First Mass Extinction Event Was Caused by Evolution of Early Animals

October 21, 2015 | Sarah Tse

Diorama of the Ediacran sea.
Photo credit: Ryan Somma/Flickr (CC BY-SA 2.0)

Although we usually blame apocalyptic phenomena like giant asteroids for every previous mass extinction, it turns out humans are not the first jerks to spoil the party.

We’re in the midst of the sixth mass extinction event in the history of life on Earth, and it’s pretty much all our fault. As a species, we are the most potent ecosystem engineers to come along, and we have made an irreversible impact on the environment and the world’s biodiversity. Previous mass extinctions came about as a result of huge calamities, like the asteroid impact that ended the Mesozoic Era and as well as 75 percent of all the existing plant and animal species. We’ve ushered in a new age of environmental degradation caused by a single species.

But before we drown ourselves in self-loathing and guilt, we can take solace from the recent discovery that the first mass extinction event was also caused by biological factors, rather than a hellish natural disaster. This event caused the downfall of the first massive group of multicellular organisms, called the Ediacarans, about 540 million years ago.

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The Ediacarans flourished for a period of about 60 million years, during which they dominated the planet-wide oceans in all sorts of strange forms: disks, tubes, fronds, quilts. They were a passive lot and simply fed on food particles that diffused their way. These simple creatures owed their success to the photosynthetic cyanobacteria that filled the atmosphere with oxygen and provided a new energy source.

Dickinsonia Costata
Caption: The only traces left by the soft bodies of Ediacarans are these imprints in sand. Credit: Verisimilus/Wikipedia (CC BY 2.5)

But then came the Cambrian Explosion, which introduced the first animals to the scene. Unlike the peaceful Ediacarans, animals weren’t happy with just sitting around and waiting for food to come to them. They actively sought out sustenance by preying on other creatures and disrupting the ecosystem to carve out their own niches. Just like us, they altered their environment to suit their needs, and this ecological engineering ended up souring the livelihoods of the Ediacarans.

According to analyses of a preserved Ediacaran community in Namibia, these pioneering multicellular organisms experienced significant ecological stress and reduced biodiversity after the first animals appeared. All other chemical and physical factors compared between this site and older sites stayed consistent, indicating that the Ediacarans suffered as a result of the meddling caused by the massive Cambrian diversification.

The devastation wrought to the Ediacarans by the incoming animal species reflects recent events caused by human interactions with the environment. While we can at least reassure ourselves that this sort of thing has happened before, it’s important to note that this first mass extinction event occurred as a result of an entire new kingdom of organisms showing up all over the planet, rather than a single species. More importantly, the new animal species completely repopulated the planet and did not need the Ediacarans to survive, whereas we humans are painfully dependent on almost all other living organisms. On the bright side, we’re clever enough to recognize what we’re doing before it’s too late.

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