BOOK REVIEW : Clues to Extinction of the Species : EXTINCTION: Bad Genes or Bad Luck, <i> by David M. Raup</i> . W. W. Norton. $19.95, 192 pages

Once upon a time, paleontologists searched for fossils of unknown organisms so they could place them on the evolutionary ladder.

Most dwelled on the mystery of the origin of species, not the reasons for their demise. The assumption was that species became extinct because they couldn’t compete or adapt to change.

But, David M. Raup points out in “Extinction: Bad Genes or Bad Luck,” species do not simply self-destruct; unlike individuals, they do not have built-in senescence. Left to themselves, they live an average of 4 million years before their numbers fall below a critical mass and they die out.

Moreover, he says, before humans became major players in the extermination of species, most kill-offs resulted from the impact of extraterrestrial objects.

In “Extinction,” Raup, a professor of statistical paleontology at the University of Chicago, ventures even farther out on the limb he first climbed several years ago. Then, he claimed that the five major mass extinctions in the history of the Earth (during the Ordovician, Devonian, Permian, Triassic and Cretaceous periods) were caused by the periodic return of Nemesis, a companion star to the sun, whose existence is problematic.

Now Raup suggests that all extinctions, not just those five major episodes, ultimately were caused by the Earth’s impact with extraterrestrial debris. These include smaller “background” events that killed off only a handful of species.

He leads the reader rapidly, but affably, through such well-known statistical theories as the “Gambler’s Ruin,” which holds that the smaller the gambler’s pot, the sooner he will go broke. Or in this case, the larger a species’ population, the longer it will survive.

Relying on graphs and logic rather than daunting mathematical theorems, and bolstering its conclusions with evidence from computer analysis, “Extinction” is as satisfying as a good whodunit.

Raup also fleshes out his theories with examples from natural history. We read about the North American heath hen, a species whose numbers were whittled away by hunters--a first strike similar to the initial impact of a meteorite--until only a small colony remained sheltered on the island of Martha’s Vineyard off the coast of Massachusetts. By then, it was too late. The concentrated population fell victim to a disease--a second strike--and is extinct.

Many species experience what Raup calls “pseudoextinction”: they evolve into other species with many of their genes intact. These follow the rules of Darwinian evolution. But there is fossil evidence of life forms like the trilobites--marine animals with complex doubled-lensed eyes--which vanished after living 325 million years.

Raup argues that climate changes (ice ages) or geographic changes (shifting continents) occur slowly enough for species to migrate or evolve the kind of incremental accommodations Darwin described. But meteorite impacts are swift, and the craters they leave are telltale fingerprints.

These extinctions, says Raup, have been a good thing. Without them, life on Earth might have reached a steady state with minimum diversity and humans would never have evolved.

Raup credits geologists with revolutionizing the conservative science of paleontology. In 1946, R. C. Sprigg, working for the Australian government, found trilobite fossils in quartzite, a place no paleontologist would have looked. In 1959, Eugene Shoemaker, employed by the U.S. Geological Survey, discovered coesite, a form of quartz formed under the high pressure of impact, in the soil of Meteor Crater in New Mexico.

Then in 1987, Walter Alvarez, a Nobel laureate in physics suggested that a layer of iridium in rocks in Italy, Denmark and New Zealand--an element common in some meteorites but rare on Earth--was debris from the impact of an asteroid or comet that hit the planet about the time dinosaurs disappeared from Earth.

Since 1973, Shoemaker has been scanning the heavens, mapping Earth-crossing asteroids. As recently as 1989, an asteroid a third of a kilometer in diameter passed at a distance about twice that from the Earth to the Moon. (For what it’s worth, no one noticed it until three days after it had passed.)

When a ship has a 50-50 chance of capsizing, you make sure it has life boats. Intelligent denizens of a planet that has been struck so often by meteors might want to plan ahead.

Since there is no way to abandon spaceship Earth and nowhere to go anyway, it might be wise to find a way to destroy such an object before impact. (For those who enjoy designing Star Wars-like weapons, this might be a healthier outlet than aiming them at other human beings.)

Another massive impact would certainly give evolution on Earth a fresh chance, but there is little reason to expect that our kind of intelligent life would ever evolve again.

Next: Jonathan Kirsch reviews “Minority Party” by Peter Brown (Regnery Gateway) .