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The Cretaceous-Paleogene (K-T) boundary is a geological signature that marks the end of the Cretaceous period and the start of the Paleogene period about 66 million years ago. This boundary is significant because it coincides with a mass extinction event that saw the demise of approximately 75% of plant and animal species on Earth, including the dinosaurs.

The evidence of this boundary comes in the form of a distinct layer of sediment worldwide that contains a higher concentration of the metal iridium. Iridium is rare on Earth's surface but abundant in extraterrestrial objects like meteors and asteroids. Hence, the elevated iridium levels at the K-T boundary suggest an extraterrestrial event, such as an asteroid impact, could be responsible for the sudden and widespread extinction.

The asteroid impact hypothesis posits that the extinction of the dinosaurs and many other species at the K-T boundary was due to the effects of a colossal asteroid or comet striking Earth. Scientists Walter and Luis Alvarez, while examining the K-T boundary layer, found an excess of iridium, leading to this groundbreaking hypothesis.

A massive impact would have ejected enormous quantities of dust and aerosols into the atmosphere, blocking sunlight and disrupting the global climate. This could lead to a 'nuclear winter' scenario with drastically lowered temperatures, affecting photosynthesis and leading to a food chain collapse. The hypothesis is supported not just by the iridium anomaly but also by the discovery of the Chicxulub crater beneath the Yucatán Peninsula in Mexico, which dates to the same period.

Mass extinctions are periods in Earth's history when abnormally large numbers of species die out simultaneously or within a limited time frame. These events are often caused by catastrophic changes to the environment, such as volcanic eruptions, climate change, or, as in the case of the dinosaurs, asteroid impacts.

The extinction that occurred at the K-T boundary is one of five major mass extinction events identified in the fossil record. Each of these events fundamentally altered the course of life on Earth. The asteroid impact hypothesis for the K-T mass extinction helps explain not only the sharp increase in iridium but also the rapid environmental changes that led to the end of the reign of the dinosaurs and set the stage for the rise of mammals, including humans, to dominate the planet.