Evolution’s Missing Link
John Grotzinger has devoted his career to solving one of the most hotly debated mysteries in evolutionary biology-what happened hundreds of millions of years ago to cause the spontaneous burst of life that spawned the early ancestors of all animals on earth today. He returned to MIT in January 2002 after a year spent in the Middle East, where he collected fossil-containing rocks dating back to this so-called Cambrian Explosion. On the basis of those fossils, Grotzinger, a sedimentologist at the Institute’s Department of Earth, Atmospheric, and Planetary Sciences, believes he has found the answer.
According to the fossil record, life on earth arose 3.5 billion years ago in the form of tiny photosynthetic bacteria. For close to three billion years, the planet was devoid of anything larger than bacteria, plankton, and microscopic seaweed. Then suddenly, about 540 million years ago, in the dark depths of the ocean, a rich community of tiny animals sprang into being. From long spiny worms to five-eyed creatures with grasping hoses for mouths, they completely transformed the ocean floor about 10 million years ago-the blink of an eye in evolutionary time. These were the first representatives of virtually all major groups of animals on earth today and perhaps still other groups that disappeared long ago.
Scientists have come up with various explanations for the Cambrian Explosion. According to one theory, the appearance of predators might have triggered a period of rapid evolution: early prey animals, in their struggle to adapt, acquired defense mechanisms such as hard shells. Another possibility is that increases in atmospheric oxygen fueled the evolution of more complex body structures: oxygen is a requirement for skeletal development and growth. To clarify what actually did happen, Grotzinger went to Oman in January 2001 to look for fossils in rocks extracted from oil fields five kilometers below ground. “The benefit of these fossils is that they are very well preserved,” says Grotzinger.
When he brought his rock samples back to MIT for analysis, Grotzinger was astounded by what he saw. Together with two of his colleagues-geochronologist Sam Bowring and geobiologist Roger Summons-he reconstructed in three dimensions what the ocean basin would have looked like prior to the Cambrian period, the geological period during which the explosion of life occurred. Fossils contained in the rocks indicated that before the Cambrian period, the ocean reefs harbored early prototype animals with soft shells-Cloudina, a wormlike creature, and Namacalathus, a tiny animal shaped like a wine goblet. Once the Cambrian period began, however, these organisms simply disappeared.
By studying carbon isotopes in the rock, Grotzinger and Bowring determined that just before the Cambrian period began, a major environmental catastrophe occurred: the oceans suddenly stopped circulating. This event led to massive extinction of those early animals-possibly through a surge in such greenhouse gases as carbon dioxide and methane, released from the oceans at the moment the oceans started to circulate again. The demise of the early animals, Grotzinger maintains, cleared the playing field and allowed a new, more diverse, and better adapted group of animals to emerge.
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.
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