The discovery that Australopithecus had arched feet suggests this human
ancestor had already abandoned life in the trees

http://www.guardian.co.uk/science/2011/feb/10/fossil-foot-bone-ancestors-walking

The transition to full-time terrestrial bipedality is a hallmark of human
evolution. A key correlate of human bipedalism is the development of
longitudinal and transverse arches of the foot that provide a rigid
propulsive lever and critical shock absorption during striding bipedal
gait. Evidence for arches in the earliest well-known Australopithecus
species, A. afarensis, has long been debated. A complete fourth metatarsal
of A. afarensis was recently discovered at Hadar, Ethiopia. It exhibits
torsion of the head relative to the base, a direct correlate of a
transverse arch in humans. The orientation of the proximal and distal ends
of the bone reflects a longitudinal arch. Further, the deep, flat base and
tarsal facets imply that its midfoot had no ape-like midtarsal break.
These features show that the A. afarensis foot was functionally like that
of modern humans and support the hypothesis that this species was a
committed terrestrial biped.

http://www.sciencemag.org/content/331/6018/750.abstract

--Laura