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Many early animal species died out just over 540 million years ago, but not for the usual reasons. A new study suggests that there was no external disaster: no supervolcano or climate change. Instead, the die-off happened as a result of increasing competition between the newly evolved animals as they diversified.

“When we think of mass extinctions, we think of them as externally generated,” says at the University of Cambridge. Famously, the dinosaur extinction 66 million years ago was triggered by an asteroid hitting Earth.

The earlier extinction was different, however. “It is kind of an intrinsic, almost internal, extinction event,” says Mitchell. “Things are going extinct, but it’s because they’re evolving and changing.”

The die-off happened towards the end of the Ediacaran period, which lasted from 635 to 541 million years ago. The earliest known complex animals lived at this time, including many soft-bodied creatures that look strange compared with modern animals. In the subsequent Cambrian period, there was an explosion of animal evolution, building off what happened in the Ediacaran.

Mitchell and her colleagues compiled data from three sets of Ediacaran fossils. The Avalon assemblage, named for the Avalon peninsula on Newfoundland, dates from 575 to 565 million years ago. The second is between 558 and 550 million years old and is named for the White Sea on Russia’s north coast. The third is the Nama assemblage, named for a site in Namibia, and is the youngest, at 549 to 543 million years old.

Previous studies have shown that the Nama assemblage had fewer species than the earlier two assemblages, suggesting that some disaster had struck the Ediacarans, although there was no evidence for anything like a meteorite strike. The drop in diversity is “one of the greatest longstanding enigmas of the Ediacaran fossil record”, says at Yale University.

Mitchell’s team looked at the patterns of species found in the three assemblages. They wanted to see if sets of species tended to occur together, suggesting they relied on each other – or if some sets of species were never seen together, suggesting they were competitors.

The team found that the oldest Ediacaran communities – represented by the Avalon assemblage – were quite simple, with few interactions between species. What’s more, although there were many species, they often lived in similar ways, suggesting there was little competition.

However, things changed as time went on. In the White Sea and Nama assemblages, the species started interacting more, both cooperatively and competitively. They also became more specialised for certain types of food or environment.

The result was that every organism’s habitat started out quite broad but gradually narrowed as the competition heated up. This competition drove many species to extinction. “If one species colonises an area where [there’s] a better competitor, it can’t survive,” says Mitchell.

“It’s a very thought-provoking new take,” says Tarhan, adding that the analyses used are “much less anecdotal and much more quantitative” than previous attempts to explain the Ediacaran extinction.

It may be that the extinction really was caused by processes intrinsic to the ecosystem, says Tarhan, but this doesn’t mean the environment didn’t have a role. The earliest Ediacarans lived on the sea floor in deep-sea environments, but as time went on, some of them moved up into shallower regions that were much more changeable and also richer in oxygen. The move into the shallows may have enabled some of the evolutionary diversification that took place – setting the stage for the die-offs that followed.

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