Would we survive a mass extinction?

Thomas Halliday, author of Otherlands, discusses the traits that help determine whether species become extinct or survive, and how the rules change during mass extinctions

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Interview by Bryony Cottam

Almost all of the species that have ever lived are already extinct. The vast majority died out sometime other than during one of the five mass extinction events that have occurred in the Earth’s history, each of which resulted in a rapid and widespread loss of biodiversity. Extinction is a natural process, one that’s essential to evolution; the Earth’s background, or ‘normal’ extinction rate (in the times between mass extinctions) is thought to be between 0.1 and one species per 10,000 species per 100 years.

According to Thomas Halliday, there are various traits that tend to prevent species from becoming extinct. Being small is associated with helpful traits such as rapid reproduction; larger animals typically mature later and hence need to survive for far longer before they can reproduce. Generalists, species that thrive in a range of environments and can eat a varied diet, typically survive the longest. ‘If you go down a specialist route, evolutionarily, then you’re far more likely to find yourself tied to other species,’ says Halliday. ‘If they go extinct, then your fate is tied to theirs.’

Halliday is a palaeobiologist who specialises in the science of reconstructing the evolutionary history of life on Earth, known as phylogenetics. His recently published book Otherlands describes 16 extinct ecosystems from the Earth’s deep past and the mass extinctions that ended a few of them. 

Those who’ve read Otherlands will be familiar with Hell Creek, a fossil-rich formation of Cretaceous and Paleogene rocks in Montana, and the setting for Chapter Six: Rebirth. The site is famous for its fossil record of the changes that took place following the most recent mass extinction, 66 million years ago, including the disappearance of the dinosaurs, thought to be caused by the impact of the Chicxulub asteroid. Halliday points to the Hell Creek fossils as clear evidence of the advantages that more generalist insect species in the area had over those with a less varied diet.

Today’s specialists include hyper-carnivorous mammals such as cats, which depend on a diet that consists of at least 70 per cent meat; koalas, which feed only on eucalyptus leaves; and diving birds such as puffins and penguins, which have developed a highly specialised foraging strategy. All are relatively poor at adapting quickly to environmental change. In comparison, humans are the ultimate generalists, says Halliday. ‘We are able to survive even uninhabitable environments because we can technologically modify them. We built the International Space Station to survive in a vacuum.’ 

Species that are able to live in an environment that’s cushioned from change certainly stand a better chance of survival. Halliday is surprised by the insulation offered by a simple burrow. ‘One of the most remarkable things I discovered while writing Otherlands is that there are ground squirrel burrows found in Los Alamos at the US nuclear testing grounds,’ he says. ‘Despite their burrows being only one or two feet below the surface, these squirrels survived repeated nuclear bombs and were buffered from the environmental devastation that was going on above.’ Burrowing is exactly the kind of trait we might expect to find in the small mammals that survived the Chicxulub impact, he adds.

However, in the event of a mass extinction, even being a generalist might not be so advantageous. ‘Mass extinctions are different from “normal” extinctions in the sense that the rules of survival tend to be suppressed – there’s a greater emphasis on luck,’ Halliday says. To an extent, he explains, the traits that will help you to survive a mass extinction are contingent on the form that it takes. 

David Jablonski, a paleontologist at the University of Chicago and a research assistant at the Smithsonian National Museum of Natural History, has sought to answer the question of which species will live or die in the aftermath of a mass extinction. Through his research, he has identified one rule that comes into play: species that have broader geographic ranges are more likely to survive. Halliday agrees that range has played an important role in extinctions caused by events that were, initially, very localised. ‘When the Chicxulub asteroid struck what is now the Yucatan Peninsula in Mexico, any species endemic to that area would simply have been obliterated right there and then. Whereas if you are a geographically widespread species, there’s a greater chance of just happening to be in the part of the world that is not directly affected by that mass extinction.’ 

However, even large geographic ranges offered little insurance against the mass extinction at the end of the Permian period, 252 million years ago. A surge in volcanic activity released carbon dioxide, methane and toxic chemicals into the atmosphere, causing rapid global warming and the loss of 76 per cent of the ocean’s oxygen. Known as the ‘Great Dying’, this cataclysmic event was the most devastating mass extinction to have occurred, obliterating 96 per cent of life on Earth. 

Halliday explains that, unlike background extinctions, mass extinctions are caused when too many connections between organisms are lost and the ecosystem itself stops functioning. ‘So a mass extinction ends up being the wholesale destruction of not just individuals, but the processes that allow those individuals to survive.’ Our present-day extinction rates – estimated to be 1,000–10,000 times the background extinction rate – are caused by human activities that similarly disrupt ecosystems: land-use change, burning fossil fuels and pollution.

Contrary to popular thinking, we’re not the only species to have had a significant impact on the rate at which species become extinct. During the  Devonian period, when the first forests began to emerge, the evolution of complex tree root systems likely flooded the oceans with excess nutrients, causing massive algal blooms that depleted most of the oceans’ oxygen, triggering the second mass extinction. Some scientists have also speculated that the evolution of methane-emitting microbes may have played a significant role in the Permian-Triassic extinction. What sets humans apart, however, is agency. 

Near-term mass extinction isn’t inevitable, says Halliday, but we ought to be careful. We’re damaging the planet in ways that simulate some aspects of past extinction events, and we’re in the unique position to be able to reflect on that and recognise the possible consequences. Besides, despite humans’ extreme adaptability, we also have traits that make us poorly suited to surviving a mass extinction, such as our size, diet and slow reproductive cycle.

Even if we were to survive a mass extinction, the world that would only start to return to ‘normal’ some 20,000 years later would be fundamentally changed. Across the chapters of Otherlands, the one thing that Halliday says all 16 ecosystems have in common is that, at the time that they existed, they were as vibrant and as seemingly fundamental to the Earth as the ecosystems that are around us today. ‘There is ultimately nothing intrinsic about humans, or the ecosystems we depend on, being on Earth,’ he says. ‘There are many ways in which life can survive and can function.’ 

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Click here to read our review of Otherlands: A World in the Making by Thomas Halliday