Approximately 66 million years ago, a catastrophic event known as the Cretaceous-Paleogene (K-Pg) extinction marked the end of the Cretaceous period. This mass extinction resulted in the disappearance of about 75% of all plant and animal species on Earth, bringing an abrupt end to the 150-million-year reign of the non-avian dinosaurs. The event closed the Mesozoic Era and began the Cenozoic Era, the age of mammals. The geological record shows this transition with a distinct sediment layer found worldwide, pointing to a sudden environmental crisis.
The Asteroid Impact Theory
The leading explanation for this mass extinction is the asteroid impact theory, first proposed in the 1980s by the father-and-son team Luis and Walter Alvarez. Their hypothesis stemmed from a peculiar discovery at the K-Pg boundary: a thin layer of clay enriched with the element iridium. Iridium is scarce in the Earth’s crust but is significantly more abundant in asteroids and other cosmic debris, suggesting an extraterrestrial origin for the layer.
Further investigation led scientists to estimate the object was a comet or asteroid approximately six to nine miles wide. For years, the theory lacked a piece of evidence: the impact crater. The “smoking gun” was located in the 1990s, buried beneath the Yucatán Peninsula in Mexico. Known as the Chicxulub crater, this massive structure is about 110 miles in diameter and dates precisely to the time of the extinction.
The energy released by this impact would have been immense, equivalent to millions of nuclear weapons detonating at once. The discovery of a second, smaller crater in Boltysh, Ukraine, which predates the Chicxulub event by a few thousand years, has raised the possibility that the extinction was the result of multiple impacts.
Widespread Volcanic Activity
Extensive and prolonged volcanic activity is also implicated as a contributor to the environmental stress of the late Cretaceous period. In modern-day India, a massive volcanic province known as the Deccan Traps was spewing enormous quantities of lava, ash, and gas for thousands of years leading up to the extinction event.
These eruptions were on a scale difficult to comprehend, covering vast areas with molten rock and releasing significant amounts of climate-altering gases into the atmosphere. The sustained release of sulfur dioxide could have led to acid rain and periods of global cooling, while emissions of carbon dioxide would have contributed to long-term greenhouse warming and ocean acidification.
Many researchers now believe that this intense volcanism did not act as a competing theory but rather as a significant contributing factor. The long-term environmental degradation caused by the Deccan Traps likely weakened global ecosystems. This pre-existing stress would have made many species more vulnerable to the sudden and catastrophic effects of the asteroid impact, setting the stage for a more severe mass extinction.
The Immediate Aftermath
The initial blast wave from the Chicxulub impact would have obliterated everything for hundreds of miles, triggering earthquakes of immense magnitude and widespread volcanic eruptions. The impact, occurring in a shallow sea, also generated mega-tsunamis that inundated coastlines far from the point of collision.
As superheated rock and debris were ejected into and above the atmosphere, they rained back down across the globe, igniting widespread firestorms. The soot and ash from these fires, combined with the pulverized rock from the impact itself, filled the sky. This cloud of aerosols enshrouded the planet, blocking sunlight for months, and possibly even years, initiating a phenomenon known as an “impact winter.”
With sunlight unable to reach the surface, photosynthesis ceased on a global scale. This caused a near-total collapse of food chains, starting with the plants and marine plankton that form the base of most ecosystems. Herbivores that relied on these plants quickly starved, followed by the carnivores that preyed upon them. The plummeting global temperatures and lack of food created a hostile environment that few organisms could survive.
Survival and the Rise of Mammals
In the wake of this global catastrophe, survival depended on size and adaptability. The creatures that survived were small, had versatile diets, and could find shelter from the harsh surface conditions. This is why many early mammals, birds (the direct descendants of avian dinosaurs), amphibians, and some reptiles like turtles and crocodilians were able to endure. Burrowing underground or living in aquatic environments offered protection from the initial blast effects and subsequent environmental fallout.
The extinction of the dinosaurs created an enormous ecological void. The sudden absence of these dominant terrestrial animals opened new opportunities for the survivors. With the dominant predators and herbivores gone, mammals, which had previously been small, nocturnal creatures living in the shadows, were free to diversify and evolve.
This moment set the stage for the Age of Mammals. Surviving mammalian species began to fill the newly vacant ecological niches, growing in size, diversity, and complexity. This evolutionary radiation eventually led to the emergence of the vast array of mammals we see today, including primates and, ultimately, humans. The end of the dinosaurs’ reign was the beginning of our own.