Aaron Clauset of the Santa Fe Institute and (SFI External Professor) Douglas Erwin of the National Museum of Natural History in Washington, D.C., created the most accurate computer model yet to predict how mammal species' body sizes change over time. Using fossil data from up to 60 million years ago to specify the form of the model, they were able to accurately reproduce the distribution of 4,000 known mammal body sizes in the last 50,000 years. Crucially, their model assumes that that when a new species appears, its size, on average, is slightly larger than its ancestor species. So why aren't all mammals the size of elephants by now? Because there's an opposing force at work, Clauset said. While evolution favors larger creatures, extinction seems to favor the small. The larger a species' body size, the more likely the species is to go extinct. "The tendency for evolution to create larger species is counter-balanced by the tendency of extinction to kill them off," Clauset told LiveScience. "The distribution of sizes over time is stabilized because these processes balance out."