TIME: 65 million years ago.
PLACE: What is now the Yucatan Peninsula in eastern Mexico.
EVENT: The crash of an asteroid 20 kilometers across.
EFFECT: The end of the dinosaurs and most other life forms on Earth at the time.

Suppose you were a lumbering triceratops. What would your walnut-sized brain have registered when this fiery crash occurred? Answer: Basically, almost nothing beyond an unbelievably intense light in the sky before you were instantaneously reduced to a heap of ashes, or even obliterated entirely if you happened to be in the impact zone. Here’s the scenario.

A huge fireball suddenly appears in the sky—for a second or two—fol- lowed by an ear-splitting impact. For thousands of miles around, animals like our friend the triceratops are incinerated, and those in the immediate vicinity of the impact simply vanish. The Earth seems to be ablaze, as a gigantic col- umn of dust and fire extends miles up into the stratosphere.

The asteroid impact sends a massive shock wave cycling to the other side of the planet, the wave moving both on the surface via massive tsunamis and beneath the ground as a wave in the Earth’s crust. This ground wave generates huge earthquakes, as well as activating numerous volcanoes. As the two shock waves traveling around the Earth in opposite directions collide, the Earth’s crust is forced up into a mountain ridge of Himalayan proportions. And this is just what’s going on in the first few hours following the impact!

The seas soon calm down and the fires go out. But the dust and smoke don’t clear away. Instead, the massive amount of debris thrown up into the atmosphere by the impact encircles the Earth in a dense black cloud, block- ing out the Sun for months—or possibly even years. It is literally midnight in winter—everywhere. Temperatures drop by more than 20 degrees centigrade worldwide and the surface of the Earth freezes solid. Photosynthesis by plants is stopped dead in its tracks and virtually all plant life dies. Animals quickly die too, at least those relying on plants for their sustenance, which means just about everything larger than a bacterium. Basically, the only thing left alive after a few months are bacteria, mosses, and perhaps a few insects, rat-like rodents, and fish.

Oddly enough, a strike by an asteroid or one of its cousins, a meteor or comet, didn’t raise much attention and was not regarded as a threat to life on

Earth until the work of Nobel-prize winning physicist Luis Alvarez and his son, Walter, both professors at the University of California, Berkeley. In Gub- bio, Italy in 1978, they discovered a thin layer of the rare mineral iridium in the soil. Iridium is not an element found in abundance in Earthly nature. But it does make up a very visible fraction of the material in the type of rock making up asteroids and meteorites. So it was a huge surprise to find this layer in the geologic record at just the point on the timeline where the dinosaurs departed and the age of mammals began.

Later, the same iridium layer was found at the very same place in the soil record in other parts of the world, leading paleobiologists to conjecture that the huge cloud of debris from the shattered asteroid impact at that time settled from the stratosphere to form that iridium layer. From here it was but a small step to conclude that this asteroid strike was the causal agent for the demise of the dinosaurs. The only missing element was the “smoking gun,” geologic evidence of the crater created by the impact.

The controversy as to whether or not there was an asteroid of the right size that struck and left a trace of iridium raged on for a decade or more until 1991 when a NASA satellite spotted the crater. What the satellite saw was a huge indentation underneath the Yucatan Peninsula, near the town of Chicxulub, which had been covered over by millions of years of weather erosion, shifts in the Earth, continental drifts, and other such phenomena that unfold only on a geologic timescale.

From the size of the crater, scientists estimated that the asteroid creating it must have been about 30 thousand feet in diameter—just about the height of Mount Everest. In short, the Earth was hit by a massive mountain. And the asteroid struck with a force of about 100 million megatons. That’s right, 100 million million tons of TNT. This amounts to five trillion atomic bombs the size of the one that leveled Hiroshima during World War II. Now that’s an explosion!

So ended the reign of the dinosaurs when this asteroid strike created the Chicxulub crater on the Yucatan Peninsula 65 million years ago. Or, at least, this is the conventional wisdom today for how the dinosaurs departed this Earth after ruling it for more than 150 million years. And thus opened an eco- niche for our distant ancestors to slip into. But things are never simple in sci- ence, and regardless of what the current fashion happens to be, there is always a counter faction arguing something different.

In this case, the counterargument is that it was not the Chicxulub impact that leveled the dinosaurs, but one that took place in India about 300,000 years later. This counterclaim says that the Yucatan strike just weakened the dinosaurs, and it took the second strike to actually finish them off. For our purposes, such academic quibbling is meaningless. Either strike serves admirably to illustrate how human life and society as it exists today can be wiped away in an instant by an indifferent cosmos.

One might wonder if perhaps the dinosaurs were just unlucky, and that the asteroid strike ending their reign (Mexican or Indian version) was a fluke. But as Goldfinger phrased it in the famous eponymous film, “No, Mr. Bond. I expect you to die.” The Chicxulub strike was neither the first, nor the biggest, nor the last such strike from outer space. Many believe that in the very early days of the Earth, over four billion years ago, a massive object struck the planet breaking it apart, one of the parts becoming what we now see as the Moon, while what remained is the planet we now call home. The next major strike, about 500 million years later, was most likely a cluster of asteroids so dense that the Earth’s crust actually melted, destroying all geologic evidence of the planet up to that time.

Similar strikes from the blue seem to have taken place every 200–300 mil- lion years, recycling the flora and fauna much like a Las Vegas dealer recycles the cards at the blackjack table, each “shuffle” removing dominant species, thus opening up opportunities for others to take their place. And the beat goes on even in modern times.