Hi Edmund
A recent press release from South Africa has revealed the fossilized remains of a hominid which dates from about 3.6 million years ago (see link below ) . Called "Little Foot " , the skeleton is almost complete and would have belonged to an animal which still had a foot suitable for living in the trees but walked on the ground as a biped .

So it might be fair to say that the arrangement of the pelvis , foramen magnum and femur needed for bipedalism evolved through brachiation and that the skeletal arrangement arrived at could be used for developing ground up, rotational , high energy kinetic energy in hand held objects .

This leg driven use of tools might have given sufficient advantage to be selected for , resulting in the evolution of a more terrestrial group of hominids with a non divergent first ray in the foot which would be better suited to bipedalism .

So brachiation leads to upright posture giving high energy tool use then commitment to bipedalism ., Simple !


link -Little Foot skeleton unveiled in South Africa - BBC News

www.bbc.co.uk/news/world-africa-42250530


6 Dec 2017 - One of the oldest and most complete skeletons of humankind's ancestors has been unveiled in South Africa. A team spent more than 20 years excavating, cleaning and putting together the skeleton of Little Foot. Its exact age is debated, but South African scientists say the remains are 3.67 million years old.