If you had been standing in what is now the Yucatán Peninsula in Mexico, you would not have had time to wonder.
A mountain-sized asteroid—roughly 10 to 12 kilometres across—tore through the atmosphere at extraordinary speed, brighter than the Sun and hotter than anything on Earth’s surface. In a heartbeat, it struck with a force equivalent to billions of nuclear bombs.
This was the Chicxulub impact.
The collision blasted a crater over 180 kilometres wide, vaporising rock, igniting forests, and sending a shockwave racing across continents. The sky itself seemed to fall. Molten debris—ejecta—was hurled high into the atmosphere before raining back down across the globe, each fragment glowing with the heat of re-entry. For a brief and terrible moment, much of the planet’s surface may have experienced searing temperatures, as if placed beneath a planetary broiler.
But the true devastation unfolded more slowly.
Fine particles—dust, ash, and sulphate aerosols—were lofted into the upper atmosphere, forming a thick veil that blocked sunlight. Photosynthesis faltered. Plants withered. Food webs, delicately balanced and deeply interconnected, began to collapse. The great non-avian dinosaurs, who had dominated Earth for over 160 million years, found themselves in a world that no longer supported them.
This was not a single bad day. It was the beginning of a prolonged ecological crisis.
We call this mass extinction event the Cretaceous–Paleogene, or K–Pg boundary. It marks the end of the Age of Dinosaurs and the dawn of a new world—one that would eventually be shaped by mammals, birds, and, much later, us.
The evidence for this catastrophic impact is written in stone.
All around the world, from Italy to Alberta, a thin layer of sediment marks this boundary. Within it lies an unusually high concentration of iridium, a rare element on Earth’s crust but common in asteroids. Shocked quartz—minerals fractured under immense pressure—and tiny glass spherules formed from vaporised rock tell the same story: something extraordinary happened here.
For decades, scientists debated alternative explanations. Volcanic activity, particularly the vast eruptions that formed the Deccan Traps in India, certainly played a role in altering the climate. These eruptions released enormous quantities of greenhouse gases and aerosols, stressing ecosystems long before the asteroid arrived.
But the consensus today is clear. The asteroid impact was the decisive blow—the coup de grâce in an already struggling world.
And yet, not everything perished.
Some creatures endured. Small mammals, tucked into burrows. Crocodilians, patient survivors of changing waters. Birds—the living descendants of theropod dinosaurs—weathered the storm and carried their lineage forward into the skies of a quieter, recovering world.
It is a humbling thought.
The forests returned. Life, as it always does, found a way to reassemble itself—different, reshaped, but resilient. The absence of the great dinosaurs opened ecological space, allowing mammals to diversify and, over millions of years, to evolve into forms both strange and familiar.
Including us.
So when we ask, “What killed the dinosaurs?” the answer is both simple and profound. A rock from space changed the course of life on Earth.
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