Life on Earth is entering the greatest mass extinction since the death of the dinosaurs, according to a major new study – and humans may be among the casualties. Such a catastrophic loss of species would leave a huge hole in the world’s ecosystems, and all sorts of weird and wonderful life would evolve into the vacancies left behind.
To consider what life after a mass extinction might involve, we can look to the past. There have been five major mass extinctions in Earth’s history – though colleagues and I recently proposed a sixth – and comparing current rates of change to the geological record of the “Big Five” extinctions suggests that this time the warning signs are real.
So let’s be pessimistic, and assume the apocalypse is going to happen. What does Earth look like afterwards?
The greatest crisis in history
The Permian-Triassic boundary (251m years ago) saw the greatest crisis in Earth’s history, when at least 90% of species died off. Even insects suffered huge losses – the only mass extinction in their long history.
The event is widely attributed to the effects of the Siberian Traps – huge volcanic outpourings of lava and associated greenhouse gases, in what is now northern Russia. This lead to global warming, ocean acidification and acid rain, marine oxygen depletion and poisoning by toxic metals such as mercury. Imagine today’s gloomiest climate predictions, but cranked up a few notches.
The few species that survived gave rise to all life thereafter and there has not been such a profound restructuring of ecosystems since, perhaps because this “survival of the fittest” rendered their descendants more tolerant to global change.
What did the planet look like in the Early Triassic? It was hot – hot as hell – and seemingly lifeless over vast areas. Sea-surface temperatures reached up to 45°C in the tropics. In the vast Pangaean desert it was probably even hotter.
The heat caused land animals, marine reptiles and fish to disappear from the fossil record in all but the high latitudes, which were presumably a little cooler, for millions of years. In fact, there are several “gaps” in the Early Triassic.
The bulk of the world’s coal today derives from vast swathes of the Permian seed fern Glossopteris – a prominent casualty, whose loss led to a “coal gap” of at least 12m years.
A series of Early Triassic “fungal spikes”, where rocks contain greatly enhanced numbers of spores, has been attributed to huge amounts of dead plant and animal matter available for fungi to feed upon. The heat, and acid rain-induced destruction of soils (which would have smelled of vanilla), must have rendered the planet largely uninhabitable.
Without plants there are no plant-eaters. Without herbivores there were no carnivores. One of the few “big” survivors on land was the “shovel lizard” Lystrosaurus, an odd-looking vegetarian which, in the absence of predators and competitors, diversified with some success during the Triassic.
The carnage was worse in the oceans, where up to 96% of species went extinct. The loss of all reef-building corals led to a 10m year Early Triassic “reef gap”. Think of it: a world without reefs – and without all the diverse and abundant life they support.
But Earth wasn’t quite lifeless – and as well as Lystrosaurus there were marine success stories amid the horror. Claraia was an opportunistic genus of scallop-like bivalve that survived the end-Permian, and then quickly diversified to fill the vacant niches left by the almost total annihilation of the dominant Permian sea-floor dwellers, the brachiopods. Claraia was tough and could withstand very low oxygen levels – a trait that came in very handy when most sea-bed life was being starved of oxygen.
Perhaps the most famous and eye-catching extinction saw the death of the (non-avian) dinosaurs around 66m years ago at the Cretaceous-Tertiary boundary. As well as picture-postcard victims such as T. rex, the turnover in tiny plankton at the other end of the food chain saw an end to the formation of the famous Cretaceous chalk cliffs that are so widespread across Europe (the period’s name comes from the German “kreide”, meaning chalk).
Whether it was a meteorite, more massive volcanic eruptions, or a bit of both that did the damage, in comparison to the Permian-Triassic scenario, the death of the dinosaurs was more modest (around 75% of global species lost) and the recovery was more rapid. Either Earth sorted itself out more quickly, or, following the “Great Dying” 185m years previously, life had become better at adapting to, and evolving with, stress.
Of course, dinosaurs are not exactly extinct. Birds are highly evolved dinosaurs that derive from the few dinosaurian survivors of the Cretaceous-Tertiary (K-T) event and nobody can deny their evolutionary success in the 66m years since the demise of the chicken-like T. rex.
Crocodiles and alligators – the closest living relatives of birds – are among the other prominent survivors. While it’s clear that birds’ ability to fly to oases of calm and plenty allowed them to flourish amid the upheaval of the K-T boundary, it is not obvious why crocodilians survived. Theories suggest their cold-blooded bodies (vs. the supposed warm-blooded theropod dinosaurs), their [fresh or brackish water habitat](http://www.bioone.org/doi/abs/10.1671/0272-4634(2008)28%5B409%3ATOACPP%5D2.0.CO%3B2) or even their high IQ enabled them to flourish.
The good news amid all this death and destruction is that life on Earth always recovers, even when it has been really badly damaged. Without extinction, there is no evolution – the two are intrinsically linked.
The earliest dinosaurs evolved 20m years after the Permian-Triassic losses. Their evolution was almost certainly driven by a freshening of climate during the “Carnian Pluvial Event” (when it rained, a lot), new-found lush vegetation and the swathes of ecospace available to colonise.
Dinosaurs lived for 165m years before their demise, but without their death, humans probably wouldn’t be here today to do their damage. Mammals, of course, were the great beneficiaries of the dinosaurs’ downfall.
If humans are indeed doomed then we won’t be around to see what evolves to replace us. But rest assured, we geologists don’t take ourselves too seriously – we know that Earth is bigger than us, and it will bounce back.