The first mass extinction was caused by animals, not catastrophe

It wasn’t a massive asteroid crashing into the Earth’s surface or the eruption of an enormous volcano that led to the first known mass extinction, according a new study – rather, the evolution of complex biological organisms capable of altering their environment was the cause.

The research, which was published Wednesday in the journal Proceedings of the Royal Society B, analyzed the events that resulted in the extinction of the world’s first multicellular lifeforms, the Ediacarans, roughly 540 years ago. The authors of that paper concluded that early animals caused dramatic changes to the prehistoric environment that led to the Ediacarans’ demise.

Simon Darroch, an assistant professor of earth and environmental sciences at Vanderbilt University, and his colleagues believe that their research has provided the first quantitative palaeoecological evidence suggesting that evolution, along with ecosystem engineering and biological interactions, was the root cause of the first mass extinction of complex life.

The rise and fall of the Ediacarans

For more than three billion years, the only type of life that existed on Earth was single-celled microogranisms. Eventually, some of those microogranisms began to photosynthesize, which resulted in the production of oxygen, and since oxygen was toxic to them, they had to develop new ways to protect themselves while harnessing this new energy source.

This additional energy allowed them to adopt multicellular forms, the researchers explained in a statement, and about 600 million years ago, there was a warm period following a prolonged spell of glaciation that resulted in the evolution Ediacarans, which Darroch called “a mysterious bunch of proto-animals” that may not be related to modern animals at all.

“However, whatever the Ediacarans were, our new data shows that their communities become stressed, and their diversity drops, coincident with the appearance of modern-looking animals,” he explained to redOrbit in an email. “The evidence suggests that extinction happened through ‘ecosystem engineering.’ Modern animals (unlike the Ediacarans’ burrow into the sediment, and exhibit a variety of novel behaviors, such as predation and filter feeding.”

These new behaviors, Darroch said, “fundamentally altered” the environment, and since the Ediacarans were “adapted to the environmental status-quo,” they wound up going extinct. The changes made to the environment by the development of modernly-recognizable animals made survival increasingly difficult for the Ediacarans, until they ultimately met their demise.

“The nature of what the Ediacara biota were is very controversial,” added study co-author Marc Laflamme, an assistant professor in the Department of Chemical and Physical Sciences at the University of Toronto. “Some are likely primitive animals, while others were most likely direct competitors with animals, but ultimately unrelated. What our research demonstrates is that the evolution of complex animals… drastically altered their environments and shifted how nutrients and oxygen were cycled within the water column and underlying sediments.”

Findings offer lessons for modern-day complex animals

As part of their research, Darroch and his colleagues conducted an extensive geochemical and paleoecological analysis of a site in southern Nambia that is home to the youngest community of Ediacarans discovered to date. They found a much lower diversity of species and evidence of increased ecological stress than observed at comparable sites 10-15 million years older.

The presence of an increased diversity of tracks and burrows found at the site presented a link between the evolution of the earliest complex animals and the extinction of the Ediacarans, and the study authors worked extensively to eliminate factors such as a lack of essential nutrients at the site as a possible cause of the mass extinction event.

“These findings illustrate that ‘ecosystem engineers,’ organisms that can fundamentally alter their environments can cause mass extinction events; you don’t necessarily need a meteorite impact, or an episode of mass volcanism to wipe out large swathes of life,” said Darroch, noting that there are parallels with these findings and modern-day Earth.

“Unlike all other ‘mass extinctions,’ believed to be caused by catastrophic events like major climate change, poisonous oceans, or giant bolide impacts, this first mass extinction was biologically driven,” Laflamme told redOrbit via email. “Extensive filter feeding and burrowing outcompeted the dominant organisms (i.e. the Ediacara biota) and changed the ecosystems at that time, thus paving the way for animals as we know them.”

“Humans are incredibly powerful ‘ecosystem engineers’ and yet have been slow to accept that they can cause ecological crises (such as a mass extinction event), even while the evidence is mounting up in front of our eyes,” he told redOrbit. “Our study provides an example of where evolutionary innovation (i.e., the appearance of modern animal groups) appears to have caused the mass extinction of the Ediacarans. By extension, it shows us that humans are certainly capable of driving a mass extinction in the future.”

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