In the autumn of 1983 a mysterious phenomenon swept quietly but quickly through the waters of the Caribbean. One of the world's most beautiful species of sea urchin -- a once-abundant coral-reef denizen with long black spines -- almost became extinct in a matter of weeks.

There was no oil spill, no overkill by skin divers, no human involvement at all, as far as scientists can make out. More likely, it was a natural event, perhaps a viral epidemic, that killed all but a few of the creatures.

Whatever the cause, evolutionary biologists say the event gave them a rare glimpse of a natural phenomenon that is coming to be regarded as one of the most important forces governing the evolution of life: extinction.

New life forms arise, biologists are beginning to believe, largely because old life forms die out. With that somewhat negative idea, biologists are challenging the more positive mechanisms of evolution put forth 125 years ago today -- on Nov. 24, 1859 -- when Charles Darwin's epochal book, "The Origin of Species," was published. Darwin proposed that evolution was a response to competition for scarce resources such as food and habitat. Ironically, the debates among evolutionists are heating up just as their battles with biblical creationists seem to be cooling down.

State laws requiring that creationism be taught in public schools have been struck down in several states. Where they remain, the laws are conspicuously not enforced.

As a result, evolutionists say, children are now free in every state to learn not only the ideas Darwin published but also to follow the deeper challenges that evolutionists themselves are posing.

The new emphasis on extinction stands in contrast to Darwin's proposition that evolution was a response to competition for scarce resources such as food and habitat. Individuals possessing advantageous traits -- a stronger wing, perhaps, or a sharper tooth -- were more likely to survive than those without them.

Over the eons, Darwin held, a gradual accumulation of advantageous traits slowly transformed the population of one species into a new species. Inherent in his book was the idea that competition led to progress as species steadily became more perfectly adapted to their habitats.

Current challenges to orthodox Darwinism suggest that competition has less to do with major evolutionary change than does extinction, which evicts the tenant of a habitat and makes it available to any new species that happens to be able to move in.

Central to the emerging view is an analysis of the extinction of dinosaurs 65 million years ago. There is evidence that it was caused by a giant asteroid or comet colliding with Earth. There is even a suggestion that the Earth may undergo periodic bombardment by asteroids that trigger mass extinctions every 26 million years.

Revising Darwinian theory is, of course, nothing new. Like almost all great scientific ideas, the theory of evolution itself has evolved. Recent years, however, have witnessed a near-revolution.

In addition to the focus on extinction, evolutionary theorists these days are examining radical new ideas about how species arise in the first place.

Biologists still believe that this happens as the result of mutations, alterations in the genetic messages that parents bequeath to their offspring. However, the explosion in genetic-engineering research has revealed some surprising natural mechanisms that could produce mutations.

Perhaps the most dramatic involves a newly discovered type of virus that can pirate genes from one species, infect another, and then take up permanent residence in the infected animals' cells. Its cargo of foreign genes, each conferring some new trait, then may be passed to descendants exactly as if they were native genes.

These ideas are arising even before an older challenge to Darwinian orthodoxy has been resolved.

In that instance, biologists proposed in the 1970s that evolution proceeds not in the steady, gradual manner Darwin suggested but in brief, but big, jumps that produce a large change in a comparatively short time. Except for these jumps, the new theory asserts, species evolve very little.

"This is turning out to be an exciting decade," said David Raup, a paleontologist at the University of Chicago and a prominent figure in the debates.

Orthodox Darwinism, he said, "stood us in good stead for several decades but an awful lot of good ideas were squashed down by that dogma. Now we're seeing a lot of new and extremely stimulating ideas. Many of them are probably wrong but, overall, we're moving toward a better understanding of evolution."

The current ferment developed about eight years ago when two young paleontologists, Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History in New York, proposed their theory of "punctuated equilibria."

Species, they said, do not change gradually throughout their existence. Most of the time they do not change much at all. They usually remain in equilibrium for millions of years.

Then, for unknown reasons, a small, isolated population of the species begins to evolve rapidly. Within the geologically brief span of perhaps 100,000 years, the population changes into a new species and then stabilizes, changing little for the next few million years.

Gould and Eldredge said their view emerged from an analysis of the fossil record. If gradualism had been the rule, it should be possible to arrange the bones of extinct species in a series of finely divided gradations from one major type of animal to another.

Instead, new fossil forms tend to appear suddenly in the sequence of geological layers, persist with little or no change for several million years, and then disappear.

Darwin was aware of this but believed that the remains of intermediate forms had not been found. More hunting and digging, he said, would fill in the missing links.

A century later most of the gaps remained. To Gould and Eldredge it was evidence for their new theory. New species, they claimed, apparently evolve into existence so quickly that the chance of finding any fossil remains are almost nil.

The sediments that trap and preserve fossil bones form only in scattered patches and then only in sporadic intervals. A period of 100,000 years may yield only an inch or two of accumulated sediment, or none.

The theory of punctuated equilibria was met with a storm of controversy that raged through the late 1970s and early '80s. Stalwarts of Darwinian orthodoxy, representing a majority of specialists in evolution theory, argued as Darwin had that the fossil record was incomplete.

They cited the generally accepted belief that new species arise in small, geographically isolated populations of an old species. The sudden appearance of a fossil species in the fossil record, they suggested, could be the result of the new species having migrated into the region being sampled in a dig.

