Stone tools, big brains, and long legs made for walking—the hallmarks of humanity—arose at different times and in patchwork fashion in Africa millions of years ago, not in one big jump as previously thought, evolution experts suggested on Thursday.

Scientists have long thought that as forests retreated and savannas expanded in East Africa more than two million years ago, our apelike ancestors adapted to a  more terrestrial lifestyle, including a dedicated bipedal gait that freed up the hands for toolmaking.

But the report in the journal Science suggests that rapid fluctuations between wet and arid conditions, rather than a steady progression from wet to dry, may have set the stage for the emergence of the genus Homo. (Related: "Oldest