375 million years ago, Earth suffered through its worst extinction event ever: 95% of all species perished. But here's the paradox: extinctions weren't actually any more common then than they are now. The real secret is that Earth's biodiversity collapsed.
At the end of the Devonian Period, around 378 to 375 million years ago, animal species went extinct. That, in itself, isn't unusual - part of the natural ebb and flow of our planet is that some species die out and new species evolve to take their place. The problem wasn't with the first part of that equation, but the second: no new species emerged to take the place of the old ones.
Over millions of years, the constant extinction rate coupled with no replacement species translated to 90 to 95% of all species disappearing. Ohio University researcher Alycia Stigall puts it succinctly:
"We refer to the Late Devonian as a mass extinction, but it was actually a biodiversity crisis. The main mode of speciation that occurs in the geological record is shut down during the Devonian. It just stops in its tracks.""
So why didn't any new species emerge? The researchers suspect it's all about vicariance. A natural driver of evolution, vicariance is when a species becomes geographically separated into multiple groups, say by the emergence of a mountain range or new river. Faced with new and different environmental conditions, the now isolated groups diverge into separate species.
In general, Earth's shifting topography will always favor greater species diversity, but in the late Devonian the normal state of affairs was reversed. The sea levels rose and the continents merged into huge connected landmasses, essentially flattening out Earth's features and giving species access to new areas without restricting their flow back to their old habitats. A few basic species invaded and wiped out the more locally adapted species. Relatively speaking, a few species took the place of what had once been hundreds of different forms of life.
The effects were dramatic. The Devonian reef system had been the largest in Earth's history, but the environmental disaster wiped out the reefs and it took 100 million years for them to return. Giant fish, the dominant form of life of the Devonian age, were all wiped out, as were many of their much smaller marine cousins.
This could be bad news for our current ecosystems. Earth's biodiversity again appears to be in crisis, and the extinction rate now is greater than it was in the Devonian or at the last great extinction of the dinosaurs. What's worse, human activity has moved invasive species, like cane toads and kudzu, all over the planet. It's a potentially grim picture, but Stigall argues this latest research could give us useful knowledge to prevent a real biodiversity catastrophe: