Full Text:
spoiler
The end-Permian mass extinction was the deadliest event in Earth’s history. Also called the Great Dying, it is thought to have nearly wiped out all life on Earth 252 million years ago. Yet, earlier this year, we learned of an ancient ecosystem at South Taodonggou, a geological site in what is now China, where plants and animals were thriving just 75,000 years later – a blink of the geological eye. You might call it an isolated miracle.
Surprisingly, palaeontologist Hendrik Nowak at the University of Nottingham, UK, doesn’t see it that way. He points to fossil pollen from other sites that also suggests “little or only short-lived disruption” from the end-Permian event. In fact, Nowak argues that the impact was so minimal that – for plants, at least – there simply was no mass extinction then.
This conclusion is controversial. Nevertheless, studies on two other major groups of organisms – insects and four-limbed land animals – echo the findings in plants. The emerging picture means Nowak isn’t the only palaeontologist questioning whether the impact of the end-Permian mass extinction was as colossal as we thought. Spencer Lucas at the New Mexico Museum of Natural History & Science goes even further – he suspects life on land has never experienced a mass extinction. “I think that you’ve got a better chance of beating a big extinction if you’re on land than you do if you’re in the sea,” he says.
This revolutionary rethink could rewrite the history of life on Earth. It would upend the idea that the continents have witnessed five mass extinctions – and it even has implications for how we frame the current human-induced biodiversity crisis.
The most famous victims of a mass extinction are the dinosaurs that died out around 66 million years ago, but much of what we know about such events comes from studying marine life. Indeed, the idea that Earth has experienced five mass extinctions came from a 1982 analysis of the marine fossil record. Two palaeontologists, the late David Raup and Jack Sepkoski, tracked changes in marine biodiversity over the past half a billion years and noticed that the record was punctuated by five crashes. These were at the end of the Ordovician (445 million years ago), the late Devonian (372 million years ago), the end-Permian (252 million years ago), the end-Triassic (201 million years ago) and the end-Cretaceous periods, the latter being when most dinosaurs went extinct. These events came to be known as the big five.
It remains beyond doubt that these mass extinctions devastated ocean life, but – dinosaurs notwithstanding – it wasn’t initially clear that they had also rippled through ecosystems on land. Mike Benton at the University of Bristol, UK, recalls that textbooks from the late 1980s “stated quite categorically” that there was little evidence of an end-Permian mass extinction of four-limbed animals, or tetrapods, that lived on land. Modern tetrapods include all reptiles, amphibians, birds and mammals. That, says Benton, was largely down to a lack of data. It is relatively easy for a dead marine organism to be buried in mud and begin the fossilisation process, whereas land organisms are less likely to become fossils.
There are, however, a few sites that do capture a reasonably good fossil record of life on land during mass extinction events. Over the past 30 years, researchers have spent countless hours collecting and analysing tetrapod remains from such sites. A clear picture has emerged, says Benton: there were tetrapod mass extinctions on land to match those in the sea. This makes sense, given that the big five were driven primarily by a combination of rapid climate change and massive environmental upheaval, triggered by things like asteroid impacts and volcanic activity. “There are considerable feedbacks between the land and the oceans,” says Benton. Runaway global warming, for instance, puts stress on both marine and terrestrial life. As such, he argues it is difficult to imagine a mass extinction affecting only one of the two realms.
So clear is this connection that many researchers now make it central to their understanding of these dramatic events. “Mass extinctions happen everywhere, all at once, on land and in the sea,” says Paul Wignall at the University of Leeds, UK. Nevertheless, some have begun to express doubts and Lucas is prominent among them. In a 2017 paper, he examined the claim that there was an end-Permian mass extinction of land-based tetrapods. There were extinctions, he concluded, but fewer than 20 genera disappeared – hardly evidence of a catastrophic loss of diversity, given that there must have been many hundreds or even thousands of tetrapod genera at the time. “There was no big extinction of tetrapods on land at the end-Permian,” he says.
Since then, Lucas has taken a critical look at the rest of the big five. In a review published in 2021, he concluded that land-based tetrapods were barely affected by any of them. “I think that there’s a lot of hyperbole involved in this,” he says. “It’s a big deal that the non-avian dinosaurs go extinct at the end of the Cretaceous. That said, I don’t think it’s really a mass extinction.” He points out that plenty of other large, land-living tetrapods, including the crocodilians, survived. And, of course, we now know that one group of dinosaurs – the birds – didn’t go extinct, nor did the mammals. Lucas argues that tetrapods on land are in a better position to avoid extinction because air has a lower viscosity than water, which makes migrating to new regions following the deterioration of the local environment energetically less costly for land animals than for their marine counterparts.
Unsurprisingly, the claim that there has never been a mass extinction of land-based tetrapods has faced pushback. For instance, Benton maintains that the group did face a massive die-off at the end-Permian, with the disappearance of major branches including the sabre-toothed gorgonopsians – but that the extinction occurred over 1 million years. He says Lucas has “missed the bigger picture” by zooming in on the very end of that protracted extinction. Wignall is another critic. “I think it would be fair to say that Lucas’s viewpoint is not mainstream,” he says.
However, Lucas is not a lone voice in questioning the big five mass-extinction paradigm. Beyond the tussle over tetrapods, researchers who focus on other major groups of land-based organisms are coming to similar conclusions.
Take insects, of which there are millions of species today. In 2021, Sandra Schachat, now at the University of Hawaiʻi at Mānoa and Conrad Labandeira at the National Museum of Natural History in Washington DC assessed the fossil record of insects and concluded that the tiny animals seem never to have suffered a mass extinction. This doesn’t mean they have had a crisis-free existence. Most notably, insect communities changed dramatically near the end-Permian, says Schachat. Important groups, including the dragonfly-like Palaeodictyoptera, vanished. Others, such as the Hemipteroidea – which includes the true bugs – rose to dominance. But crucially, she says, we have no idea how these changes came about because the insect fossil record is extremely patchy, with a gap of about 20 million years near the end-Permian. Over such a long period, insect communities can change gradually, but drastically, through evolution by natural selection alone. “When the fossil record is so incomplete that your best snapshots of a group of organisms come tens of millions of years apart, you’re going to expect to see big changes, with or without a mass extinction,” says Schachat.
Researchers including Wignall argue that it makes the most sense to tie the insect community changes near the end-Permian to a mass extinction. That is a possibility, says Schachat, but it ignores an important point: insect species are impressively well equipped to survive tough times. In their 2021 paper, she and Labandeira pointed out that insects occur in vast numbers and have short generation times. This means natural selection can proceed exceptionally quickly, helping insect species adapt to rapidly changing conditions. Moreover, faced with an acute crisis, individual insects can enter a period of dormancy called diapause until conditions improve.
Arguably, some marine invertebrates have similar features. But the fact that they live in the ocean may leave them more vulnerable to extinction, according to Schachat and Labandeira. Most notably, changes in atmospheric oxygen and carbon dioxide levels can trigger ocean stagnation, leading to the death of marine creatures through asphyxiation. Organisms on land don’t face that problem. “We see tremendous changes in marine communities that correlate with drops in atmospheric oxygen, and then if we look at the record on land, we don’t see anything like that,” says Schachat.
The fossil record of land plants also fails to conform to the big five narrative. In 2013, Borja Cascales-Miñana, now at the University of Lille in France, and Christopher Cleal, now at the University of Bristol, took a close look at the record and concluded that plant mass extinctions are surprisingly rare. For instance, no family of vascular plants, a group that includes things like ferns and conifers, died out during the supposed fifth mass extinction at the end of the Cretaceous. In fact, they concluded, only one of the big five – the end-Permian – coincided with a mass extinction of plants. And as this year’s study of the South Taodonggou fossils makes clear, even that is now questioned by some.
