How many mass extinctions have there been?

Over the last 500 million years, during which complex, multicellular life has existed on Earth, there have been periodic extinctions events, when up to 90% of all lifeforms died off. Some say that we’re in the midst of another one now. So what do we know about these episodes, and how often did they happen? Speaking with Chris Smith, Charles Marshall is a palaeobiologist at UC Berkeley…

Charles - Over the last decades, there has been a sense that there are five big mass extinctions. They're known affectionately as of our Big Five. And what I really wanted to do is find out whether they really are a thing and just what are their properties? Do they have things in common? Do they have things that are quite different from each other? Related to that, the Big Five have become canonized in popular parlance with the notion that the current biodiversity crisis is called the sixth mass extinction, making reference to those five. So there's relevance to that issue as well.

Chris - So in that regard, how did you actually approach this then to test the validity of this claim?

Charles - So mostly it consisted of just systematically going back and reading all the papers that were relevant, the initial proposal of the Big Five, and the methods and quantitative tools that we used to identify them and even define what a mass extinction was

Chris - And how far back in time are we talking here with the Big Five?

Charles - The Earth's about 4.5 billion years old, and then starting about 500, 540 million years ago, we start to see a rich fossil record. And that's when we can start to measure origination and extinction evolution in the fossil record well, and so we are talking really about the last five, 540 million years. And in that interval, the Big Five mass extinctions have been identified.

Chris - And if we look at other things that are going on at the same time and we look at other records, other sorts of measures of what the climate was doing, how the planet was behaving, how the biosphere was behaving, are there any clues as to why we got extinctions on a mass scale when we did?

Charles - That's a tricky question. One thing that is clear that the mechanisms of extinction are different for each. So the end Cretaceous, 66 million years ago, many but not all would agree that a giant meteorite the size of Mount Everest slammed into the earth causing short term intense climate change. The biggest mass extinction that everybody agrees upon is the end Permian about 250 million years ago, before the dinosaurs, before the rise of mammals. And that seems to be a longer period of extended vulcanism that would've flooded the lower 48 states in the United States to the height of Sears Tower in Chicago, massive outpouring of basalt that caused long-term climate change. The end order vision, which is the first one, looks like it was related to a major glaciation event that perhaps strange shallow oceans off the continents where a lot of life was. Today's world is strange in that the continents sit mostly high and dry, but for much of earth history, the continent have been flooded with very, very shallow oceans, which have been replete with marine life.

Chris - When one looks for evidence of these mass extinction events, are we just seeing what we've got in the fossil record and saying, well, there's a lot there. Then when we look a bit later, there's not so many. Is that reliable? Do we know that they genuinely were big die-offs or could it just be a sampling error?

Charles - Ah, that's a very good question. In many ways, the idea of mass extinctions didn't take proper hold until the end Cretaceous meteorite impact theory came along because it was very unclear, one, what the duration was. And then the second one, which you stated explicitly, is the incompleteness of a fossil record. And now it appears in fact that I might have the oldest fossil and then the youngest fossil of a species. But the time of extinction may post date that youngest fossil by tens, hundreds of thousands of years, maybe half a million years. So there's a great deal of uncertainty as to when things actually became extinct and whether or not they became extinct simultaneously or not.

Chris - And do we have an idea as to how many species or what fraction of species were lost in these mass extinctions?

Charles - So it varies. The higher estimates are in the order of 90% of species, but that's the biggest of all. And there's a continuity of extinction intensities a little bit like there's a continuity of earthquake intensities and the intensity of extinction we see in the fossil record runs up to the order of 90% and all the way down to just a few percent. So there's a continuity of extinction intensity.

Chris - And how about today, because that's the critical thing, isn't it? People are very concerned about environmental degradation and so on, loss of species, now. How does the trajectory of loss today compare with some of these historical events?

Charles - That's a very tricky and interesting question. The mass extinctions in the fossil record were identified primarily by the magnitude in comparison to those numbers. We aren't even close to a mass extinction yet. Relatively few things have actually gone extinct at our hands, which is fantastically good news. What is disturbing is that all the other ones are large scale interactive effects where this one is caused by one species and one species alone. Us and the rate of disappearance of species already, is far higher than all of the mass extinctions except for the end Cretaceous mass extinction with a meteorite impact. So if we do nothing, I'm pretty sure we will be in a mass extinction, but we're not there yet. So I've proposed that we call the sixth mass extinction something quite different. I don't like the number six and it hasn't happened yet. So I like to call it the incipient Anthropocene, identifying us as humans as the cause of it mass extinction, to evoke those big extinction events that we do see in the fossil record.