How did our early hominid ancestors begin to utilize bipedal locomotion?

Austrolipthecine Species

Ardipithecus ramidus 

Australopithecus afarensis

Australopithecus anamensis

Australopithecus robustus

Australopithecus aethiopicus

Australopithecus boisei

Australopithecus africanus

Australopithecus garhi

Evidence for Bipedalism: 

Vestibular System, 

Laetoli Footprints, 

Foramen Magnum, 

Metatarsals, & 

Pevlis