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Re: To climb or not to climb

At 08:45 AM 12/16/98 -0500, Norton, Patrick wrote:
>are both viable locomotive options for cursors.  My basic point is that I   
>can't think of one example of an animal that has evolved an obligatory   
>bipedal stance while occupying an arboreal niche (hominids came down from   
>the trees before they walked fully upright.)

I think this is, at best, missing the point.  I would say that hominids
became bipedal the *moment* they left the trees, not after.  Indeed the
smaller living apes (gibbons, young/female chimpanzees, female orangutans)
are *all* normally bipedal when moving on the ground, one might even say
that gibbons are *obligatory* bipeds on the ground.  It is really a small
step from this to the human condition - mostly what is required is *loss*
of climbing adaptations, and a subtle modification of the hip joint to make
walking more efficient (which can come much later anyhow).

> There are no obligatory   
>bipeds in the trees, except birds.

No, but several arboreal quadrupeds are bipeds when *out* of the trees.  (I
suspect that spider monkeys may also fit this description, and I wonder
about the origin of bipedality in kangaroos, given the frequency of
arboreal forms in the larger group of which they are a member).

>  And if birds evolved obligatory  bipedalism in an arboreal niche, they
> appear to be unique example.   

The *arboreal* bipedality probably derives from flight modifications.

But an arboreal quadruped that is a ground biped is not unknown, or even
that unusual.

May the peace of God be with you.         sarima@ix.netcom.com