Many traits unique to humans, such as large brains, long legs and ability to craft tools may have originated much earlier than thought - between three and four million years ago.
Such traits were long thought to have originated in the genus Homo between 2.4 and 1.8 million years ago in Africa.
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 as African grasslands expanded and Earth's climate became cooler and drier.
However, new climate and fossil evidence analysed by a team of researchers suggests that these traits did not arise as a single package.
Rather, several key ingredients once thought to define Homo evolved in earlier Australopithecus ancestors between 3 and 4 million years ago, while others emerged significantly later.
The team's research takes 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).
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 Smithsonian Institution paleoanthropologist Richard Potts developed a new climate framework for East African human evolution that depicts most of the era from 2.5 million to 1.5
million years ago as a time of strong climate instability and shifting intensity of annual wet and dry seasons.
This framework suggests that multiple coexisting species of Homo that overlapped geographically emerged in highly changing environments.
"Unstable climate conditions favoured the evolution of the roots of human flexibility in our ancestors," said Potts.
"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 said.
The team reviewed the entire body of fossil evidence relevant to the origin of Homo to better understand how the human genus evolved.
Comparison of these fossils with the rich fossil record of East Africa indicates that the early diversification of the genus Homo was a period of morphological experimentation.
Even though all of the Homo species had overlapping body, brain and tooth sizes, they also had larger brains and bodies than their likely ancestors, Australopithecus.
The research was published in the journal Science.