Grasslands dominated the cradle of humanity in east Africa longer and more broadly than thought, says a study published Thursday, bolstering the idea that the rise of such landscapes shaped human evolution.

According to the so-called "savannah hypothesis", the gradual transition from dense forests into grasslands helped drive the shift toward bipedalism, increased brain size and other distinctively human traits.

First outlined in the 1920s, the theory suggests that our most ancient upright ancestors learned to walk on two feet, in part, to peer over tall grass in search of prey and predators.

Rather than simply plucking fruit from trees, they had to become shrewd hunters and move longer distances in order to survive.

The notion has been debated for more than a century, however, with some scientists saying other forces were more important in driving humans to assume their signature posture.

They also point to studies showing that the landscapes of the two regions of east Africa richest in hominid fossils -- the Awash Valley and The Omo-Turkana Basin, both in Ethiopia -- were in fact quite diverse in terms of tree cover.

One of the most complete early hominin species yet discovered, Ardipithecus ramidus, for example, may have lived primarily inside woodlands and patches of forest, they argue.

Hominins include early humans and pre-humans, along with the early ancestors of chimpanzees and gorillas.

The new study, published in Nature, will not settle the debate, but it offers evidence that savannahs -- with their limited tree cover -- stretched back even beyond the five million-year boundary widely assumed up to now, especially in areas populated by our distant forebear.

"There have been open habitats for all of the last six million years in the environments in eastern Africa where some of the most significant early human fossils were found," said Thure Cerling, a professor at the University of Utah and lead researcher of the study.

"Wherever we find human ancestors, we find evidence for open habitats similar to savannahs -- much more open and savannah-like than forested," he said in a statement.

Combining an analysis of soil samples and satellite photos of tropical regions around the world, the researchers created vegetation chronologies for the regions home to many hominin fossils, including Ardipithecus, Australopithecus, Paranthropus and our own genus, Homo.

During the past 7.4 million years woody cover has ranged from 75 to five percent, they found.

But significant areas of savannah -- below 40 percent wood cover -- were consistently present "all the time for which we have hominin fossils in the environments where the fossils were found during the past 4.3 millions years," thus including the oldest human ancestors, Cerling said.

Up to now, many scientists believe that East Africa was forested up until two million years ago, he added.

"This study shows that during the development of bipedalism -- about four million years ago -- open conditions were present and even predominant," Cerling said.

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