Today's Reading

Battered quartz crystals, vast amounts of prehistoric soot, and a rare metal called iridium, found just at the geological levels where the fossil record of non-avian dinosaurs disappears, suggested that some kind of extraterrestrial body had slammed into our planet. First proposed in 1980, at the fevered height of a new scientific interest in dinosaur biology, the idea set off an academic firestorm akin to the very impact it described. Paleontologists, geologists, and astrophysicists fought as fiercely as tyrannosaurs in conferences and journals over the proper interpretation of the results. But the discovery of an enormous impact crater in the Yucatán Peninsula in the 1990s settled the debate: a massive asteroid about seven miles across had struck Earth at just the moment the extinction becomes apparent in the strata. Nothing like this had ever been seen before. Physicists calculated that the initial impact that created the Chicxulub crater in Central America would have been powerful enough to blow many terrestrial dinosaurs in the vicinity off into space. But it wasn't just the initial hit that sparked the extinction. The aftereffects of this dramatic event tipped the scales against the terrible lizards and many, many other forms of life.

Often, this is about as far as the discussion goes: an immense rock smacked into the planet and myriad species were summarily snuffed out. Simple as that. The asteroid becomes a cosmic bullet shot into the Earth's gut. Yet there have been other impacts of similar or greater scale throughout our planet's history—impacts that did not trigger biological disasters. About 35 million years ago, another large asteroid struck ancient Siberia and carved out the Popigai crater, sixty-two miles across. That's more than ten miles wider in diameter than the impact crater in the Yucatán. But this more recent strike did not cause a mass extinction. There was local upheaval and damage, certainly, but life on the rest of the planet kept trotting along much as before. Not all impacts are equal.

The Cretaceous killer thus becomes a special case, standing out from other impacts through time. The size of the K-Pg asteroid, its speed, its angle, and the nature of the rock it struck all came together in the worst possible way for life on Earth—a set of complete happenstances that coalesced into nothing short of an apocalypse. It wasn't just that Earth was hit by a massive asteroid. It's that the aftermath of the impact played out in such a way that life was pushed to the breaking point, with many organisms unable to cope with the rapid changes. Earth swung between a world of fire and ash and one of withering, persistent cold and darkness. Dinosaurs didn't just collapse when the asteroid struck. The real extinction played out over hours, days, months, and years in a constant state of flux as a new world emerged from the cosmic shake-up.

The K-Pg disaster was a global event, its story told through evidence gathered from many places across the planet. But the fossil record is uneven, yielding a collection of pinholes to look through to try to ascertain the whole. As the naturalist Charles Darwin famously observed, the world's geological strata are like a book that lacks entire pages, paragraphs, sentences, and words from the story, leaving us to piece together the narrative from what might seem like isolated parts. By luck, good or bad, some chapters are richer than others. So far as the K-Pg transition goes, the best of these is in the western United States among the Hell Creek Formation beds of central Montana and the Dakotas. This relatively narrow expanse of our planet documents the last days of the dinosaurian reign up through the earliest days of the Paleogene period that followed. The impact boundary is clearly visible in the rock record itself. Sections of these strata displayed in museums look like the world's most regrettable chunk of chocolate cake, dark brown and deadly. In this place we know the cast of characters who ambled across this ancient stage well and can track their fates across time and their changing environment. Their stories tell us how life suffered greatly, yet still survived.

But the reason we've gone back to this place and this one infamous moment is to understand not only why there are no Ankylosaurus descendants at the zoo but also how and why we came to exist. The Age of Mammals, a marker literally set down in stone, would never have dawned if this impact hadn't allowed for evolutionary opportunities that were closed for the previous 100 million years. The history of life on Earth was irrevocably changed according to a simple phenomenon called contingency. If the asteroid's arrival had been canceled or significantly delayed, or if it had landed on a different place on the planet, what transpired during the millions of years that followed the strike would have unfolded according to an altered script. Perhaps the non-avian dinosaurs would have continued to dominate the planet. Maybe marsupials would have held sway as the most common beasts. Perhaps some other disaster, like massive volcanic eruptions in ancient India that picked up around the same time, would have sparked a different sort of extinction. It's likely that the Age of Reptiles would have marched on unimpeded, but without the origin of any species introspective enough to engage in such ruminations about time and its flow. This day was as critical for us as it was for the dinosaurs.

Now, after decades of fierce scientific debate, our picture of what transpired is starting to become clearer. Paleontologists, geologists, astronomers, physicists, ecologists, and others have assembled a more detailed tableau of what happened to the planet following the collision. It wasn't the impact itself that caused such dramatic damage, but the long-lasting aftereffects that permanently reshaped the nature of life on Earth and allowed for the eventual and unintended emergence of humans. By imagining ourselves in the heyday of the dinosaurs at Hell Creek, on Extinction Eve and what follows, I'm going to walk you through what happened in the seconds, days, months, years, centuries, and millennia after the impact, tracking the sweeping disruptions that overtook this one spot and imagining what might have been happening elsewhere on the globe.

This excerpt is from the hardcover edition.

Monday we begin the book DAMN LUCKY by Kevin Maurer.

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