Re: Initial bipedalism
Phil Nicholls (pnich@globalone.net)
Tue, 04 Jul 1995 15:09:17 GMT
Elaine Morgan <Elaine@desco.demon.co.uk> wrote:
Points on which agreement is reached:
>
>Sure, bipedalism must have been initially awkward. The measure of
>how awkward it was is the extensiveness of the remodelling needed to make
>us as efficient at it as we are after 5 million years of practice.
Awkwardness is a relative term. Awkward compared to modern humans who
are specialists, yes. Awkward compared to primates in general, I
suspect that protohominids were somewhat preadapted to bipedalism.
>I would imagine most scientists nowadays accept that the latest common
>ancestor did a lot of climbing and armswinging in the trees
>but probably less actual brachiation than the other African apes.
>Evidence for that is the knuckle-walking. They cannot walk
>with their palms flat because they evolved too far in the
>direction of hooking on to,branches, so they are incapable
>of extending wrist and elbow at the same time. No sign our ancestore
>ever knuckle-walked.
Evidence suggest that protohominids didn't knuckle-walk. As to how
arboreal there were I would suspect that without a specialized mode of
terrestrial locomotion they were more arboreal than modern African
apes, none of which are true brachiators.
>Agreed the half-way house is hard to imagine. For an ape unadapted to
>bipedalism, running on two legs (though not apperently walking on two)
>is much more energetically expensive as well as much slower. Why would
>they persist in doing it long enough to become better at it?
A difficult question since we were not there at the time. We know
that arboreal primates, those that rarely come to the ground are
bipeds exclusively when they come to the ground. Here is my
hypothesis. Protohominids were a marginalized group living at the
peripherial forest ecozones. Bipedalism allowed them to effective
exploit resources on the savannah. Initially this exploitation too
the form of brief excursions. As they become more efficient bipeds
the excursions became longer. Australopithecines probably maintained
the forest/savannah niche and hominids did not move onto the savannah
for good until the emergence of the genus Homo.
>The only extant animal model for a wild primate which spends much time
>in the trees but at ground level often walks on two legs is the
>proboscis monkey, wading through its mangrove habitat. Bipedalism is
>initially so inefficient that the initial stages would only have
>been embarked on under duress, as by the proboscis monkey.
I have seen no data on the precentage of time the proboscus monkeys
spend bipedally. I suspect that the bonobo is a much better model.
>When you told your son that the explanation of bipedalism is now being
>hotly debated, did you mention that it has been hotly debated for over
>a hundred years, and did he ask you why it was taking so long? It is
>because of that hidden premise in the question : Why did bipedalism
>evolve (on the savannah, understood) ? There will never be an answer
>to that because on the savannah it never would have evolved.
It has never been "solved" because it is not a problem that is likely
to generate a final answer. There are limits to what scientist can do
in terms of reconstructing past behaviors. The notion that our
evolution is tied to exploitation of the savannas is tied two facts:
[1] The paleoclimatological evidence shows that the total land area
in Africa covered by forest declined and that the total land area
occupied by savanna environments increased. It is therefore
reasonable to assume that our evolution is tied to this environmental
change.
[2] We know for certain that later hominids were exploiting savannas.
Now let us compare our scenerios here:
Yours:
Trees -------------> Water ---------------> Savannah
Mine:
Trees ------------> Trees/Savannah --------> Savannah
My scenerio has the advantage of being more parsimonious than yours.
Unfortunately for you there is not "suntan" in evidence.
==========================================================
Phil Nicholls "To ask a question you must first
pnich@globalone.net know most of the answer.
Semper Alouatta! - Robert Sheckley
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