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Early animals might have caused a mass extinction event, says new evidence

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Fossils found in Namibia support coexistence of ancient organisms

Simon Darroch / Vanderbilt University

New fossils discovered in the southwest African country of Namibia reinforce a theory that Earth's first mass extinction was caused by the planet's earliest animals, known as metazoans. These animals, which comprise most common forms of life today including vertebrates and arthropods, arrived on the scene roughly 540 million years ago. The effects of the diversification and spread of animals across the globe is known as the Cambrian explosion, and scientists now think it may have also led to the extinction of Earth's first multicellular organisms, known as Ediacarans.

"These new species were 'ecological engineers' who changed the environment in ways that made it more and more difficult for the Ediacarans to survive," said Simon Darroch, an assistant professor of earth and environmental sciences at Vanderbilt University, in a statement. Ediacarans are thought to have evolved from the planet's single-celled organisms and populated Earth about 600 million years ago.

Earth's first multicellular organisms may have been wiped out with the arrival of animals

Now, Darroch's team have come upon a set of fossils supporting the idea that Ediacarans and metazoans coexisted, and that the ecological introduction of one may have pushed the other to extinction. "Until this, the evidence for an overlapping ecological association between metazoans and soft-bodied Ediacaran organisms was limited," Darroch said. The discovery should help scientists gain a deeper understanding about the shared communities of metazoans and Ediacarans, allowing room for speculation as to how they interacted with one another.

The findings, laid out in a paper titled, "A mixed Ediacaran-metazoan assemblage from the Zaris Sub-basin, Namibia," were published in in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. Just because these two types of organisms lived together in the same place doesn’t mean that the extinction theory is confirmed, Darroch points out. But scientists are now one step closer in piecing together how modern life may have shaped Earth's ecosystem and radically affected its earliest multicellular organisms.

"Humans are the most powerful ecosystem engineers."

For instance, some of the burrow fossils found in Namibia are thought to have been formed by sea anemones, which may have sought out Ediacaran larvae as a source of food. "In general, these new fossil sites reveal a snapshot of a very unusual 'transitional' ecosystem existing right before the Cambrian explosion," Darroch said. Across the planet, he added, less advanced multicellular organisms may not have been able to withstand drastic changes to the environment and the introduction of new predators. This could have created a situation where Ediacarans were, as Darroch puts it, clinging on to "grim death, just as modern-looking animals are diversifying and starting to realize their potential."

There's an analogy here, too. Just as Ediacarans were wiped out by the behavior of Earth's earliest animals, the Earth too is undergoing rapid change due to modern humans' contributions to the atmosphere. "The end-Ediacaran extinction shows that the evolution of new behaviors can fundamentally change the entire planet," Darroch said, "and today we humans are the most powerful 'ecosystems engineers' ever known."