Evolution of tree roots nearly ended life on Earth

In the 2012 computer-animated film The Lorax (based on Dr. Seuss’ book of the same name and now streaming on Peacock!) the people of Thneedville live in a treeless world. 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 therefore need a defender like the Lorax. In the Devonian, however, the power structure was reversed 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 that occurred between 419 and 358 million years ago. During the Devonian, animal life hadn’t yet made its way onto the earth, but plants had, and things were about to get weird. During the Devonian, the world went through a series of extinction events – including one of the five major mass extinctions – which devastated marine ecosystems. By the time it was all said and done, more than two-thirds of all species on the planet had been wiped out.

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

You would think that extra nutrients in the water would be a net good, but plants may have placed their leafy thumbs so firmly on the scale of the ecosystem that it almost completely collapsed. In modern times, when nutrient supply increases dramatically – whether by natural or anthropogenic means – we see massive algal blooms that kill thousands or millions of fish in a relatively short period of time. . Algae release toxins into the water, and as they break down and are eaten by bacteria, they deplete the oxygen in the water column and literally suffocate any animals unfortunate to live nearby. The process is known as eutrophication, and if it had occurred on a global scale it would very quickly have become very difficult to survive, perhaps even destroying entire species or ecosystems. At least that was the assumption, but the researchers needed hard evidence.

Researchers from Purdue and the University of Southampton speculated that if such a nutrient surge occurred, there should be a sign in the geological record showing a nutrient spike above the background level. They examined geochemical records from ancient lake deposits in Greenland and northern Scotland and found precisely the signal they were looking for.

Geological records reveal high levels of nutrients, especially phosphorus, at the same time plants were evolving and growing in the Devonian. High nutrient levels also coincided with fossil evidence of trees, including early species with deep root systems. In two cases, the identified nutrient inputs were also consistent with marine extinction events, including the Late Devonian mass extinction.

The discovery could also have solved the mystery of why the Devonian saw so many extinction events. The researchers found that there was a periodic nature to plant expansion at the time, and that this cycle appears to be related to the wet-dry climate cycle. These jolts of plant expansion meant that the world would go through periods of lower and lower nutrient release, triggering extinction events again and again until things stabilized.

This spurt of plant growth and evolution would produce incredible amounts of oxygen for our planet, not to mention deposit planet-wide carbon stores that modern humans mine for things like coal and oil. Before that, however, they redistributed nutrient resources so dramatically that life on Earth almost came to an end.

Suddenly pulling weeds doesn’t feel so bad.