Although the evidence remains tenuous, the geological record provides some backing for panspermia. The earliest signs of microbial life, from roughly 4 billion years ago, coincide with a period of high cometary impacts. Adherents continue to search for additional proof. Wickramasinghe's group has used high-altitude balloons to collect microorganisms from 41 kilometers above Earth. They found many more than expected, suggesting, says Wickramasinghe, that the material fell from above instead of upwelling from the lower atmosphere. But the real tests, still pending, will be the measurements of carbon isotope ratios in the microorganisms. "If they turn out to be different from Earth [organisms]," he says, "then the game is up."