Lucy walked upright, but her shoulders resembled the chimpanzee. It becomes obvious that brachiating ambulation "teaches" the hand to form an opposed thumb. In turn, the development of bipedalism then freed the hand to carry the branch -- now club and spear -- which had previously carried the creature.

very well written; page turner. Got it from a friend who owned two copies; finished it the nexth day. ( )

I read this at least a decade ago - probably closer to 20 years ago - and I don't remember much of the information in it, but I do remember really, really enjoying reading it.