Extinction Through Geologic Time
Mechanisms capable of causing extinction include climatic change (including glaciation), sea level change, continental drift, changes in the chemistry of ocean (e.g., availability of oxygen) and atmosphere (e.g., ozone layer), and awesome messages from outer space (supernovae, cosmic radiation, impactors). Impact craters on land (such as the ones studied around the world by Gene and Caroline Shoemaker) demonstrate that large rocks can occasionally hit the Earth. If they are large enough, the consequences for living things are dismal, as we now know.
During the Phanerozoic, roughly the last 600 million years, the history of life is punctuated by a number of mass extinctions. The extinction at the end of the Cretaceous (extinction of dinosaurs and ammonites, K-T event) was but one of about half a dozen of such events, several of which were even more severe than the K-T event. Perhaps the largest ever, the end-of-the-Permian extinction, is estimated to have eliminated as many as 95 percent of the marine species of that time. (An estimate made by the American paleontologist David Raup, based on a census of the extinction of families.)
The mass extinctions through the Phanerozoic have reset, many times, the Earth's evolutionary systems, like a forest-fire or a hurricane may reset the ecologic system of a given region. Whereas in regional disturbances, re-population proceeds through immigration from nearby areas, global disturbance can only be remedied through evolution of new species. In the race to fill the newly available ecologic space, new players get a chance to show what they can do. Thus, dominance of life forms can move from one group to another (say, from trilobites to ammonites, or from reptiles to birds and mammals).
If the environment had not changed since the Devonian, (and there had not been major and multiple catastrophes wiping out major groups of organisms since then,) our seas might still be patrolled by armored fish, chambered cephalopods and scorpion-like arthropods with enormous claws, while the bottom would be crawling with trilobites. None of these creatures would be the same now as then, of course, as they would have adapted to a changing scenery of alliances and enmities between each other, and have become more efficient in maintenance and reproduction. However, they would be a lot more similar, as an entire community, to the life forms of the Devonian period than to what we see today.
All this is speculation, obviously. Yet, it is clear that the big changes in the life forms, at least those seen in fossils, have come with catastrophe. One reason the world developed the way it did, for the last 65 million years (and one reason why we are here) is the impact of a large asteroid on the Yucatan peninsula of Mexico, which set off a series of events killing half of the life on Earth, including the dinosaurs and the ammonites. This insight we owe to the work of the father-and-son team Luis Alvarez and Walter Alvarez, and their collaborators [see exhibit]. Luis Alvarez, a physicist, recognized the importance of finding the rare metal iridium at the Cretaceous-Tertiary boundary, sampled in Gubbio, Italy by the geologist Walter Alvarez. Iridium points to an extraterrestrial source. Walter Alvarez picked this particular place for sampling because it had previously been identified as an excellent place to study the transition (by the Italian geologist Isabella Pemoli-Silva and the Swiss geologist Hans Luterbacher).