>What use it is evolution-wise for adults to live beyond the age where
>their children have become independant adults?

1)To take care of the grandchildren while the parents are out hunting
or gathering.  2) To pass on the wisdom, knowledge and perspective that
follows with old age - all which is vital for the survival of the
tribe.

It has been shown that the brain in healthy individuals never stops
developing.  If there was no evolutionary advantage to such an on-going
process it would seem that such redundance would have been eliminated a
long time ago by the efficiency of mother nature.

Best, Peter
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