The authors provide the following graphic to explain their idea.
They suggest that early humans would have found the uneven terrain of broken ground attractive, both for safety and for opportunities to trap prey. Thanks to volcanoes and shifting tectonic plates, the landscape of East and South Africa would have included plenty of gorges and caves for hominins to scramble over.
Needless to say, this hypothesis is only one in a large number of ideas about how we came to walk upright. There’s even some evidence that we became more or less bipedal before we left the trees. Still, it’s interesting to think about our ancestors actually being cavemen.Isabelle C. Winder, Geoffrey C.P. King, Maud Devès, & Geoff N. Bailey (2013). Complex topography and human evolution: the missing link 2013 Antiquity. 87:336 p333-349.