“It’s pretty evident if you look at the fossil record from a broad perspective that something happened: the terrestrial flora changed quite a bit,” says Nowak. For instance, forests dominated by the genus Glossopteris vanished in the end-Permian. “But can you call that a mass extinction?” In a 2019 study, Nowak and his colleagues argued that you can’t. They concluded that this event affected some plants, including ferns, but had little impact on others. Conifers even appear to have increased in diversity. Cascales-Miñana and Cleal stand by their finding of an end-Permian plant mass extinction, pointing out that Nowak’s team focused largely on pollen and spores, which are released in the billions by one tree, and so could create an impression of many plants even amid a large decline. “If you are counting spores, you are not counting plants,” says Cascales-Miñana. But this controversy shouldn’t detract from the broader message, which is that plants weren’t badly affected by most of the big five mass extinctions. “I think that idea is pretty well accepted among palaeobotanists,” says Cleal.
Again, the lack of mass extinctions among plants probably comes down to biology. They have several strategies for withstanding disaster. The most significant, says Cleal, is that they can survive for decades or even centuries as seeds and spores. “Imagine shooting all the elephants in the world: 10 years later, there are still no elephants,” he says. “Now imagine cutting down all the oak trees in the world: 10 years later, there are the beginnings of new oak forests because the acorns germinated.”
The fact that plants were largely unaffected by most – and potentially all – of the big five extinctions leads to an intriguing philosophical question, one that was first posed by Cascales-Miñana and Cleal in their 2013 paper. Should we label an event a “mass extinction” if it only affects a limited set of organisms and has little impact on other major groups? Lucas, for one, thinks we shouldn’t. “How would you create a mass extinction on land?” he asks. “You would kick the floor out from under the food pyramid, take out the plants. But wait a second: the plants aren’t going extinct at these events. Then how does the animal community collapse?”
The growing uncertainty about what counts as a mass extinction has implications for the way we think about the biodiversity crisis unfolding today because of human activities. Many researchers have begun labelling it Earth’s sixth mass extinction, but, for life on land, it may arguably be the first. However, either label may not be warranted. For instance, Schachat and Labandeira argued in their 2021 paper that we need to see the disappearance of entire branches of the insect evolutionary tree to declare that a mass extinction of these animals has begun. They are undoubtedly experiencing catastrophic losses of abundance and biomass right now. “[But] there are no indications that we are anywhere near a crisis of this severity,” the pair wrote.
It may seem unwise to question the sixth mass extinction idea – especially as it is framed as a rallying call to urgently conserve biodiversity, says John Wiens at the University of Arizona. Nevertheless, he thinks conservationists would benefit from abandoning this rhetoric. “Many people are now saying: we’ve got to stop the sixth mass extinction. But, actually, that is not an ambitious or even an urgent conservation goal,” he says.
To understand why, we need to go back to basics. Surprisingly, there is no precise definition of the term “mass extinction”, but there is a general consensus that these events see the loss of at least 75 per cent of species over the course of several thousand years to around 2 million years. Meanwhile, estimates indicate that over the past 500 years, less than 0.1 per cent of known species have become extinct. In a paper published earlier this year, Wiens and his colleague Kristen Saban at Harvard University point out that these figures suggest avoiding the sixth mass extinction will be a breeze. “We could lose half the species on the planet over the next 3000 years and still say, ‘Yeah, we did it! We prevented the sixth mass extinction,’” says Wiens. Targeting such an easily achievable goal risks doing more harm than good. If we really want to conserve biodiversity, we should aim to prevent human-induced extinctions rising to 0.2 per cent, not 75 per cent or even 50 per cent, he says.
No doubt, the debate about whether life on land has experienced five mass extinctions, one or even none will continue. Whatever palaeontologists conclude, that doesn’t change the urgent need to address our current crisis, says Wiens. “It’s popular right now to talk of a sixth mass extinction. But it’s just the wrong way to think about it.”
