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«Once one dismisses
The rest of all possible worlds,
One finds that this is
The best of all possible worlds.»

– Leonard Bernstein, Candide

65.5 million years ago the Chicxulub bolide, a mountain of rock some 12 km across, struck the Earth. The asteroid’s origins are unknown, but fate had tied the destiny of our home, Earth, to that of the rock. When the rock struck, our Earth was changed forever.

The impact left several legacies: the enormous Chicxulub crater just off the coast of Mexico’s Yucatán Peninsula, a layer of iridium-rich clay that forms the boundary between Mesozoic and Cenozoic in the Earth’s rock strata, and the extinction of some 75 % of all fossilisable species.

In short, the impact marked the end of the Mesozoic with the great Cretaceous-Palaeogene (or K-Pg) mass extinction event. It ushered in a new era in the evolution of vertebrate life on Earth. Many groups of organisms, for example the ammonoid cephalopods, the bennettitalean plants, and the enantiornithean birds along with most other dinosaurs, were completely obliterated by this, the greatest extinction in almost 200 million years. The mammals, those furry, lactating therapsids, the creatures that had lost the original battle for ascendancy after the last great extinction to the dinosaurs, suddenly found themselves in an empty world almost devoid of competition.

In the wake of the K-Pg extinction, mammals exploded into dazzling array of giant forms. Within 10 million years, almost all of the modern groups of mammals (and then some) had already appeared. The mammals conquered the land, the water, and even the air (although the last remaining dinosaurs, the neornithean birds, are unquestionably dominant at least in the air – they actually contain about half of all amniote species). Today, furry creatures occupy every continent and, only 65 million years after the asteroid struck, have produced organisms of astonishing variety, from blue whales to shrews, from hyaenas to humans, from elephants to bats. The (non-neornithean) dinosaurs, once the largest land animals on the planet, have been reduced to mere stone and imagination.

But what if none of this history were true?

What if the rock had missed?

One can speculate.