Rugged landscapes in East and South Africa may have helped evolve humans' earliest ancestors from tree-dwelling quadrupeds to upright bipeds, scientists say.

Researchers at Britain's York University, working with French colleagues, say humans' upright gait may have its origins in prehistoric landscapes shaped during the Pliocene epoch by volcanoes and shifting tectonic plates.

Early hominins would have been attracted to the resulting terrain of rocky outcrops and gorges because they offered shelter and opportunities to trap prey but those same landscapes would have required more upright scrambling and climbing gaits, prompting the evolution of bipedalism, the researchers said.

That theory challenges traditional hypotheses that humans' early ancestors were forced out of the trees and onto two feet when tree cover declined because of climate change.

"Our research shows that bipedalism may have developed as a response to the terrain, rather than a response to climatically-driven vegetation changes," York researcher Isabelle Winder said in a university release.

"The broken, disrupted terrain offered benefits for hominins in terms of security and food, but it also proved a motivation to improve their locomotor skills by climbing, balancing, scrambling and moving swiftly over broken ground -- types of movement encouraging a more upright gait."

That would have left the hands and arms of upright hominins free to develop increased manual dexterity and tool use, supporting a further key stage in the evolutionary story, the researchers suggested.