This was not the result Grotzinger thought he would find. “Our initial theory was that the appearance of all the diverse Cambrian fauna was more a result of the invention of hard shells, which allowed these organisms to be suddenly preserved in the fossil record,” he says. “I had no idea we were going to come up with an extinction story.”In the early 1990s Grotzinger had discovered Precambrian fossils of Cloudina and Namacalathus in Namibia. But the fossil record at the boundary between the Precambrian and the Cambrian is poorly preserved, and the fate of the early creatures remained an enigma. Grotzinger speculated that they were merely transitional animals between the very first prototype animals and the diverse Cambrian fauna. But on the basis of his data from Oman, he believes that it was the extinction of the early animals that stimulated the Cambrian Explosion. “It’s a great story, but opposite to what I had expected,” he says.
Grotzinger’s finding is “really the smoking gun,” says Andrew Knoll, an evolutionary biologist at Harvard University who a few years ago was among the first to propose an extinction theory to explain the Cambrian Explosion. And although Knoll and many other scientists have found evidence of a major environmental shift at the Precambrian-Cambrian boundary, none has been able to correlate the event directly with the disappearance of early prototype animals. The new data from Oman join these two events together for the first time, making the extinction theory, says Grotzinger, “a complete slam dunk.”
The new evidence is also consistent with the patterns of other major extinctions in the history of animal evolution. For instance, dinosaurs and mammals coexisted 150 million years ago. “However, in all that time, mammals never got out of the sort of ecological box into which they were confined by the dinosaurs,” says Knoll. “Instead, they remained little ratlike things skittering around in the trees.” However, a leading theory now holds that when a meteor struck the earth 65 million years ago, wiping out the dinosaurs, the mammals survived. In this new, open environment, they quickly evolved, grew large, and filled empty ecological niches. Similar variants of mammals had probably been produced through genetic mutations before, but they would have been eliminated by natural selection during the reign of the dinosaurs. After the dinosaurs’ extinction, the larger mammals survived.
“There’s always a tension between genetics and environmental possibility,” says Grotzinger, who was recently elected to the National Academy of Sciences. Although genes dictate what an organism looks like, he says, physical forces shape the course of the evolution of a species. And when those forces are great, they can open the door to entirely new forms of life.
To verify the results from Oman, the MIT researchers have now moved on to China, where they have found rocks that date back to the Precambrian-Cambrian boundary. “China has lots of rocks of the right age, and their preservation appears to be spectacular,” says Bowring. “In the coming year, we’re going to produce a lot of interesting data.” More fossils and signatures of ancient environments will continue to shed light on how those intriguing creatures that thrived in the Cambrian oceans hundreds of millions of years ago arrived on earth.