Although the following hypothesis has gained a lot of traction..."Such geobiological hypotheses sometimes persist based largely on the strength of their novelty, without sufficient predictive testing."
"The first trees...could not grow very tall because they didn't have lignin or cellulose, the tough fibers that make the woody trunks of trees. Trees eventually evolved lignin and cellulose and began to grow very tall – up to 160 feet or more."
"Although trees had evolved lignin and cellulose, no bacteria that could digest these woody substances had yet evolved. In fact, those bacteria would take another 60 million years to evolve."
Not so fast. The above "evolutionary lag" hypothesis didn't hold water when examined more closely.
"Boyce and his colleagues took a closer look at this "evolutionary lag" hypothesis, examining the idea from various biochemical and geological perspectives. "Our analysis demonstrates that an evolutionary lag explanation for the creation of ancient coal is inconsistent with geochemistry, sedimentology, paleontology, and biology...showed that shifts in lignin abundance in ancient plant fossils had no obvious impact on coal formation. In fact, many Carboniferous coal layers were dominated by the remains of lycopsids, an ancient group of largely unlignified plants...The scientists instead argue that the waxing and waning of coal deposits during the Carboniferous period was closely tied to a unique combination of tectonics and climate conditions that existed during the assembly of Pangea...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...forces were herding several large land masses together into what would eventually become the massive supercontinent Pangea...as the mountains rose, the basins deepened, and even more plant material could pile up."
Furthermore,
"only ∼70% of the coal organic matter is consistent with the possibility of a lignin origin, the rest being broadly attributed to microbes or algae..."
It would have taken only about a thousand years to accumulate the world's coal reserves, not 60 million years - as follows: "Even if terrestrial productivity were only 25% of the modern levels of ∼55–60 gigatons per year and lignin accounted for 20% of that production [lignin content generally ranges from 5% to 35% in most extant tracheophytes], carbon deposition in the form of lignin would have amounted to ∼3 gigatons per year. The extremity of this number is placed in perspective by considering that modern global coal reserves spanning the entire 420-million-year history of lignified vascular plants are only on the order of a few thousand gigatons. Actual rates of organic accumulation are thought to be at least two orders of magnitude lower, even in the Carboniferous. Despite feedbacks with weathering rates, much smaller imbalances would have resulted in the complete removal of atmospheric CO2 in less than a million years. Without evidence of such dire consequences, lignin production in the absence of lignin decay for more than 100 million years into the early Permian is untenable. Most organic matter decays, regardless of composition, and only accumulates where local stagnant waterlogging results in substrate anoxia"
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