IN the Western world B.D. (Before Darwin), few questioned the Book of Genesis description of how God made every living thing. But it was more than a little difficult to understand (or explain) the fossil remains of all those animals no longer visible in Europe, North America, South America or Africa. There was evidence of gigantic swimming lizards, reptiles with wings, huge hairy elephants, sharks with teeth the size of your hand.
Some people thought the fossils' living counterparts just hadn't turned up yet, that with a little more exploration, we would find them in the Amazon jungle, on a remote South Seas island or lurking in the depths of the ocean. Another explanation, propounded by no less an authority than Georges Cuvier (1769-1832), the founder of comparative anatomy and vertebrate paleontology, was that they had all been washed away in the biblical flood. "Numberless living things were victims of such catastrophes," Cuvier wrote. "[S]ome, inhabitants of dry land, were engulfed in deluges; others, living in the heart of the seas, were left stranded when the ocean floor was suddenly raised up again; and whole races were destroyed forever, leaving only a few relics which the naturalist can scarcely recognize."
Evolution, as defined by Charles Darwin and his successors, can explain the presence of the world's diverse flora and fauna. Extinction, evolution's handmaiden, can be used -- although not very effectively -- to explain the absence of so many life forms. It has been estimated that 99% of all the species that have lived on Earth are extinct. What happened to them? We really don't know. Well, that's not exactly true. We do know what happened to the dodo, the passenger pigeon, the Carolina parakeet, the ivory-billed woodpecker and the Tasmanian tiger (we killed every last one), but we cannot easily explain what happened to the billions of living creatures that are long gone.
Because of their fossils, we know about trilobites, jawless fishes, dinosaurs, pterodactyls, ichthyosaurs and countless other "prehistoric" creatures. Why they are no longer with us is not so easily explained. Were they knocked off by the impact of a gigantic asteroid? Did the climate change, causing them to roast or freeze to death? Did they become too specialized to survive? Did they get to be too big? Too small? Or did their time just run out?
To answer the question of extinction, we need to address the parallel question of evolution, and that is what Edward J. Larson does in "Evolution: The Remarkable History of a Scientific Theory." He points out that the idea of evolution did not originate with Darwin, but rather with those early paleontologists -- like Cuvier -- who realized that there must have been myriad earlier life forms that somehow led to those we observe today. Larson begins his book with the men whose contributions were responsible for the run-up to Darwin, but he doesn't stop there; he rolls right on to the development of genetics -- the mechanism for evolution that Darwin never even suspected -- and those who would apply the new science to modify human evolution.
Larson's book is a fascinating, straightforward view of the development of evolutionary theory that alas, is not a straightforward story. It is a complex mystery, often with tantalizingly elusive clues. Detectives are still working on it.
Evolution is a progression, but its description is not easily encapsulated as "this happened and then that happened," Larson writes. That is how it works generally, of course, but there are so many gaps in the fossil record (perhaps one in every 100,000 extinct species has been found, and scientists theorize that there may have been as many as 20 billion) that sometimes the best we can do is to assert that this creature lived before (or after) that one.
The record of the rocks, however, is much more reliable than the record of the fossils, largely because the rocks greatly outnumber the fossils. The ages of rocks can be ascribed to their relationship to each other (older rocks are usually underneath younger ones), but the surface of the Earth is not a cake carefully arranged in layers. It is more like a cake that has been dropped, and sometimes the chronological relationship of one layer to the next has to be deduced by some pretty fancy geological detective work. It is the fossils found in those rocks that sometimes talk to paleontologists.
Fossils are evidence of extinct life forms, unquestionable in their rock-hard reality, but lacking eyewitness accounts of how these animals walked, swam or flew, paleontologists have to speculate on the animals' lifestyle. Some of this speculation is fairly easy: An animal with big sharp teeth was probably a carnivore; one with wings probably flew. But after the obvious, things get a little more complicated.