Much of what we know about the catastrophic events in Earth's history that saw mass die-offs of hundreds of species spanning the entire planet is thanks to one man - Jack Sepkoski. Back in the 1970s, the Chicago-based palaeontologist pored over all the relevant scientific literature to come up with a record of every single ocean-dwelling creature known to science. The data this record yielded gave rise to what could be the most iconic chart in the history of palaeontology, and it revealed the extent of some of Earth's most devastating events.

As the latest episode of MinuteEarth above explains, by tracking the entire fossil record of every ocean-dwelling creature of Earth (that we know about), scientists were able to map out five catastrophic, global biodiversity crashes - known as the Big Five. They're thought to be the biggest die-offs in our planet's history, and include the wiping out of the world's non-avian dinosaurs, and that time when as much as 90 percent of all Earth's species disappeared.

But as great as Sepkoski's work was, these five big extinction events might not have been the only such instances in Earth's history, and they might not have even been the biggest. 

The problem with using the fossil record to make assumptions about mass extinction events is that a whole bunch of life forms are simply too minute to make that sort of impression. Evidence from ancient rocks has revealed that 2.3 billion years ago, there were significant spikes in the atmospheric oxygen on the planet, which scientists think led to the mass extinction of the oxygen-intolerant microbes that had dominated Earth for at least 1 billion years.

We have enough evidence to suggest this Great Oxygenation Event did actually happen, but because the affected creatures aren't in the fossil record, it's virtually impossible to compare the scale of the event to, say, the extinction of the non-avian dinosaurs, says Emily from MinuteEarth. 

The fossil record is a far from perfect representation of what happened in Earth's history before humans were around to write it all down, but scientists are way too smart to settle for imperfect. Thanks to some really clever statistical models, they've figured out how to adjust for the inconsistencies of the fossil record.

So how many mass extinctions have there been then? Well, despite clever statistics, we still can't really be sure. Some say five, some say 11, some say, "What exactly is a mass extinction, anyway?" I'll let Emily explain why in the MinuteEarth video above, but let's just say when it comes to mass extinctions, it's not how many animals you kill that matters, it's the number of species.