Re: First Family and AAT
Gerrit Hanenburg (ghanenbu@inter.nl.net)
Thu, 28 Sep 1995 19:51:30 GMT
hubey@pegasus.montclair.edu (H. M. Hubey) wrote:
>>Why should the AA walk upright on land instead of moving on all fours like
>>it probably did before going aqua?
>Head up is the natural position for any land animal that gets
>thrown in the water. I assume that the legs would hang down and
>that in shallow water they'd be in contact with the bottom. If the
>body is more upright (i.e. so-called streamlined) it would be able
>to stay in deeper water for the same length appendages.
This is not really an answer to my question.An upright position may be good
for an ape in the water,this doesn't mean it is also a good position for an
ape on land.The AAT has to answer the same question that other theories
concerning the origin of bipedalism have to answer.
For what reason did the aquatic ape maintain its wading position on land?
>>The point isn't how long they have been aquatic or how long ago the
>>transition took place.The point is the duration of the transition from
>>being terrestrial to becoming aquatic.This can be relatively short.
>>About 10 myr in the case of Cetaceans.
>Since we cannot perform the experiment of throwing some animal
>into the water and keeping it there for 10 million years we can't
>tell if any animal will lose its limbs and develop like Cetaceans.
The experiments have already been done by mother nature. All *we* have to
do is gather the data and do the analyses.In the case of Cetaceans and
manatees the outcome seems to be loss of hindlimbs while in the case of
pinnipeds there is reduction of the hindlimbs which also have become much
less efficient in terrestrial locomotion.In the case of hippopotamus the
experiment is still going on,so keep watching carefully. :-)
>>As far as I now it has been fairly well established that Cetaceans have
>>descended from Eocene mesonychid condylarths about 45-55 myr ago.
>>The mesonychids were terrestrial quadrupeds with fully developed hindlimbs.
>Full developed as in dogs, humans, horses, or say crocs and lizards?
As in dogs.A species like Mesonyx had limbs specialized for cursorial
locomotion.
>OK. I still find it difficult to believe that humans can assume
>the shape of whales.Maybe webbing will develop on hands and feet.
>maybe the digits will get even longer. But this is more like
>science fiction.
I already told that I do not expect the anatomical changes in the aquatic
ape to be as pronounced as in whales or pinnipeds.Far from it.
But since in an aquatic environment gravitational forces are counteracted
by the upward pressure of water I do expect changes that deal with this
physical phenomenon.
Biomechanical adaptations to wading are not adaptations to bipedal walking
since during wading in at least chestdeep water gravitation has much less
influence on the body.
So my problem is (for example):why should the kneejoint in an aquatic ape
begin to become adapted to weightbearing while in fact it didn't have to
bear much weight?
Why should it begin to develop a relatively (compared to apes) high
bicondylar angle in order to bring the knee closer to the midline of the
body so that the centre of gravity of the body need only move a short
distance to bring it above the stance leg while such a problem of gravity
was hardly present in the water? etc.etc.
Gerrit.
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