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>Another question is: why would an arboreal animal that climbs with its 
>forelimbs become a bipedal animal?  

I think that the answer to this would depend on HOW it climbs.  Apes do not
just climb in the way that a hoatzin does; they brachiate, so that the
forelimbs become the primary locomotor limbs in trees (watch a gibbon
sometime).  This seems to have resulted in a major disproportion of the limb
elements, with the front limbs longer (much longer on gibbons and orangs) than
the hind and the hands specialized to varying degrees as grasping hooks.

This has two results when such animals descend to the ground.  The first is
that even in a quadrupedal pose, the back may be more or less erect (orangs
especially).  The second is that the modified hands must either be used in an
unusual manner for walking (the "knuckle-walk" of the great apes) or held out
of the way altogether (as in gibbons, which are almost obligate bipeds on the
ground).  Coupled with further selection for the modification of the ape hand
as a manipulating organ, these features seem to me to be exaptations for a
bipedal mode of walking.

Of course such a pathway is totally irrelevant to considerations of the
evolution of tree kangaroos, kangaroo rats and (I suspect) dinosaurs and
birds.  However, it may be worth noting that the only other mammals I can
of that brachiate, the sloths, also had terrestrial relatives that were (or
usually restored to be) more or less bipedal.
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net