People often say that we’re in the middle of the sixth mass extinction event on Earth. Turns out, they might be wrong.
Nope, it’s not because it isn’t an extinction event (though that is under question, that’s not what we’re questioning here!). If we are in the middle of a mass extinction, then some recent research suggests that it wouldn’t be the sixth, but the fifth.
The extinction that got bumped is the one at the end of the Devonian period, which put paid to trilobites (among other things). There’s no real doubt from the fossils we have that there was a large reduction in biodiversity over time, but the new calculations suggest it wasn’t a mass extinction. The new calculations were done via a machine learning process, using a new(ish) statistical approach on 100,000 fossil records, and they suggest that the loss of biodiversity was more of a slow bleed. When we’re talking mass extinctions, we’re usually talking about simultaneous extinction of a lot of different species due to a single catastrophic event or combination of catastrophic events, and their knock-on effects — not a long term decline that could have dozens of contributing factors at different times.
(However, “simultaneously” sounds very immediate and the actual geological time period we’re talking about is actually quite long. The end-Permian extinction took ~63,000 years. The putative Devonian extinction is way less concentrated, though the loss in diversity is real.)
This isn’t totally out of the blue: researchers had been suggesting this interpretation for a while. The new calculations do suggest the other four theorised mass extinction events (including the one that wiped out the dinosaurs) are genuine.
Source: News coverage on some of the results of ‘A high-resolution summary of Cambrian to Early Triassic marine invertebrate biodiversity’, by Jun-xuan Fan et al, in Science Vol. 367, Issue 6475. Unfortunately I did not have access to the full article in writing this.