Progress Unchained reinterprets the history of the idea of progress using parallels between evolutionary biology and changing views of human history. Early concepts of progress in both areas saw it as the ascent of a linear scale of development toward a final goal. The 'chain of being' defined a hierarchy of living things with humans at the head, while social thinkers interpreted history as a development toward a final paradise or utopia. Darwinism reconfigured biological progress as a 'tree of life' with multiple lines of advance not necessarily leading to humans, each driven by the rare innovations that generate entirely new functions. Popular writers such as H. G. Wells used a similar model to depict human progress, with competing technological innovations producing ever-more rapid changes in society. Bowler shows that as the idea of progress has become open-ended and unpredictable, a variety of alternative futures have been imagined.