Evolution of Tree Roots Triggered Mass Extinctions 300 Million Years Ago, UK Study Reveals

According to a study conducted by scientists at IUPUI and colleagues in the United Kingdom, the evolution of tree roots may have set off a series of mass extinctions that shook the Earth's oceans over 300 million years ago.

Devonian Period: Evolution of Tree Roots and Algae Growth

Gabriel Filippelli, Chancellor's professor of Earth Sciences at IUPUI's School of Science, said that according to their team's analysis, the evolution of tree roots probably filled earlier oceans with too many nutrients, which led to massive growth in algae. The majority of the oceans' oxygen would have been depleted by these destructive and quick-growing algae blooms, leading to disastrous mass extinction events.

Before the evolution of life on land, during the 419 to 358 million-year-old Devonian Period, several mass extinction events are thought to have killed off roughly 70% of all life on Earth.

The process described in the study, known scientifically as eutrophication, is strikingly similar to the contemporary, but somewhat smaller-scale, a phenomenon currently causing broad "dead zones" in bodies of water including the Gulf of Mexico as well as the Great Lakes. Here, too much nutrient from fertilizers as well as other agricultural runoff causes massive algae blooms that obliterates all of the oxygen in the water, fueling the Gulf of Mexico in addition to the Great Lakes' "dead zones."

The distinction is that these historical occurrences were probably largely driven by tree roots, which, during periods of growth, drew nutrients from the land and then hastily dumped them into the water of the Earth.

Chemical Perspective

Filippelli claims that a combination of current and historical data lends support to the theory.

The scientists were able to confirm cycles of higher and lower concentrations of phosphorus, a chemical element present in all life on Earth, based on a chemical analysis performed on stone deposits sampled from ancient lake beds. The samples collected for the study from locations in Greenland and along the northeast coast of Scotland are among the many remnants of the stone deposits that can still be found today.

Additionally, they were able to distinguish between wet and dry cycles based on the signs of "weathering," or soil formation, which is caused by root growth. Less weathering suggested dry cycles with fewer roots, while more weathering suggested wet cycles with more roots.

Most importantly, the team discovered that the dry cycles were accompanied by higher levels of phosphorus, indicating that during these times, the nutrients from dying roots were released into the planet's water.

Matthew Smart, a Ph.D. student in Filippelli's lab at the time of the study, said that Although looking back 370 million years is difficult, there are still locations on the planet where chemistry can act as a microscope to reveal the secrets of the ancient world because rocks have long memories.

The researchers were able to identify the decay of tree roots as the main culprit behind the Devonian Periods extinction events due to the phosphorus cycles taking place in parallel with the evolution of the first tree roots, a characteristic of Archaeopteris, which was the first plant to grow leaves as well as reach heights of 30 feet.

New Threats to Oceans

Fortunately, Filippelli said, nature has since developed systems to counteract the effects of rotting wood, so modern trees don't cause similar destruction. In comparison to the thin layer of soil that covered the ancient Earth, modern soil is deeper and retains more nutrients.

The findings of the study on dynamics, however, gave information about other, more recent threats to marine life. The study's authors note that other researchers have argued in a prior study that was published in the journal Science that pollution from organic material, including sewage, fertilizer, manure, has dangerously close the oceans of the Earth to losing all oxygen.

Filippelli said that these fresh insights into the devastation caused by natural disasters in antiquity could send a strong message about what might happen today if similar conditions result from human activity, Phys Org reports.

The GSA Bulletin reports evidence in support of this novel interpretation of a strikingly volatile period in prehistoric Earth. The study was led by Gabriel Filippelli and Matthew Smart.