Evolution of tree roots may have led to mass extinctions

Evolution of tree roots may have triggered a series of mass extinctions that rocked Earth’s oceans during the Devonian period more than 300 million years ago, according to a study by scientists from the IUPUI, with colleagues from the UK.

Evidence for this new view of a remarkably unstable period of Earth’s prehistory is reported in the GSA Bulletin. The study was led by Gabriel Filippelli, Chancellor Professor of Earth Sciences in the IUPUI School of Science, and Matthew Smart, Ph.D. student in his laboratory at the time of the study.

“Our analysis shows that the evolution of tree roots likely flooded past oceans with excess nutrients, causing massive algal growth,” Filippelli said. “These rapid and destructive algal blooms would have depleted most of the oxygen in the oceans, triggering catastrophic mass extinction events.”

The Devonian Period, which occurred 419 million to 358 million years ago, before life on earth evolved, is known for mass extinction events, during which it is estimated that nearly 70% of all life on Earth perished.

The process described in the study – known scientifically as eutrophication – is remarkably similar to the modern phenomenon, albeit on a smaller scale, which currently fuels vast “dead zones” in the Great Lakes and the Gulf of Mexico. , as excess nutrients from fertilizers and other agricultural runoff. trigger massive algal blooms that consume all the oxygen in the water.

The difference is that these past events were likely fueled by the roots of trees, which pulled nutrients from the earth during periods of growth, then dumped them abruptly into the Earth’s water during periods of decay.

The theory is based on a combination of new and existing evidence, Filippelli said.

Based on a chemical analysis of stone deposits from ancient lake beds – remnants of which persist across the globe, including the samples used in the study from sites in Greenland and off the North Coast – east of Scotland – researchers were able to confirm previously identified cycles of and lower levels of phosphorus, a chemical element present 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 root growth, with greater weathering indicating wet cycles with more roots and less soil. weathering indicating dry cycles with fewer roots.

More importantly, the team found that dry cycles coincided with higher phosphorus levels, suggesting that dying roots were releasing their nutrients into the planet’s water during these times.

“It’s not easy to look over 370 million years into the past,” Smart said. “But rocks have a long memory, and there are still places on Earth where you can use chemistry as a microscope to unlock the mysteries of the ancient world.”

In light of phosphorus cycles occurring at the same time as the evolution of the first roots of trees – a hallmark of Archaeopteris, also the first plant to sprout leaves and reach heights of 30 feet – researchers were able to identify the decay of tree roots. as the prime suspect behind the Devonian extinction events.

Fortunately, Filippelli said, modern trees don’t cause similar destruction since nature has since developed systems to offset the impact of decaying wood. The depth of modern soil also retains more nutrients compared to the thin layer of soil that covered ancient Earth.

But the dynamics revealed in the study shed light on other, more recent threats to life in Earth’s oceans. The study authors note that others have made the argument (as in Science in 2016) that pollution from fertilizers, manure, and other organic waste, such as sewage, has put Earth’s oceans on the “edge of anoxia,” or a complete lack of oxygen.

“This new information about the catastrophic results of natural events in the ancient world may serve as a warning about the consequences of similar conditions resulting from human activity today,” Fillipelli said.