University of Delaware Office of Public Relations The Messenger Vol. 6, No. 1/1996 Evolution by Gluttons-When Productivity swamps an ecosystem For more than 200 years, Earth scientists have argued the reasons behind the explosion and extinction of life-forms on the planet. Paleontologist Ron Martin champions a theory that the ocean's ecosystem collapsed under its own success. Martin's view was among three theories on the evolution of life featured in Discover magazine earlier this year, as part of the magazine's survey of the top 100 science stories. After looking at fossil and isotope records, Martin, an associate professor of geology, says there is some evidence that large increases in nutrient productivity can "blow out the ecosystem, making way for the surviving organisms to try something different." Discover magazine called it evolution by "gluttony"-in which plentiful food results in bursts of extinctions followed by a surge of new, more efficient organisms. Life was slow on Earth for the first 3 billion years, Martin says. But, some 500 million years ago, in the Cambrian Period, a riotous array of marine animals in different shapes and anatomical designs appeared. "We see very odd-looking creatures in the Burgess Shale in British Columbia," he says. "Some look like a shrimp crossed with a worm; others are crab-like creatures; none exist today. Most all of the experimental forms created during the Cambrian died out." According to Martin, there are two basic hypotheses for why Earth had this burst of new life-forms in the Cambrian, followed by the failure of most of the experimental types. Internal control, or the genome hypothesis, suggests that genetic constraints are set in place in the organisms, keeping them from evolving into very different life-forms through the eons. The ecospace hypothesis suggests external controls were more important, with ecological constraints ranging from predators to environmental changes. Some of the external controls that theorists suspect may have affected these cyclic explosions of life include an increase in ocean warmth from undersea volcanoes that sped up biological activity; increased oxygen in the atmosphere diffusing into the ocean, allowing more organisms to dwell in the sea bottom; and, increased predation, which caused bottom dwellers to produce shells to protect themselves. "Explosions and extinctions of life can be caused by a variety of factors," Martin says. "Continents pull apart, volcanism increases (which pumps more carbon dioxide into the atmosphere) and temperatures rise. Continents come together and destroy habitats. Sea levels rise and fall. But, one common thread runs though it all: Nutrient levels go up and go down. "A modern example occurred in the Great Lakes. When sewage and industrial wastes were dumped into the lakes, there were algal blooms that swamped the lakes. Other aquatic creatures died, and the whole ecosystem was upset." Plentiful food can knock over delicate ecosystems, Martin says. "The organisms that reproduce the fastest will do so, and they'll overcome the ones that reproduce more slowly. Ultimately, they swamp the system and it collapses. "This appears to have happened in the Cambrian and many other extinctions, as well." After the collapse and the consequent extinctions, Martin says, the system begins to reorganize itself from the survivors. As he said in the Discover article, "I think most people would agree that somehow you have to break down the old ecosystem so that organisms have a chance to try something that's different." Recognizing that other theorists have suggested that extinction is associated with lower productivity and lower nutrient levels, Martin says that this may be the case in some extinctions. For example, the popular theory today is that dinosaurs died out because dust from a meteor blocked sunlight and rapidly cooled the Earth. "Decreased sunlight would have also drastically decreased photosynthesis in the oceans, so marine ecosystems would have collapsed, a situation called a 'Strangelove Ocean,' in reference to the movie of the same name," Martin says. "We have some chemical indicators of increased productivity in the carbon isotopes," he says, "but we must keep in mind that there are different ways to interpret what we see in the fossil and isotope record. It's always hard to prove any scientific theory, but, in Earth science, it can be especially difficult. "What happens at one point in time is dependent on what happened hundreds of millions of years earlier. It takes a tremendous amount of time for things to happen, and it is very difficult to measure the rates of processes that occur so slowly that they appear to be constant over many, many human generations. It's not like working in a controlled laboratory setting. That's why historical sciences like geology and paleontology are so important." Martin's version of what happened will appear in his book, One Long Experiment: Scale and Process in Earth History, to be published next year by Columbia University Press. -Cornelia Weil