Biologists have long debated how the planet's first mass extinction took place 540 million years ago. Was it a huge meteorite or terrifying volcanic eruption, typical of Earth's other mass extinctions? Nope, per a recent paper that says it offers the first "critical test" of the theory that the first animals simply gobbled up or trampled on the species that preceded them, IFL Science reports. The arrival of Earth's first animals—creatures like jellyfish, vertebrates, and sponges—occurred during the so-called "Cambrian explosion" and spawned most of the world's animal phyla. Their apparent victims: the Ediacarans, multi-celled marine organisms with odd shapes like fronds, tubes, and discs, according to a press release. For 60 million years, many of them survived by just passively ingesting the water's chemicals via their membranes.
It seems the new animals easily dominated their motionless neighbors. "Modern animals are ecosystem engineers," study co-author Simon Darroch of Vanderbilt University told New Scientist in April. "They alter the environment, burrow into sediments, and prey on each other." How does he know that's what happened? Well, fossil and sediment evidence from a site in Namibia, southern Africa, suggest that Ediacarans were "ecologically stressed" at the time and saw their diversity declining, he says. And rocks from the period show increasing tracks and burrows by Earth's first animals, "presenting a plausible link between their evolution and extinction of the Ediacarans," per the release. Also interesting: "There is a powerful analogy between the Earth’s first mass extinction and what is happening today," Darroch says, because "new behaviors can fundamentally change the entire planet, and we are the most powerful 'ecosystem engineers' ever known." (One of those early predators was a sea scorpion that grew "as big as Tom Cruise.")