Causes of Hominid Evolution

What causes organisms to evolve? What advantages were attained by the apelike ancestor Australopithecus when it progressed into modern Homo sapiens?  Erect posture, bipedal locomotion, different hand and feet structure, and reduced canine teeth must have ensured a greater survival for the hominid lineage than a retention of its former attributes.  Why is it that hominids evolved such features?  Why did they/we gain a larger brain, language and what seems to be an unparalleled capacity for reason in the animal realm?
Obviously, there are no easy or even provable answers to these important questions which to some people beg for philosophical, mythical or religious explanations.
The  ideas brought forth to attempt at explaining the evolution of hominid characters are very speculative and also very hard to test.
  Anthropologists bring forth evidence from inferences from studies of other primates and the study of contemporary cultures.  They use material evidence such as anatomy and artifacts as evidence.

It is guessed that human evolution is greatly due to the social circumstances of early hominids.  Studies are made on the behaviour of other primates and contemporary hunter and gatherer cultures.  These studies have attempted to provide insight into the social structure of early hominids.