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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
I think this answer from Ronald Orenstein is quite to the point. I would
say arborealism is responsible for some of the 'weirdest mechanics' in
locomotion and a kind of erect posture that is not comparable with ground
sloths or similar 'passive' semi-erect postures.
Tim White who is currently studying the most ancient australopithecine
remains claimed that his results will show an animal "whose locomotion
didn't resemble anything we could have ever imagined" (needless to say, I'm
really waiting for his paper on Australopithecus anamensis).
Regarding the Australopithecus locomotion and evolution, the August
National Geographic has a good summary on current thinking about
Australopithecus. As PTJN@aol.com has just commented:
>It appears that we may have lived an arboreal life longer than previously
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