Re: Bipedalism and Arboreality.

["Jaime A. Headden" <qilongia@yahoo.com>]

Betty Cunninggoat -- I mean, "ham" -- wrote: ( :) )

<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.>

  Okat, this is how I see it, pardon for not listing
refs beforehand, but I have to back search to find the
key studies that have led to this hypothesis:

  1. Orangutans and gibbons represent the brachiating
phase, using primarily the arms for locomotion;
gravity, as everyone knows, pulls thing downwards, and
with dangling legs, this has one recourse, unless one
wants to uselessly spend the energy to maintain a
curved distal thoracid spine to keep the the lower
limbs perpendicular to the pectoral region. So, in
brachiating apes, the legs are pulled downward, the
spine is essentially straitened, though not
completely. Orangutans still knuckle-walk, in part due
to their elongate arms, but what have you.

  2. Chimpanzees and gorillas are the real
knuckle-walkers, gorillas slightly more arboreal than
chimps, and again while orangutans knuckle-walk, all
three can maintain limited fully bipedal motion on the
ground (waving their arms about for balance). Chimps,
I believe, have even been seen to "walk" on a
horizontal branch for a step or two. Whatever. (I
_will_ track these refs down.) Gorillas don't even
have to wave their arms in the arm for motion, as can
chimps, but more so the bigger blacker lugs.

  3. Anyway, along comes *Australopithecus* (this is
not meant to be a precise ancestry analysis, so pardon
for the mistakes) who has come down out of the trees,
though it still possesses some arboreal and ancestral
characteristics, including the semi-erect spine, the
long arms, but now has longer legs and the ankle has
changed to best enable ground walking (at the time)
given a semi-scansorial capability. These are the
scansor phase, in case anybody's keeping score.

  4. *Homo* developed, in succession, though not
neccessarily in this order, and certainly with many
sidelines and probable reversals (as did
*Australopithecus*) the ability to stand upright given
a fully bipedal hip and spine; to stride, given a
stronger knee, ankle, and hip; shorter arms for
reduced arboreal activity (but don't tell that to your
kids clammering about in the elm next door -- though I
hear oaks are better for that sort of thing).

  By the earliest hominoids, the tail has already been
reduced, if not lost, in a theroretically fully
arboreal animal. There is no use for it, essentially,
that hominines, hominids, or hominoids cannot manage
to do with an arm or a leg, or our own intellect to
invent something else. Man's ancestors, successively,
brachiated, dropped to the ground in a scansorial
mode, became upright, given whatever reason (lack of
opportunity as an arboreator, increased opportunity as
a terrestriator, to explore, etc.), developed the long
stride to move over ground swiftly (only in a
non-forested habitat, this, since striding is hampered
-- by experience -- by changes in terrain caused by a
forest), and we have arrived at us, fully bipedal and
terrestrial (by adaptation, not by obligation).

<In the case of theropods to birds we'd have theropod
(bipedal but not upright) going to bipedal arborial
(unknown upright or not) and ending up at bipedal bird
(semi-upright) -OR- Dinogeorge's bipedal arborial
(unknown upright or not) and splitting off to theropd
bipedal (not upright) and bird bipedal (semi-upright)

Can you support a case why the bipedal arborial
sequence 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?>

  Not to cause a furror, but Feduccia has proposed
that birds possess their sprawled hips (the knee is
not held parallel to the vertical [sagittal] plane)
but factor of their having developed from a sprawling
ancestor. Not that I see this as the most likely
possibility of an evolutionary phenomenon, it does
raise the question -- why does the hip sprawl in
birds? Is the adaptation to the air and the increase
in girth at the knee (due to the knee being oriented
forward) the cause, or was there a connection between
the need to sprawl in lizards and the wider knees than
hips in birds? Just a biomechanical question.


=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

Qilong---is temporarily out of service.
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