Relevance to the Frequency of Life on Other Planets

Is the long wait for animals—the story of animal life on Earth—the exception or the rule for any other planets with emerging life? Could either oxygenation or the evolutionary steps necessary for completion of the complex animal body plan have occurred more rapidly on some other planet, and if so, under what conditions? The lesson of Earth's Cambrian Explosion is that two parallel preparatory steps must be taken if complex metazoans—animals— are to appear. First, an oxygen atmosphere must be constructed. This is surely the most critical environmental step. Second, a very large number of evolutionary adaptations must be concluded to allow the evolution of an ocean liner—our animals—from the toy sailboat—the bacteria—that began it all.

Both of these parallel tracks require time. There do not appear to be any shortcuts. On Earth, one or both required several billion years. And during that time, Earth had to maintain a temperature that allowed the presence of liquid water and avoid what we might call "planetary disasters" of sufficient magnitude to sterilize the evolving root stocks of animals. In the next chapter, we shall see why no such disaster put an end to animal evolution on planet Earth.

Mass Extinctions and the Rare Earth Hypothesis

Much of the work we do as scientists involves filling in the details about matters that are basically understood already, or applying standard techniques to new specific cases. But occasionally there is a question that offers an opportunity for a really major discovery.

—Walter Alvarez, T. Rex and the Crater of Doom

Imagine that we are in a spaceship orbiting Earth 65 million years ago— about 500 million years after the Cambrian Explosion described in the last chapter—on the day that an asteroid enters the atmosphere and streaks down toward what is now the Yucatan region of Mexico. We are about to witness the collision that will eliminate the dinosaurs (and 60% of all other species) from the rolls of the living.

The asteroid (perhaps it is a comet) is between 6 and 10 miles in diameter, and it enters the Earth atmosphere traveling at a rate of about 25,000 miles an hour. At such speed the body takes 10 seconds to pass through the atmosphere and then smashes into Earth's crust. Upon impact, its energy creates a non-nuclear explosion at least 10,000 times as strong as the blast that would result from humankind's entire nuclear arsenal detonating simultaneously. The asteroid hits the equatorial region in the shallow sea then covering the Yucatan and creates a crater as large as the state of New Hampshire. Thousands of tons of rock from the ground-zero impact area, as well as the entire mass of the asteroid itself, are blasted upward. Some of the debris goes into Earth orbit, while the heavier material reenters the atmosphere after a suborbital flight and streaks back to Earth as a barrage of meteors. Soon the skies over the entire Earth glow dull red from these flashing small meteors. Millions of them fall back to Earth as blazing fireballs and, in the process, ignite the verdant Late Cretaceous forests; over half of Earth's vegetation burns in the weeks following the impact. A giant fireball also expands upward and laterally from the impact site, carrying with it additional rock material that clots the atmosphere as fine dust is transported globally by stratospheric winds. This enormous quantity of rock and dust begins sifting back to Earth over a period of days to months. Great dust plumes and billowing smoke from burning forests also rise into the atmosphere, cloaking Earth in a pall of darkness. From space we begin to lose sight of the surface of the planet and can see only darkening gauze obscuring Earth's once green and blue surface. It is a vision from Dante's Inferno, a nightmare of red fires and black soot.

The impact creates great heat both on land and in the atmosphere. The shock heating of the atmosphere is sufficient to cause atmospheric oxygen and nitrogen to combine into gaseous nitrous oxide; this gas then changes to nitric acid when combined with rain. A prodigious and concentrated acid rain begins to fall on land and sea, and before it ends, the upper 300 feet of the world's oceans are acidic enough to dissolve calcareous shell material. The impact also creates shock waves spreading outward through the rock from the festering hole in the crust; Earth is rung like a bell, and earthquakes of un-

precedented magnitude occur. Huge tidal waves spread outward from the impact site, eventually smashing into the continental shorelines of North America, and perhaps Europe and Africa as well, leaving, when they recede, a trail of destruction and a monstrous deposit of beached and bloated dinosaur carcasses skewered on uprooted trees. The surviving scavengers of the world rejoice. The smell of decay is everywhere.

For several months after this fearsome day, no sunlight reaches Earth's surface,- the atmosphere is darker than the oil-fueled miasma that blanketed Kuwait following the Gulf War. After the initial rise in temperature from the blast itself, the ensuing darkness causes temperatures to drop precipitously over much of the planet, creating a profound winter in a previously tropical world. The tropical trees and shrubs begin to die,- the creatures that live in them or feed on them begin to die,- the carnivores that prey on the smaller herbivores begin to die. The Mesozoic era, which began 250 million years after the Cambrian Explosion featured in the last chapter, comes to the end of its nearly 200-million-year reign.

Following months of darkness, Earth's skies finally begin to clear, but the extinction—the death of myriad species—is not yet finished. The impact of winter comes to an end, and global temperatures begin to rise—and rise. The impact has released enormous volumes of water vapor and carbon dioxide into the atmosphere, which now create an intense episode of greenhouse warming. Climate patterns change quickly, unpredictably, and radically around the globe before Earth's temperature regains some equilibrium. From tropical to frigid, then back to even more tropical than before the impact, all in a matter of a few years. The temperature swings produce more death, more extinction.

All of this havoc creates death: the death of individuals, the death of species, the death of entire families of organisms. This event is a planetary catastrophe. Had the impacting object been only twice the size it was, it might have sterilized the surface of planet Earth. It was a narrow escape for complex metazoans.

Just 65 million years ago, such an impact event did end the Mesozoic era, and it ended the Age of Dinosaurs as well. It was but one of many impacts and other assorted global catastrophes that have imperiled complex life on Earth over the past 500 million years. Such events must happen on planets elsewhere in the Universe, and they would surely be the greatest obstacle to the continued existence of any complex metazoan that might exist there. Extinction events are an important aspect of the Rare Earth Hypothesis. Although the animals and plants of Earth have suffered grievously in the assorted mass extinction events through time, the damage could have been worse—and on many other planets where life may have evolved, it probably has been, or will be. If hit at an inopportune time, a planet's higher life might be snuffed out—or it might never be allowed to evolve in the first place.

As we saw in the last chapter, the Earth of 500 million years ago was teeming with complex animals and plants. Attaining such a world, for the first time populated by animals, required a large number of evolutionary and environmental changes and took 3 to 3.5 billion years. Maintaining these organisms required other conditions. Complex metazoans tolerate a far narrower range of environmental conditions than do microbes; there are no extremophile or anaerobic complex metazoans, for example. Complex metazoans are also far more susceptible to extinction caused by short-term environmental deterioration.

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