Being four-legged has its perks. As a quadruped, your center of gravity is lower, there's less wind resistance when you're running, and, best of all, you can use your hind foot to scratch your ear.

All of this raises a big question: What were our apelike ancestors thinking when they started walking upright?

A prevailing hypothesis is that they were prompted by climate change. As African forests declined due to temperature fluctuations some 2.5 million years ago, the hypothesis goes, our australopithecine ancestors descended from the trees and ventured out into the open savanna, an environment thought to be friendlier for those standing on two feet.

The savanna hypothesis has its critics, however. There is some evidence that bipedal primates evolved before the biggest temperature swings kicked in, that some australopithecines ancestors lived in forests, and that they were adapted to both tree-climbing and upright walking.

Now a new study suggests that walking on two legs was a result of geology, not climate. In a study published in this month's issue of the archaeological journal Antiquity, a team of archaeologists makes the case that our forebears' transition to bipedalism was prompted by volcanoes and shifting tectonic plates in East and South Africa, which produced rocky outcroppings and steep gorges.

“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,” said University of York archaeologist and study co-author Isabelle Winder, in a press release.

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This development would have conferred benefits that extend far beyond locomotion. Walking on two legs frees up the hands, allowing for the use of tools and, eventually, bigger brains. And the complex landscape could have made our ancestors smarter, says Dr. Winder.

“The varied terrain may also have contributed to improved cognitive skills such as navigation and communication abilities, accounting for the continued evolution of our brains and social functions such as co-operation and team work."