Re: Bipedalism and Arboreality.

[NJPharris@aol.com]

In a message dated 9/27/99 10:50:14 AM Pacific Daylight Time, 
bettyc@flyinggoat.com writes:

> In the case of humans, could I suggest that proto-humans were >upright<
>  first due to brachiating (not tree-climbing per-se but rather swinging
>  from branch to branch by the arms), and then retained the uprightedness
>  as they went to terrestrial locomotion.

Except that humans didn't go directly from brachiating to terrestrial 
locomotion.

Gibbons are the only living (adult) apes that habitually brachiate; 
orangutans clamber, chimps and gorillas knuckle-walk on the ground, and 
humans walk upright.  There was likely a clambering stage in the evolution of 
all the great apes (hominids sensu lato, = orangs, chimps, gorillas, humans). 
 This motion involves all four limbs, including walking motions with the 
hindlimbs, and may, I suppose, have been a precursor to human upright walking.

Cladistically, though, the relationships among the living members of the 
African ape clade most likely look like this: (gorillas + (chimps + humans)). 
 Thus, either humans are ancestrally knuckle walkers (which introduces a 
whole new set of issues), or all three clades became terrestrial 
independently, most likely deriving from a clambering form.


>  Can you support a case why the bipedal arborial sequence [from non-avian 
>  theropods to birds] would need to
>  develop semi-uprightedness?  Do you see this stage brachiating as
>  proto-humans did, and if so, why are birds only SEMI-upright?

I can see absolutely no evidence of the long arms, extremely mobile 
shoulders, and strong, hook-shaped hands that would have been required for 
protobirds or other theropods to brachiate.

--Nick P.