Mass Extinctions Are Always Around 30 Million Years Apart

The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: Ordovician, Devonian, Permian, Triassic, and Cretaceous. We tend to think of an event as a catastrophe only if it affects us humans. But the worst disasters ever to befall the earth happened millions of years before anything remotely resembling Homo sapiens came along. Our planet has suffered at least five great mass extinctions, or die-offs, plus about twenty smaller ones.

Scientists who favor the catastrophe theory have observed that Earth's mass extinctions seem to follow a sort of timetable: the intervals between them are always around 26 to 32 million years. This has led astronomers to formulate what is familiarly called the "Shiva hypothesis," named after the Hindu god of destruction and renewal.

According to the Shiva hypothesis, every 30 million years or so something disturbs the trillions of comets that circle the sun at the outer limits of the solar system, in a formation known as the Oort cloud. A large number of the icy bodies, dislodged from their orbits, hurtle toward the sun. Along the way, a few strike the planets.

There are all sorts of conjectures about what it is that shakes the comets loose. Probably the most popular theory is that our sun has a dim companion star whose path periodically takes it through the Oort cloud. Although this celestial body has yet to be found, it's been given the Star Trekish name of Nemesis. Another possibility is Planet X, a theoretical tenth planet that follows an erratic orbit somewhere out beyond Pluto.

It may be wrong, though, to pin the blame on an undiscovered planet or star. The real culprit may be something less substantial. As our solar system circles the Milky Way, it drifts up and down, a movement known as galactic oscillation. In doing so, it periodically passes through clumps of dust and gas called interstellar clouds. Some astronomers feel these clouds may be dense enough to deflect comets from their orbits. There's no real proof that they do, but it is intriguing to note that we encounter the heaviest concentration of interstellar clouds once every 33 million years.

The earliest mass extinction took place about 440 million years ago, at the end of the Ordovician period; it involved mostly simple sea creatures. A second die-off 365 million years ago brought the Devonian period to a close. At the end of the Permian period, 250 million years ago, perhaps 90 percent of the plants and animals on Earth also met their end. A mere 45 million years later, a fourth extinction killed off, among other things, some of the early dinosaurs.

But the "terrible lizards" made a big comeback. According to Dr. Robert Bakker, "If we measured success by longevity, then dinosaurs must rank as the number one success story in the history of land life." They were undisputed rulers of the earth for at least 130 million years. Mammals, by contrast, have been top dog for less than 70 million. As for humans ... well, we've been on Earth for a paltry 100,000.

Most dinosaur species didn't hang around for the whole 130 million years, of course. They kept evolving and adapting. By the end of the Cretaceous period, about 65 million years ago, there were perhaps twenty species, possibly more, in the western United States alone, including the famous triceratops and tyrannosaurus. What is now the Great Plains was then covered by a shallow, warm inland sea, creating a semitropical climate in which the huge creatures flourished.

Then something happened that radically changed conditions on the earth and brought on what scientists call the "Time of Great Dying." Dinosaurs weren't the only victims, of course. But because they were so big and so well established, it's especially hard to imagine how they could be wiped out so suddenly and completely. In the words of paleontologist John R. Horner, they "seem to have disappeared in the geological equivalent of an instant."

How long it actually took them to die out is a matter of much dispute among scientists. At one end of the spectrum are the so-called catastrophists, who believe it could have happened in ten years or less. At the other end are the gradualists, who suspect it was more in the neighborhood of two million years.

One of the earliest catastrophists was the eighteenthcentury French scientist Pierre-Louis de Maupertuis, who suggested that, if a large comet struck the earth, it might well destroy entire species. Half a century later another Frenchman, Baron Georges Cuvier, became an outspoken champion of this theory.

Most of the scientific community, however, sided with English geologist Charles Lyell, who preached that all changes in nature happen in small increments, over long periods of time. The gradualists managed to keep the upper hand for the next century or so.

Then, in the mid-1970s, Luis Alvarez, a Nobel Prize-winning physicist, began analyzing samples of rust-colored clay that's found worldwide, sandwiched between limestone from the Cretaceous period and stone formed during the period that followed, the Tertiary - "like jam in a sponge cake," as one writer puts it. Alvarez discovered that the clay contained startlingly high amounts of iridium, a dense element that's rare in the earth's crust but abundant in extraterrestrial bodies - meteors, comets, and asteroids.

