Evolution is driven by adaptation, and a new study offers fresh clues as to the role our ancient ancestors' habitat played in encouraging them to stretch their legs and begin walking upright.

The forerunners of modern humans first emerged in East Africa about 2.5 million years ago, and scientists have speculated that they may have been spurred to come down from their trees as open grasslands called savannas came to replace dense forests throughout the region. A University of Utah geochemist has discovered that savannas predominated long before humans emerged, adding further clues to how environment and evolution are intertwined.

"Wherever we find human ancestors, we find evidence for open habitats similar to savannas - much more open and savanna-like than forested," geochemist Thure Cerling said. This is true "for all of the last 6 million years in the environments in eastern Africa where some of the most significant early human fossils were found."

Cerling and his team analyzed 1,300 samples of ancient soil from Kenya and Ethiopia for evidence of what the landscape looked like then. The grasses common to savannas employ a type of photosynthesis that involves carbon isotoped with either six or seven neutrons, whereas trees and shrubs found in forests use a form of photosynthesis that relies heavily on the six-neutron carbon isotopes.

The vast majority of samples reflected an environment with minimal tree cover, while less than one percent indicated a forested environment. That suggests that when humans began walking on two legs, they were more likely to be strolling across savannas.

It has been a matter of considerable debate whether Ardipithecus ramidus, the earliest known human ancestor, inhabited savannas or forests. Cerling said there is evidence that ardipithecus had access to a wide variety of different habitats and had savanna grasses in its diet.

"It was going into the savanna, unless it was eating takeout food," he said.