A large brain, long legs, the ability to craft tools and prolonged maturation periods were all thought to have evolved together at the start of the Homo lineage between 2.4 and 1.8 million years ago in Africa.
These traits did not arise as a single package, an analysis of new climate and fossil evidence shows.
Several key ingredients once thought to define Homo evolved in earlier Australopithecus ancestors between three and four million years ago, while others emerged significantly later, the study noted.
"Unstable climate conditions favoured the evolution of the roots of human flexibility in our ancestors," said paleoanthropologist Richard Potts, from the Smithsonian Institution in the US.
"The narrative of human evolution that arises from our analyses stresses the importance of adaptability to changing environments, rather than adaptation to any one environment, in the early success of the genus Homo," he added.
The team's research took an innovative approach to integrating paleoclimate data, new fossils and understandings of the genus Homo, archaeological remains and biological studies of a wide range of mammals (including humans).
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The synthesis of these data led the team to conclude that the ability of early humans to adjust to changing conditions ultimately enabled the earliest species of Homo to vary, survive and begin spreading from Africa to Eurasia 1.85 million years ago.
The findings appeared in the journal Science.