In the United States wealthy tycoons funded fossil-hunting expeditions and new natural history museums to display their discoveries. Dinosaurs from the Western states dramatically transformed the way the ascent of life could be represented because they were quite unlike any living reptiles and confirmed that the ‘tree of life’ had many more branches, some of which had disappeared completely. There was increasing evidence of relatively abrupt transitions in the earth’s history, forcing geologists and evolutionists to reconsider their impression that change had been more or less continuous. As the tree of life became more complex, the assumption that the human species was the inevitable outcome of progressive evolution became less plausible. Although non-Darwinian theories were retained by some authorities, the new vision of evolution came to seem more compatible with Darwin’s vision of an open-ended and less predictable process.

In the eighteenth century naturalists had already begun to realize that the diversity of living forms was so great that they could not be arranged into a single linear hierarchy. The relationships were best represented as a two-dimensional map, or ─ when the time dimension was added ─ as a branching tree. Darwin used the 'tree of life' but because his theory of natural selection explained evolution in terms of adaptation to local environments his tree had no central trunk leading to humanity. Evolution was a utilitarian process in which success depended on gaining an advantage in dealing with the environment. Progress towards higher levels of organization occurred in many different ways, although only a few adaptations resulted in an overall increase in complexity. In the early twentieth century palaeontologists studying the history of life showed that progress was episodic, occurring only when a major biological innovation led to the emergence of a new class of organisms. These events were unpredictable on the basis of previous trends and could be compared to the inventions that have transformed human societies. Darwinism was a form of 'creative evolution' in which innovations were multifarious and unpredictable.