The evolution of tree roots nearly ended life on Earth

In the 2012 computer animated movie The Lorax (based on the Dr. Seuss book of the same name and now streaming on Peacock!) the residents of Thneedville live in a world without trees. Unfettered greed leads to deforestation and pollution, and the outside world becomes a barren wasteland. In the end, the characters restore balance with nature by heeding the words of the titular Lorax.

Today, trees and other plants are at the mercy of animals, and so have need of an advocate like the Lorax. In the Devonian, however, the power structure was flipped, and the world needed someone to speak for everyone else, lest the trees destroy us all.

According to a recent study published in the Geological Society of America, plants may have been responsible for a series of extinction events which occurred between 419 and 358 million years ago. During the Devonian, animal life hadn’t yet made its way onto land, but plants had, and things were about to get weird. Over the course of the Devonian, the world went through a series of extinction events — including one of the five big mass extinctions — which devastated marine ecosystems. By the time everything was said and done, more than two thirds of all species on the planet had been wiped out.

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Scientists proposed that extinction of marine species may have been directly caused by the evolution and expansion of early land-based plants. The idea was that as plants continued to spread onto land, they developed root systems which loosened soil and liberated nutrients which had previously been landlocked. In essence, as soon as roots evolved, the oceans experienced an incredible influx of nutrients from the land.

One might think that additional nutrients in the water would be a net good, but plants may have placed their leafy thumbs so firmly on the ecosystem scale that it nearly collapsed completely. In modern times, when nutrient delivery drastically increases — whether through natural or anthropogenic means — we see massive blooms of algae which kill thousands or millions of fish in a relatively short period of time. The algae releases toxins in the water and as it decays, and is eaten by bacteria, it depletes oxygen in the water column and literally suffocates any animals unfortunate to be living nearby. The process is known as eutrophication, and if it occurred on a global scale, it would have very quickly become very difficult to survive, maybe even destroying entire species or ecosystems. At least, that was the hypothesis, but researchers needed hard evidence.

Researchers from Indiana University Purdue University-Indianapolis — IUPUI — assumed that if such a nutrient surge did occur, there should be a sign of it in the geologic records showing a nutrient spike higher than the background level. They looked at geochemical records from ancient lake deposits in Greenland and northern Scotland and found precisely the signal they were looking for.

The geologic record reveals elevated nutrient levels, specifically phosphorous, at the same moment that plants were evolving and expanding in the Devonian. Elevated nutrient levels also coincided with fossil evidence of trees, including the first species with deep root systems. In two cases, the identified nutrient influxes also aligned with marine extinction events, including the late-Devonian mass extinction.

The discovery might also have solved the mystery of why the Devonian had so many extinction events. Researchers found that there was a periodic nature to plant expansion at the time, and that cycle appears to be tied to the wet-dry climate cycle. These fits and starts of plant expansion meant the world would go through periods of higher and lower nutrient release, triggering extinction events over and over again until things leveled out.

This surge of plant growth and evolution would produce incredible amounts of oxygen for our planet, not to mention depositing planet-wide carbon reserves which modern humans exploit for things like coal and oil. Before that, though, they redistributed the nutrient resources so dramatically that life on Earth almost ended.

Suddenly, pulling weeds doesn’t feel so bad.