Is there only one possible outcome for the evolution of mankind? When two cultures light years apart follow different evolutionary paths, that question could be answered. And many more could be raised. In the twenty-second century, Earth was dying. World population was more than twenty billion, the global environment was in ruins, and living conditions were intolerable everywhere on the planet. Many people thought the only hope for mankind lay in colonizing space. And so giant ships were built to carry brave volunteers who would settle the planets of nearby stars. Many years later, the colonists returned. The Earth they found was a far different place than the Earth of their distant ancestors. And they were not welcome.