About 65 million years ago, dinosaurs, the largest, most fearsome creatures ever to rule the planet, died off in vast quantities. Although this mass extinction didn't happen literally overnight, in evolutionary terms, it may as well have--within a few thousand years of whatever catastrophe caused their demise, the dinosaurs had been wiped off the face of the earth. (See 10 Myths About Dinosaur Extinction)
The Cretaceous-Tertiary Extinction Event--or K/T Extinction Event, as it's known in scientific shorthand--has spawned a variety of less-than convincing theories. Up until a few decades ago, paleontologists, climatologists, and assorted cranks blamed everything from epidemic disease to lemming-like suicides to intervention by aliens. That all changed, though, when the Cuban-born physicist Luis Alvarez had an inspired hunch.
Did a Meteor Impact Cause the Extinction of the Dinosaurs?
In 1980, Alvarez--along with his physicist son, Walter--put forth a startling hypothesis about the K/T Extinction Event. Along with other researchers, the Alvarezes had been investigating sediments laid down all over the world around the time of the K/T boundary 65 million years ago (it's generally a straightforward matter to match geologic strata--layers of sediment in rock formations, river beds, etc.--with specific epochs in geologic history).
These scientists discovered that the sediments laid down at the K/T boundary are unusually rich in the element iridium. In normal conditions, iridium is extremely rare, leading the Alvarezes to conclude that the earth was struck 65 million years ago by an iridium-rich meteorite or comet. The iridium residue would have scattered all over the globe--and the massive amounts of dust raised by the impact would have blotted out the sun, and thus killed the vegetation eaten by herbivorous dinosaurs. When the herbivorous dinosaurs starved to death, the carnivorous dinosaurs would have succumbed. The result: a long period of scarcity, cold, and darkness, and--when all was said and done--no dinosaurs.
Where is the K/T Impact Crater?
It's one thing to propose a massive meteor impact as the cause of the dinosaurs' extinction, but it's another thing to present convincing proof. The next challenge the Alvarezes faced was to identify the responsible astronomical object, as well as its impact crater--not an easy matter, since the earth's surface is geologically active and tends to erase evidence of large meteorite impacts over the course of millions of years.
Amazingly, a few years after the Alvarezes announced their theory, investigators found the buried remains of a huge crater in Chicxulub, on Mexico's Mayan peninsula. Analysis of sediments demonstrated that this gigantic (over 100 miles in diameter) crater had been created 65 million years ago--and was caused by an astronomical object, either a comet or a meteor, sufficiently large to cause the extinction of the dinosaurs (along with plenty of other prehistoric reptiles, including pterosaurs, plesiosaurs and mosasaurs).
Was the K/T Impact the Only Factor in Dinosaur Extinction?
Today, most paleontologists agree that the K/T meteorite (or comet) was the prime cause of the extinction of the dinosaurs. However, this doesn't mean there couldn't have been aggravating circumstances: for instance, it's possible that the impact caused or was concurrent with an extended period of volcanic activity, further polluting the atmosphere, or that new diseases may have picked off dinosaurs already weakened by hunger and cold.
It's also important to remember that the K/T Extinction Event wasn't the only such catastrophe in the history of life on earth--or even the worst, statistically speaking. For example, the end of the Permian period, 250 million years ago, witnessed a still-mysterious global extinction in which over 70 percent of land-dwelling animals and a whopping 95 percent of marine animals went kaput. Ironically, it was this Permian/Triassic extinction event that cleared the field for the rise of the dinosaurs toward the end of the Triassic period--after which they managed to hold the world stage for 150 million years.