Re: AAT
Phil Nicholls (pnich@globalone.net)
Sun, 16 Jul 1995 16:33:16 GMT
Elaine Morgan <Elaine@desco.demon.co.uk> wrote:
>Alex Duncan writes:
>A feature does not have to be an adaptation in order to exist.
>In that case we may as well close down this newgroup and take up
>knitting. Why are we naked? There doesn't have to be a reason. Why are
>we bipedal? there doesn't have to be a reason. Why the big brain? there
>doesn't have to be a reason...
According to modern evolutionary theory, Elaine, natural selection is
not the only mechanism of evolutionary change. One of the most
important, non-selective/non-adaptive mechanisms of evolution is
genetic drift. I actually saw a paper presented not long ago that
suggest that hominid brain size, at least initially, could easily be
accounted for by "random walk" (i.e., the accumulated effects of
genetic drift over time).
Certainly most of use feel that hairlessness and increased brain size
are adaptive. Unfortunately the best we can do is propose possible
explanations for the adaptive significance of these features and to
perhaps eliminate those that are less likely to be true. We will
never KNOW with any kind of certainty.
>I entirely agree that some of the features we now have are not
>adaptations to our present mode of existence (like the appendix) but I
>suggest they would not be there unless they had at some point been
>adaptations to a previous mode of life led by our ancestors at some
>point in time.
Why do humans have five fingers on each hand. It has nothing to do
with five fingers being somehow more adaptive than six or eight.
There is nothing about five fingers that somehow represents an optimum
number of fingers.
The reason we have five fingers was that the tetrapod ancestor of all
LIVING terrrestiral vertebrates had five supporting digits in its
lobe. That tetrapod survived for reasons unrelated to the number of
digits but it fixed that number in the lineage of all terrestrial
vertebrates.
>You say :"Is there a functional or adaptive reason why gibbons are
>largely bipedal when on the ground? I've never heard one suggested".
>You then go on to give a perfectly good one, which I believe to be
> accurate.i.e. because their arms have grown too long for them to able to
>walk efficiently on two legs. Why do you say that is not an adaptation?
>What do you mean by adaptation? What I mean by it is a change in either
>behaviour or physical structure in response to a change in the
>realtionship between an animal and its environment. The gibbon responds
>to the demands of brachiation by growing longer and longer arms. That
>is an adaptation.
The gibbon didn't respond to its environment by growing longer arms.
If the is a feature controlled by natural selection then the Darwinian
scenerio is that gibbons with longer arms reproduced more effectively
than those with shorter arms.
>It then has to respond to the vicissitudes of
>occasional groundwalking by abandoning the quadrupedal gait which its
>ancestors, however distant, at one time employed. That is another
>adaptation.
No, that is a constraint resulting from the gibbons specialized form
of locomotion. It is not an adaptation.
>What would happen if gibbons found themselves in a treeless habitat is
>a very good question. I suspect they are so overspecialised for
>arborealism that they would become extinct. It is just conceivable that
>they would instead become better at bipedalism and their arms would
>become shorter and voila! a biped!
>Now that is an ingenious scenario. But do you, or does anyone, really
>believe that it throws any light on human bipedalism? Do you think our
>ancestors bcame more over specialised for arborealism than the gorillas
>and chimps so that they were driven down this path? Everything I have
>read about primate evolution has stressed that the whole secret of
>human evolution is that we remained the most unspecialised of the apes,
>and thus were able to adapt more quickly to the vicissitues of change.
You have actually cited a reference in _Scars of Evolution_ , a book
edited by Shiro Kondo called _Primate Morphophysiology, Locomotor
Analysis and Human Bipedalism. Try reading some of the other articles
in that book. As a matter of fact, Jack Prost, whom you cite, also
suggests that early protohominids were more arboreal than the
ancestors of great apes.
>You say: "I don't recall anyone even suggesting that human running
>capability was all that important, " In that case remember that you
>read it here first, because I am suggesting it now, Actually Carrier
>wrote a paper on it, the one I quoted from. And maybe the !Kung can
>walk an armadillo to death, but I can't see Homo erectus walking a
>zebra to death, nor walking away from a leopard or a lion.
!Kunk hunters do not run down their prey nor do they escape from lions
or leapords by running away. If you are going to invoke an
ethnographic analogy you should read an ethnography of the group in
question.
-----------------------------------
Phil Nicholls "...it is infuriating to be quoted
pnich@globalone.net again and again by creationists --
whether from design or stupidity I
do not know -- as admitting that there
no transitional forms." S.J. Gould.
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