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Re: CLIMBING



At 07:13 AM 8/7/98 PDT, Matthew Troutman wrote:
>
><<Yes, but remember that the ultimate primate bipedalism 
><<(represented by 
><<ourselves) originated in trees, as suported by the latest 
><<research on hominids and australopithecines in general. The 
><<afarensis had recurved phalanx and ...>>
>
>I was under the impression that this was a vestige.

This is still somewhat debated.  Personally, I think the evidence favors
some remnant of arboreality in early hominids.

>  I'm not  sure whether I can support climbing as a something 
>that caused bipedalism.

Well, in hominids, it almost certainly is.

As has been pointed out, arboreal primates are arboreal quadrupeds.
However, the great apes are very specialized arboreal quadrupeds, called
brachiators.  That is, they have *greatly* elongated arms they use to swing
from branch to branch.  (This specialized form of locomotion, complete with
elongate arms is found convergently in the new-world spider monkeys as well).

Now, it so happens that these very long arms make normal terrestrial
quadrupedal locomotion almost impossible.  Great apes and gibbons, when
forced to walk on the ground, use one of two mechanisms.  They either walk
bipedally with their arms held high, or they walk on their knuckles.  The
difference seems to be weight related.  The heavy forms, male orangutans,
male common chimpanzees, and gorillas, use knuckle-walking.  The smaller
forms, the gibbons, female orangutans, and the female bonobo, use bipedal
walking.  (I am not sure of the status of female common chimps and male
bonobo).

The earliest hominids (_Australopithicus afarensis_ and its predecessors),
were within the smaller size range, being about the size of a bonobo.
Thus, in hominids, bipedality probably *did* originate due to loss of
arboreality.

However, this sequence comes from a very specialized form of arboreal
locomotion, for which I have seen no evidence in any dinosaur (including
avians).  Furthermore, I suspect that our rather *odd* form of fully erect
bipedality is due to this odd origin from brachiating apes.

Bipedal archosaurs all have a more typical "horizontal" bipedality.  The
origin of this is likely to be quite different.

[By the way, this is why I do not like the "dinosauroid"
pseudo-reconstruction - even if the descendents of _Troodon_ developed
sapience, I am quite certain they would have retained a more bird-like
posture - a humanoid fully-erect posture is simply not useful to an animal
that is *already* bipedal].

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

In the case of hominids, it was due to the overly long arms, derived from
their unusual use in locomotion.

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