PARIS (AFP) ― The rugged landscape created by volcanic eruptions and tectonic plate shifts in east and south Africa millions of years ago may be what prompted our human ancestors to start walking on two legs, a study said.

The research published in the journal Antiquity challenges the commonly-held theory that early hominins (members of the broad human family) were forced onto two feet on the ground because climate change reduced the number of trees they lived in.

According to the new hypothesis, it is not why they left the forests, but where they went, that explains the evolution.

“Our research shows that bipedalism may have developed as a response to the terrain, rather than a response to climatically-driven vegetation changes,” study co-author Isabelle Winder from the University of York’s archaeology department said.

Between 6 million and 2 million years ago, our ancestors lived exclusively in Africa ― mainly in the east and south where much tectonic activity happened.

Winder and her team compared geological changes with evolution of hominin anatomy over millions of years, and concluded it was likely that our early tree-living ancestors were attracted not to flat plains as widely thought, but rocky outcrops and gorges.

These would have offered shelter from predators and made it easier to corner pray.

But rugged terrain also required more upright scrambling and climbing gaits ― prompting the emergence of bipedalism.

“For an animal moving on rough ground, the land is made up of lots of small, broken surfaces at different heights and angles. If you use four limbs to carry your weight, the chances are higher that you will be unable to position yourself effectively or that one of your hands or feet will slip,” Winder told AFP by email.

“It is to your advantage if you can balance on just two or three limbs and use the others to steady yourself.”

Thus our ancestors’ legs came to carry most of their weight, and their hands would have been used to stabilize and pull the body up rock faces and would have become better at grasping as a result ― ultimately enabling the evolutionary leap to tool-making.

“The varied terrain may also have contributed to improved cognitive skills such as navigation and communication abilities,” said Winder.

She said the finding answers a question that has stumped scientists for decades: how did our ancestors survive the many predators of Africa when they moved from the trees to the ground?

As it turns out, the rugged terrain where they emerged made it easier to hide.

The move to flatter ground probably only started a few million years later, said Winders.

“This study is the first to successfully explain how our ancestors lived during this period and why they evolved as they did,” she said.