How many people could live on Earth? Many scientists have tried to calculate that number, with widely divergent results. Seventeenth-century biologist Antonie van Leeuwenhoek put the upper limit at 13.4 billion; in 1967, biochemist C.T. De Wit said one trillion. Population modelers now say there is no single answer. The population that Earth can ultimately carry, and the quality of life that those people will have, depends on political and environmental choices. sustainability-research organization in Boston, has made some of the most sophisticated predictions yet of how those decisions will play out. PoleStar, the computer simulation that Tellus developed, starts with projections of population and economic growth from consumption, land use and pollution. The simulation's outcomes show that how we shape policy now will determine whether the next century's population lives in a pleasant world or one of degradation and scarcity. It's not too late to choose one of the brighter paths, says Richard Rosen, the executive vice president of Tellus, "but you'd have to get going immediately. There's no leisurely way.