The mass extinction event that occurred about 66 million years ago, marking the boundary between the Cretaceous and Paleogene periods (K-Pg), remained a profound mystery for decades. Scientists struggled to explain the sudden disappearance of the non-avian dinosaurs and roughly 75% of all plant and animal species on Earth. Ideas about gradual climate change or disease were common, but none offered a definitive mechanism for such a swift, global catastrophe. The core concept of the Asteroid Hypothesis—that a large extraterrestrial impact caused the mass extinction—eventually emerged to challenge these long-held gradualist views.
The Initial Proposal and Scientific Skepticism
The first concrete evidence that sparked the Asteroid Hypothesis came from the discovery of an Iridium Anomaly in the K-Pg boundary layer across the globe. Geologist Walter Alvarez and his father, physicist Luis Alvarez, found concentrations of iridium hundreds of times higher than normal in a thin layer of clay in Italy. Since iridium is rare in Earth’s crust but abundant in asteroids, the Alvarez team proposed in 1980 that a 10 to 15 kilometer space rock had struck the Earth, creating a worldwide layer of iridium-rich dust. This radical, catastrophic explanation was met with intense scientific skepticism, fundamentally opposing prevailing gradualist theories. Critics argued the iridium layer was insufficient proof of a global catastrophe and pointed to the absence of a confirmed impact crater, forcing proponents to seek more compelling evidence.
Feedback Driving the Search for Physical Evidence
The community’s demand for geological corroboration beyond the chemical signature of iridium spurred researchers to search for physical indicators of a massive impact. This led to the discovery of shock-metamorphosed quartz, or “shocked quartz,” found in K-Pg boundary layers worldwide. These quartz grains contain microscopic deformations that form only under the extreme pressures of a massive impact, far greater than those generated by volcanic activity. Researchers also identified tiny glass spherules, known as microtektites, which are millimeter-sized droplets of molten rock blasted into the atmosphere by the impact. The presence of both shocked quartz and microtektites provided physical, globally distributed evidence that terrestrial processes could not easily explain.
Incorporating Competing Theories and Refining the Model
Despite the mounting impact evidence, the scientific community maintained a debate by focusing on the massive volcanic activity of the Deccan Traps in India, which occurred around the K-Pg event. This alternative hypothesis suggested that the sustained eruption of over a million cubic kilometers of lava spewed enormous quantities of climate-altering gases and aerosols. Proponents of the volcanism theory argued these eruptions caused severe environmental stress, including global warming and ocean acidification, which could have led to the mass extinction. This strong competing theory ensured intellectual rigor, forcing asteroid proponents to refine their model and address the volcanism data. They demonstrated that while the Deccan Traps likely contributed to long-term climate change, the impact scenario better explained the sudden, global distribution of impact-specific materials and high iridium concentrations, leading to the consensus that the impact was the ultimate trigger.
International Collaboration and the Confirmation of Chicxulub
The hunt for the physical impact site itself was the final, collective challenge posed by the scientific community. The large, circular structure beneath the Yucatán Peninsula, known as the Chicxulub crater, was eventually identified as the likely candidate. Its size—approximately 180 kilometers in diameter—and its age, precisely matching the K-Pg boundary, provided the missing piece of the puzzle. Global efforts involving geophysics, deep-sea drilling projects, and core sample analysis were organized by programs like the International Ocean Discovery Program (IODP). These cooperative expeditions recovered rock cores from the crater’s peak ring, which contained iridium and other impact materials, indisputably linking the crater to the global K-Pg layer.