The first great extinction of life was caused by an evolutionary advance, rather than a catastrophe such as an asteroid strike or supervolcano, evidence from Namibia suggests. The arrival of animals, with the newfangled advantage called movement, spelled doom for species trapped in one place.
For three billion years, the only life on Earth was single-celled microbes. Around 600 million years ago, the first multi-celled organisms, known as the Ediacaran biota, appeared. Sixty million years later, these were followed by the first animals, an evolutionary experiment with a bewildering range of body types known as the Cambrian explosion.
Around the time of the Cambrian explosion, many of the Ediacaran species disappeared. The extinction is not considered as disastrous as the “Great Dying” at the end of the Permian era, but in terms of species loss, it probably exceeded the Cretaceous-Paleogene event that extinguished non-avian dinosaurs.
Paleontologists have pondered whether some dramatic event wiped out the Ediacaran species, making way for something new, or if the arrival of animals led to the extinction of their predecessors. Resolving this question is hard, because fossil records from the period are so sparse.
"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 Vanderbilt University’s Simon Darroch, first author of the paper in the Proceedings of the Royal Society B. He argues that it was changes wrought by these animals that eventually caused so many lifeforms to die out, rather than an external event.
“We perform the first critical test of the ‘biotic replacement’ hypothesis,” Darroch and his co-authors write of their work at Farm Swartpunt, Namibia. The fossils found there date from 545 million years ago, 1-2 million years before the end of the Ediacaran. “Even after accounting for a variety of potential sampling and taphonomic biases, the Ediacaran assemblage preserved at Farm Swartpunt has significantly lower genus richness than older assemblages.” The slow decline of diverse taxa is inconsistent with a sudden catastrophe.
In sediment from the same time that the Ediacaran lifeforms were disappearing, Darroch found traces of burrows and tracks from pre-Cambrian animal species, indications that are rarer or absent in the other great Ediacaran sites in Newfoundland, South Australia and Russia.
“We found that the diversity of species at this site was much lower, and there was evidence of greater ecological stress, than at comparable sites that are 10 million to 15 million years older,” Darroch said.
Animals had a single enormous advantage over the species that preceded them: they could move. This enabled them to literally eat the opposition, wiping out all those species that did not adapt in one way or another to this new threat.
“There is a powerful analogy between the Earth’s first mass extinction and what is happening today,” Darroch said. “The end-Ediacaran extinction shows that the evolution of new behaviors can fundamentally change the entire planet, and we are the most powerful ‘ecosystem engineers’ ever known.”