Re: First Family and AAT

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ghanenbu@inter.nl.net (Gerrit Hanenburg) writes:
>szpltdwn@dale.ucdavis.edu (Gerrell Drawhorn) wrote:

>> 2) Bipedal adaptations in early hominids appear to be mechanically
>> designed to deal with increasing gravitational forces applied to the hip,
>> hindlimbs and foot. Swimming would seem to completely release the
>> hindlimbs from these verticle forces - selection would take the climbing
>> I find it difficult to envisage a situation where the lateral
>> system in the pelvic musculature, the increased surface areas and
>>angulation of the knees, the development of the longitudinal and
>>transverse arches of the foot, etc. could arise if animals were in an
>>environment where they were buoyant. Either a lightly-built knuckle-walking
>>or (more generalized) brachiator (not as specialized as hylobatines)
>>would seem to provide a better progenitor than an aquatic form.
[I'm having some trouble following your terms (what changed in the
angulation of the knee, and what effect did it have?) but, some notes:]
Wading would involve an environment that requires balancing against
a shifting current - shifting the knees relative to center of gravity,
and developing arches in the foot, could assist in maintaining balance.
We would probably expect the locking knee, also, which Lucy didn't have, so...
Modern humans do have a foot with a large (relative to primate's hand-feet)
surface area; webbing the the toes (last two joints) wouldn't be much
of an improvement over the modern solid foot. (If I weren't in a public
computer lab, I could double-check some of the joints...)

>"It (the water) is not the right environment to select in favour of an
>anatomical complex that suits bipedal locomotion because the upward pressure
>of the water takes much of the load of the body off your lumbar spine,
>pelvis and lower limbs.That load is exactly what is needed to remodel
Bouyancy doesn't overcome weight until you're at least chest deep -
this would then involve swimming rather than wading.

>Without those gravitational forces there is no selection to remodel the
>lower body in favour of bipedalism.Instead you would expect a kind
>of "degeneracy" or remodelling in the direction of wateradapted
>locomotion.(i.e swimming and/or floating).
>Other mammals that have moved to the water have lost their hindlimbs
>altogether as in whales,dolphins and manatees or they have
I seem to remember a summary of the 10 my evolution of whales; the
early 'whales' kept the four legs for foraging on land, then developed
webbed foot, presumably until the webbing became awkward on land, then
went through a phase where the hindlimbs developed into grasping arms
before they atrophied entirely. They also started with tails - a hominid
with reduced legs would be tending toward a blob with arms, and would
have a very hard time moving with any speed - there was to be something
powerful to move water around.

>become much less functional in terrestrial locomotion as is the case
>in pinnipeds,and where they kept their legs they became adapted to
>swimming as in otters".
What kinds of adaptions are you referring to? I thought otters were
basically weasel-shaped.

--

-Clara A. N. Fitzgerald     cfitzger@s.psych.uiuc.edu

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