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Catastrophes and calms

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13.08.2020

In certain places around the world – in the Badlands near Drumheller in Alberta, Canada, in the Geulhemmergroeve tunnels in the Netherlands, or in the Hell Creek formation in eastern Montana – you can touch a thin line of rock, and know you are touching the most famous mass extinction event on Earth. This Cretaceous-Paleogene (K-Pg) boundary layer is a seam of clay found all over the world, enriched with iridium – an element that appears on the Earth’s surface only when something from outer space crashes there. It represents a fleeting geological moment, separating the epoch of the dinosaurs from the world we know without them.

While dinosaurs get most of the attention, they weren’t the only ones to disappear. More than half of all life on Earth – from plankton to pterosaurs – perished following the impact of the Chicxulub asteroid that created a 150 km-wide crater in the Yucatán peninsula. It took 30,000 years for life to re-emerge in the fossil record, and another 4 to 9 million years to return to pre-impact levels.

What happened in those first 30,000 years? Some animals were still around even if their numbers weren’t great enough to be fossilised – otherwise we wouldn’t be here today. But what was it like to be a survivor in the aftermath of the catastrophic event?

It probably helped to be a creature that lived underground, given the asteroid impact shot 10 trillion tons of rock and other debris high into the atmosphere. As this debris fell back to Earth, the entire globe was bombarded with superheated projectiles. For those that survived the intense heat of this onslaught, the weather forecast for the next several months would be acid rain, dark skies and below-freezing temperatures as a layer of fine atmospheric particulates and a cloud of sulphuric acid covered the world.

Survivors of the impact – such as the small, rat-like Procerberus – would eventually have crawled out of hiding into the ultimate postapocalyptic environment. Massive fires started by the superheated bullets were still raging. Rotting carcasses and charred remains of those caught on the surface – billions of unfortunate plants and animals – littered every corner. Reliable sources of clean water would have dried up or been contaminated by rock, dust or acid rain.

Survival required ingenuity and flexibility. Normal sources of food were gone, the temperature had plummeted, members of families and social groups were dead or dispersed. Such conditions might force a typically tree-dwelling species, such as our tiny insectivorous ancestor, to find shelter in caves. Or they might have forced an omnivorous, mostly plant-eating species, such as the freshwater Neurankylus turtle, to become a dinosaur-meat scavenger. The only silver lining would have been the total annihilation of all the large predators – the relatively tiny survivors could forage for charred scraps in peace.

The resilient few not only survived, but also eventually reproduced – which must have required even more flexibility. Given the shortage of options, survivors couldn’t afford to be too picky in their choice of mates. In fact, mating between different species was probably common in the aftermath. About the same time as the K-Pg extinction event, there is genomic evidence of increased hybridisation between plant species.

This postapocalyptic period was characterised by novelty. New genetic combinations resulting from crossbreeding would have added to an array of anomalies caused by developing in a stressful environment. The first generation of the new world order wouldn’t copy the habits that had enabled their lineages’ survival for aeons; instead, they would copy habits their parents had improvised out of the necessity of the moment. Emerging from the ashes, the biotic world looked nothing like the one before the asteroid struck. It would take millions of years for pre-impact levels of species diversity to rebound and for ecosystems to stabilise.

When things did settle back down, the pace of evolution would return to a virtual standstill. That’s the pattern we observe in the fossil record: disruption, change and then long periods of stasis. However, it took many years for scientists to accept this pattern. It contradicted the Darwinian paradigm, where evolution should occur through slow and gradual changes. Under the Darwinian view, the diversity of life can be explained by simply adding up many, many small inherited changes over a long period of time. Such gradualism was believed to be a necessary part of adaptation by natural selection – the process by which some variants of traits are lost each generation, because their bearers leave no offspring.

The evolution of something new, such as eyes or feathers, requires a heck of a long waiting time

But this persistent focus on natural selection as the sole mechanism of adaptive evolution has always been a sticking point. It can’t properly explain how anything new arises. After all, natural selection is a process that eliminates unfit variants – it doesn’t create, but changes the prevalence of what’s already there. Instead, novelty must come from the purely random process of genetic mutation. The problem is that when new mutations appear, they’re usually not a good thing. They are more likely to disrupt well-adapted systems than to improve them, especially if they have a big effect. The upshot is that the evolution of something new, such as eyes or feathers, requires a heck of a long waiting time. Not only is there a long wait for a beneficial mutation to come along, but then there’s the long process of accumulating enough of them to build up, step by step, a complex new structure.

Fortunately for the Darwinian paradigm, geological time gives evolution millions and billions of years to work with. Yet in the 1970s, the American palaeontologists Stephen Jay Gould and Niles Eldredge proposed that the pattern of stasis and disruption might be something more than just imperfections in the fossil record. This punctuated equilibrium, as they called it, might reflect the uneven way that evolution actually unfolds. If Gould and Eldridge were right, then natural selection on random mutation suddenly had a much shorter timescale in which to accomplish major evolutionary changes. Because of this, punctuated equilibrium was initially met with scepticism.

However, there’s been a growing acceptance of this pattern among evolutionary biologists and theorists over the years, as new........

© Aeon


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