Moreover, critics said, fossilization preserved only an organism's hard parts: bones or shells. Much evolutionary change could be accumulating in an organism's soft tissues without showing up in the hard parts. They cited examples of living species that are vastly different in body chemistry but have nearly identical skeletons.

Although the controversy has quieted without general agreement, both sides say it has stimulated a fruitful search for new forces governing evolution. It also has opened the field to ideas from other disciplines.

"The single most exciting new theme," Gould said, "is this idea that mass extinctions were an important factor. It has certainly downgraded the classic idea of competition as a major force."

The new theme achieved prominence in 1980 when Luis Alvarez, a Nobel laureate in physics, and his son, Walter, a geologist, suggested that the extinction of the dinosaurs 65 million years ago was the result of a comet or asteroid slamming into Earth.

The scientists, both at the University of California at Berkeley, believe the impact kicked up so much dust that it darkened the skies for months, killing vast areas of plant life and the animals that depended on them for food.

The effect may be similar to the "nuclear winter" that many scientists say might follow a nuclear war that raised huge amounts of dust, cutting off sunlight.

As evidence for a prehistoric "asteroid" winter, the Alvarezes cited a thin film of unusual dust sandwiched into the geological strata and covering the layer that formed just as the dinosaurs died out.

The layer is a generally accepted boundary marking the close of the Cretaceous period, the geological moment when the dinosaurs became extinct, along with perhaps half the known species living at the time.

The unusual thing about the dust was its content of iridium, an element that is rare on Earth but much more common in asteroids. Strikingly high levels of iridium have been found in this dust layer as sampled at several sites around the world.

If an asteroid or some other celestial object slammed into Earth, it would have vaporized, scattering its iridium atoms through the general dust cloud that eventually enveloped Earth and then settled to the ground.

"I think Alvarez has proved his case," Gould said. "I think the dinosaurs would still be here if it weren't for that event. Of course, we wouldn't be here."

The evolution of mammals is generally thought to have exploded once the dinosaurs and other animals vacated their habitats. Until the dinosaurs were gone, the little ratlike mammals of the age had no opportunity to evolve into larger forms of plant-eaters and meat-eaters. The dinosaurs occupied those ecological niches.

Last year the impact theory took a startling turn. Raup and a colleague at Chicago, John Sepkoski, put forth the idea that mass extinctions had taken place roughly every 26 million years. Examinations of the fossil record suggest that there were at least five waves of extinction, of which the dinosaurs' 65 million years ago is the best known. The last one appears to have occurred around 13 million years ago.

The periodicity of mass extinctions is by no means established, but the idea prompted scientists to wonder what kind of process could have such a lengthy cycle. No earthly mechanism appears likely. Such long cycles, however, are common in astronomy.

Thus was born the hypothetical suggestion that the sun has a distant, unseen companion star whose orbit brings it close enough to the solar system every 26 million years to trigger a hail of comets or asteroids. It is known that swarms of comets beyond the planet Pluto's orbit circle the sun. An approaching companion star, astronomers speculated, might exert enough gravitational pull on some of the distant bodies to disrupt their orbits and send them careening toward the sun.

Once every 26 million years, in this scenario, one or more of the comets might happen to come close enough to the Earth to hit it and trigger a blackout of the sun.

Though there is no direct evidence of any companion star, astronomers have already tried to name it. Some call it the "death star." Others call it Nemesis, for the Greek goddess of vengeance.

For all the emphasis on extinction, however, evolutionists must still grapple with the title problem of Darwin's book, the origin of species. Extinction may open ecological opportunities but it does not create new species. Mutations -- changes in an organism's genes -- do.

As geneticists know, all of an organism's physical features and some of its behavioral patterns are dictated by genes, the master molecules of heredity. As long as an organism inherits the same genes its parents had, it will look and behave much like its parents. Modifications appear only when the offspring inherit new or altered genes, the result of mutation.

Many things -- from cosmic rays to naturally occurring chemicals -- can cause mutations. Sometimes the gonadal cells can err when making duplicates of parental genes that will go into sperm or eggs. Often the mutation is harmful, but sometimes it will produce a change that, in an appropriate environment, could confer a survival advantage and possibly launch a new species.

Most mutation processes that evolutionists know of produce only small changes. Lately, however, geneticists have learned of a natural mechanism that might produce sudden large changes that would seem to fit with the theory of punctuated equilibria.

Earlier this year, for example, James W. Valentine of the University of California at Santa Barbara, suggested that a kind of virus may do the job. These are viruses that invade a cell and cause copies of their own genes to be spliced into those of the host animal. Unlike ordinary viruses, this kind does not kill the cell. Its genes become permanent residents and are passed on to descendant cells along with the native genes.

Geneticists had suspected that these special viruses originally acquired their genes by stealing them from other animals. These viruses, in effect, carry escaped animal genes, ferrying them from one species to another.

If the virus happened to infect gonadal cells, they could produce sperm or eggs carrying not only the species' native genes but also a complete new gene or set of genes that would alter the animal's form or behavior after reproduction.

Valentine suggests that if the virus delivered regulatory genes, they could dramatically alter the growth pattern of the infected animal, producing what geneticists have long called, usually with derision, a hopeful monster, a mutant whose only hope for survival would be to find itself in an environment where its mutation conferred an advantage.

Repeated mass extinctions, of course, would make many environments available to a hopeful monster.