Australopithecus was a genus of Apes or Ape men and some of that genus gradually evolved into Humans. There was a vast difference between the early australopithecines and later australopithecines. Three Million years ago early australopithecines like Lucy walked upright but their Brains were smaller than human brains and similar to ape brains.   Later australopithecines were sometimes more like humans and some could use tools in fairly sophisticated ways.  Australopithecus sediba was a bit of an anomaly, it was a late australopithecine and had many human features. Sediba's brain was in some ways organised like a human brain but smaller than Lucy's brain. 
Perhaps scientists are wrong to think of australopithecine evolution entirely in terms of progressively larger brains evolving and intelligence progressively increasing. Some australopithecines developed large brains and were our ancestors but we don't need to assume that all did. Nerve cells and brain cells consume a great deal of energy, in modern humans the nervous system consumes 50% of the energy we eat and the brain consumes 25%.   Smaller brained individuals needed less food during times of starvation. Throughout our evolution there was a premium on more efficient brains that could pack more intelligence into a smaller size. That gave and gives the advantages of power thinking without increasing food requirement. 
In intellectually unchallenging environments smaller brained individuals may have been able to survive better even if they were less intelligent because they were less vulnerable to starvation.
- Perhaps australopithecus sediba had an efficient brain and could think in relatively human ways despite its brain being smaller than Lucy's.
- Perhaps australopithecus sediba evolved an unchallenging lifestyle, lost some intelligence and brain size but could survive starvation better than bigger brained competitors. If this is true australopithecus sediba may have kept some human ways of thinking that helped survival.
Perhaps smaller brained, less intelligent australopithecines competed successfully for tens of thousands of years or hundreds of thousands of years when the climate was relatively stable. From time to time when an ice age ended or when a warm interglacial period ended the more intelligent australopithecines with large brains or with efficient brains would have the advantage because they would be able to learn or think up ways of dealing with the novel climate. What I'm considering is that
- Every time the climate changed rapidly and drastically the less intelligent australopithecines tended to become Extinct or to become locally extinct in some areas.
- Every time the climate remained stable for prolonged periods ecological niches appeared for smaller brained australopithecines. Those australopithecines could survive and stay healthy during food shortages more easily because their brains needed less food.
- If all small brained australopithecines that filled a given ecological niche previously had died out, then intelligent australopithecines with larger brains could have evolved reduced brain size to fill that niche.
- Australopithecus sediba may be an example of an intelligent australopithecine with many human characteristics evolving reduced brain size in this way.
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