Applying the Darwinian perspective to human origins required palaeoanthropologists to abandon the assumption that the human race is the goal of the evolutionary process. Darwin himself suggested that our acquisition of higher mental powers was a by-product of an adaptive shift to walking upright. Although rejected for decades, this approach became widely accepted in the 1930s as fossil discoveries confirmed that bipedalism preceded the expansion of the brain and the plausibility of natural selection was boosted by the synthesis with genetics. Most of the biologists who pioneered the synthesis accepted that the human species could no longer be regarded as an inevitable outcome of evolution. Science-fiction authors began to explore the idea that there might be non-human aliens, recognizing that evolution could produce intelligent beings in many different ways.

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.