent. Plant growth then expanded over the globe so extensively that it changed the atmosphere. CO2 was depleted from the skies – converted into cellulose and lignin wood fibers. Large volumes of O2 were released by photosynthesis, and amphibians and insects that breathe by absorbing oxygen through their skin were able to grow to gigantic sizes.
Seams of coal hundreds of feet thick were created during the Carboniferous period. Piles of timber accumulated on the forest floor and were buried, compressed in stone layers, and eventually converted into black bituminous deposits. One explanation for this coal creation was that the trees fell in great flat swamps, and their immersion there precluded decay until they could be buried. But the land was not especially level back then – the Carboniferous was an age of mountain building.
Bituminous coal formation ceased abruptly at the end of the Carboniferous, even though lush forests would persist and expand in subsequent times. Now an international team of scientists has mounted a research effort that revises the explanation for that cessation of geological coal production. This explanation does not depend on the preservation of the woody precursors of coal by immersion until they were finally buried.
For this effort, 70 researchers* from 26 different institutions have worked together to expand and fill-out the family tree of the “white rot” fungi. They compared the sequences of genes from different members of this group of wood-rotters, some of them more evolved than others. Using a molecular clock derived from the rate at which mutations accumulate in genes, they were able to back-date the branch points in this family tree. They identified the times at which species diverged from each other in the past, interpolating all the way back to the historic origin of these fungi.
They found that the first of these wood-rotters that were able to degrade lignin – and thereby able to decompose downed trees – originated around the end of the Carboniferous period. They appear to have arisen to expand into a previously unoccupied niche – to find a reservoir of the material they could metabolize. The lignin in the stocks of downed trees was a windfall find for these fungi. It had been lying unused in piles hundreds of feet thick, for millions of years, due to the absence of an organism that could take advantage of the situation and grow by decomposing it.
Now, when we walk through the shade of a tall, old-growth forest, we do not find jumbled stacks of fallen timber. We find an occasional downed trunk melting into the ground. Along its lower margin we find the hard conks of shelf fungi – the turkey tails of the wood rotters emerging from the decaying fiber. The weight of a foot on the darkened wood may push a hole right through it and down to the ground.
The fungal mycelia penetrate throughout the layers of dead wood, digesting the lignin as well as the cellulose. They produce their hard mushrooms and send forth their spores, recycling the nutrients back into the soil and the carbon back into the air as they reproduce themselves. These fungi now preclude the accumulation of plant matter on the ground in amounts that once gave rise to the great bituminous coal measures that drove the industrial revolution.
So the coal that the industrial world now depends upon cannot be recreated – it is a non-renewable resource. When it has been all consumed, in the coming few decades, the primary source of energy that now sustains the human population will be gone. It was last created in deep dark bituminous seams 300 million years ago, and has not been created since then – since the white rot fungi arose to destroy the starting material of fossil hydrocarbon fuels before it could even be buried.
*Floudras et al (2012) The paleozoic origin of enzymatic lignin decomposition reconstructed from 31 fungal genomes. Science 366:1715