As a high school student, Andy Knoll was an avid fossil collector, but it never occurred to him that he would someday become a paleontologist. Where he came from, a small town near Reading, Pa., bright teenagers aspired to careers in medicine, law and, in Knoll's case, engineering. But one day while sitting in his dorm room at Lehigh University, Knoll realized that he hated engineering and loved biology and geology. That's when it hit him: as a paleontologist he could indulge his passion for both.
That revelation marked the beginning of Knoll's lifelong fascination with one of the most mysterious episodes in the history of our planet: the sudden appearance some 540 million years ago of a wild profusion of multicelled animals. That event, known as the Cambrian Explosion, created the evolutionary dynamic that produced most of the species that subsequently populated the earth, from insects and fish to dinosaurs and humans. Given his background, Knoll was particularly interested in how geophysical and geochemical changes (caused by powerful tectonic forces) might have set the stage for everything that followed.
In his quest to explain the Cambrian Explosion, Knoll, now a professor at Harvard, has had to probe deeply into the so-called Proterozoic, the poorly understood era that started 2.5 billion years ago and ended about 2 billion years later. Thanks in no small measure to Knoll's pioneering efforts, scientists are finally beginning to appreciate how very strange that long interval of time was. For in addition to finding and describing a multitude of fossils that came from that era, Knoll has also managed to flesh out the evolutionary context in which those fossils appeared.
Among other things, Knoll notes, the Proterozoic oceans were not as broadly oxygenated as they are today, a fact that may explain the distinctive pattern he and his colleagues have been finding in the distribution of organisms whose fossilized remains are so vanishingly small that most are not visible to the naked eye. Under a microscope, Knoll notes, the cellular architecture of these ancient life-forms looks remarkably similar to that of modern algae. But unlike their contemporary kin, which bloom all along the continental shelves, these marine algae never strayed far from the shallows.
To Knoll, this strongly suggests that the basis for a complex marine food chain was in place 1.5 billion years ago, yet for some reason never got past the square marked GO. Why not? The hypothesis Knoll favors invokes the competition between two classes of compounds for control of the biochemical environment, one based on oxygen and the other on sulfide. During most of the Proterozoic, it turns out, only the shallows were infused with oxygen. The deep oceans, by contrast, were inordinately rich in sulfides, which indirectly interfere with the ability of algae to make use of growth-promoting nitrogen.
Just before the Cambrian, however, something big happened. The deep oceans were made oxygen rich and sulfide poor, Knoll believes, when an unusual spate of undersea landslides (triggered by the breakup of a primordial supercontinent) buried megatons of oxygen-consuming debris. Virtually simultaneously, microscopic algae spread far and wide. For the first time since the planet's formation 4 billion years earlier, the oceans were capable of supporting a population of small-, medium- and large-bodied animals.