Tree root evolution may have led to mass extinctions
The evolution of tree roots may have led to a series of mass extinctions that shook Earth’s oceans during the Devonian Period more than 300 million years ago, according to a study led by scientists at IUPUI, along with colleagues in the United Kingdom.
Evidence for this new look at a remarkably volatile period in Earth’s prehistory is reported in the GSA bulletin. The study was led by Gabriel Filippelli, Chancellor’s Professor of Earth Sciences in IUPUI’s School of Science, and Matthew Smart, a Ph.D. student in his laboratory at the time of study.
“Our analysis shows that the evolution of tree roots probably along oceans flooded with excess nutrientscausing massive algae growth,” said Filippelli. “This rapid and destructive algae bloom would have depleted most of the oceans’ oxygen, causing catastrophic mass extinction events.”
The Devonian period, which occurred 419 million to 358 million years ago, prior to the evolution of life on land, is known for mass extinction events, in which it is estimated that nearly 70% of all life on Earth has perished.
The process outlined in the study — known scientifically as eutrophication — is remarkably similar to the modern, albeit on a smaller scale, phenomenon that currently fuels broad “dead zones” in the Great Lakes and Gulf of Mexico, as excess nutrients from fertilizers and other agricultural waste cause massive blooms of algae that consume all the oxygen in the water.
The difference is that these past events were likely fueled by tree roots, which took nutrients from the land during times of growth and then abruptly dumped them into the water of the earth during times of decay.
The theory is based on a combination of new and existing evidence, Filippelli said.
Based on a chemical analysis of rock deposits from ancient lake beds — the remains of which persist around the world, including the samples used in the study from sites in Greenland and off the northeastern coast of Scotland — the researchers were able to identify previously identified cycles of higher elevations. and lower levels of phosphorus, a chemical element found in all life on Earth.
They were also able to identify wet and dry cycles based on signs of “weathering” – or soil formation – caused by carrot growth, with more weathering indicating wet cycles with more roots and less weathering indicating dry cycles with fewer roots.
Most importantly, the team found that the dry cycles coincided with higher levels of phosphorus, suggesting that dying roots released their nutrients to the planet’s water during these times.
“It’s not easy to look more than 370 million years into the past,” says Smart. “But rocks have long memories, and there are still places on Earth where you can use chemistry like a microscope to unravel the mysteries of the ancient world.”
In light of the phosphorus cycles that occurred at the same time as the evolution of the first tree roots — a hallmark of Archeopteris, also the first plant to grow leaves and reach a height of 30 feet — the researchers were able to pinpoint tree root decay as the prime suspect behind the extinction events of the Devonian period.
Fortunately, Filippelli said, modern trees wreak no comparable destruction, as nature has since developed systems to offset the impact of rotting wood. The depth of modern soil also holds more nutrients compared to the thin layer of dirt that covered the ancient earth.
But the dynamics revealed in the study shed light on other newer threats to life in Earth’s oceans. The study authors note that others have advanced the argument (as in Science in 2016) that pollution from fertilizers, manure and other organic waste, such as sewage, has placed the Earth’s oceans on the “edge of anoxia” or a complete lack of oxygen.
“These new insights into the catastrophic consequences of natural events in ancient times could serve as a warning about the consequences of similar conditions created by human activity today,” Fillipelli said.
Matthew S. Smart et al, Enhanced terrestrial nutrient delivery during Devonian forest emergence and expansion: evidence from lacustrine phosphorus and geochemical records, GSA bulletin (2022). DOI: 10.1130/B36384.1
Andrew J. Watson, Oceans on the Edge of Anoxia, Science (2016). DOI: 10.1126/science.aaj2321
University of Indiana
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