Kingdon visualises something similar in our ape ancestors in the forest, turning over stones or leaf litter for insects, worms, snails and other nutritious morsels. To do this effectively they would have had to undo some of their adaptations to living up trees. Their feet, previously hand-like for gripping branches, would have become flatter, forming a stable platform for squatting on the haunches. You will already be getting a glimmering of where the argument is going. Flatter, less hand-like feet for squatting are later going to serve as pre-adaptations for upright walking.