Barringer Crater Near the town of Winslow in the heart of the Arizona desert there is a huge bowl-shaped crater 4,200 feet across and 575 feet deep. It is called the Barringer, or Meteor, Crater. The Barringer Crater was formed about 50,000 years ago when a huge meteorite - a fragment of rock from space - thundered into the Earth's surface. It may have weighed as much as 885,000 tons and measured 330 feet in diameter. The impact was such that the force threw up a bank around the crater 130 feet above the level of the surrounding desert. Most meteorites burn or break up as they meet the resistance of the Earth's atmosphere. Scientists believe that this one was so vast, and moving at such high speed, that it hardly slowed down in the atmosphere and reached the Earth in one piece. It exploded on impact, scattering fragments of rock over a large area. Many of these fragments have been found and identified, but excavations in the crater itself have revealed no large mass that could have been part of the meteorite. The scientists' estimate of its size is calculated from the size of the crater. The Barringer Crater is only about 60 miles from Holbrook, Arizona, where another amazing meteorite strike occurred on July 19, 1912. Local people were astounded to find small stones raining from the sky. A total of 14,000 stones was eventually counted. Asteroids and comets It used to seem like a simple distinction. Asteroids were chunks of rock hurtling through space. Comets were icy rocks expelling a stream of gas in the form of a head (called a coma) and a tail; they were spotted as they orbited the sun. But now we know better. The difference is becoming more amorphous as time passes and we learn more about asteroids - usually called minor planets by astronomers - and comets. As it turns out, not all comets have a coma or tail, and some large asteroids, like Ceres, which is the largest in the asteroid belt between Mars and Jupiter, contain ice. Astronomers have also traditionally relied on location to distinguish between the two bodies, but that method isn't rock-solid. Most asteroids are found in the asteroid belt, but some, like the recently demoted Pluto, are farther out. And although ice-filled comets are thought to form beyond Jupiter's orbit, scientists aren't sure where the water comes from. All this confusion is complicating scientists' attempts to label newly-identified bodies. Comets in the asteroid belt can masquerade as asteroids; some meteor showers, which normally come from asteroids, are actually comet debris; and dead comets (ones that have lost their tails and are now only a rocky nucleus) can resemble asteroids when viewed from Earth. P/2007 R5 is one such hard-to-classify object. Since its discovery in 1999, it has appeared repeatedly - much like a periodic comet - every fourth September, but it doesn't have a coma or tail. During last year's pass, it brightened by an impressive factor of approximately one million and then faded - also typical comet behavior. But according to Karl Battams, who runs the comet-discovery program at NASA's Solar and Heliospheric Observatory, "There's no conclusive proof that it is a comet. Most likely it's the nucleus of an extinct comet!"

He concluded that near the end of the Cretaceous period, a comet or meteorite roughly six miles in diameter struck the earth, throwing up an immense cloud of dust that eventually settled, creating the layer of clay that marked the K-T (Cretaceous-Tertiary) boundary. There was one flaw in the theory, though: an object that size would have left an immense crater, and no one had discovered a crater that was the right size and age.

However, oil companies drilling off the coast of Mexico had long been aware of a curious circular formation on the ocean floor, buried beneath a thick layer of sediment. By the mid-1990s, scientists had identified it as an astroblemean eroded impact crater - and dubbed it the Chicxulub (pronounced chick-shoe-lube) crater.

According to geologist Charles Frankel, "the Chicxulub crater is universally recognized as the source of the K-T boundary clay - and as the likely cause of the great mass extinction." Not quite. There are still plenty of gradualists around and, although they're willing to concede that a comet or meteorite did hit the earth 65 million years ago, they don't believe that it caused the dinosaurs' demise.

Even among scientists who do believe it, there's a great deal of disagreement over just how the impact affected the dinosaurs. Some say that the cloud of dust blocked the sun for so long that it brought on an "impact winter," a global cooling trend. As plant life, deprived of sunlight - and perhaps pelted by acid rain - died off, so did the herbivorous dinosaurs that fed on it and so, in turn, did the carnivorous dinosaurs that fed on them.

Another scenario assumes just the opposite - that the shroud of dust created a greenhouse effect, raising the temperature of the earth. As the oceans warmed, the plankton died off, as did the creatures that fed on it, and so on up the food chain.

The gradualists don't buy either of these theories. Many of them believe that the dinosaurs were already a dying breed and that the Chicxulub event, at most, simply hastened their end. They place the blame on other factors, most notably what's called the Bearpaw regression. At the end of the Cretaceous, parts of the ocean floor were sinking; at the same time, the Rocky Mountains were rising. As a result, the shallow sea that covered most of North America slowly drained away, leaving behind a virtual desert that couldn't support such large creatures.

It's possible, of course, that it was neither an extraterrestrial object nor the receding seas that did the dinosaurs in. As Robert Bakker points out, "The mass murder that marked the end of the Cretaceous Period seems to attract all manner of solutions." Some are more credible than others. Here are a few of the most interesting:

1. A supernova - an exploding star-bombarded them with cosmic radiation.
2. Because they had poor tastebuds, they ate toxic plants that poisoned them.
3. Insects or parasites spread disease among them.
4. Their eggs were eaten by the crafty mammals that had begun to appear.
5. The shells of their eggs grew too thin; stress from overpopulation.
6. Alien spacecraft brought safaris here from other planets to hunt them.
7. They drowned in the biblical flood because there no room for them on the ark.
8. They were so overcome with boredom that they committed mass suicide.
9. Gases from volcanic eruptions destroyed the ozone layer, giving the dinosaurs unhealthy doses of ultraviolet rays.

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