Re: Exaptation and cookie cutters
Phil Nicholls (pnich@globalone.net)
Mon, 02 Oct 1995 15:06:41 GMT
Elaine Morgan <Elaine@desco.demon.co.uk> graced us with the following
words:
[Discussion of "Exaptation" omitted]
>Okay, I can think of possible evolutionary examples. Face-to-face sex,
>for instance. People used to think of a rewason why this was adaptive.
>e.g. as a way of cementing the pair bond, because a mating couple can
>look into one another's eyes during coition,,and this was supposed to
>make them more aware of, or more likely to remember later, who they
>were doing it with. My own first thought was (guess what!) that it must
>be an aquatic adaptation because the ventro-ventral approach is very
>common in aquatic mammals and very rare in land ones. I now concede
>that it was a spandrel, an inevitable result of the spine and the hind
>limbs being habitually arranged in a straight line, thus altering the
>angle of the vagina. To my mind that was indirectly aquatic because I
>believe bipedalism was a result of wading; but I that if anyone comes
>up with a convincing non-aquatic explanation of bipedalism, they will
>have explained ventro-ventral sex also. The aquatics are frequently
>ventro-ventral of course because swimming also arranges the limbes and
>spione in a straight line.
I hope you read Jerry Drawhorn's excellent article on bipedalism. In
case you haven't I am going to repost it here.
>> I have three points to make regarding the AAT.
>> 1) The AL-333 Locality suggests that A. afarensis was poorly adapted
>> to dealing with even moderate levels of water. The lithostratigraphy and
>> taphonomy of this "First Family" site indicates the group was incapable of
>> escaping from a slowly rising water level while they were trapped on a
>> spit of land along an embayment or slow moving river. Clearly these hominids
>> were utilizing littoral environments- but why would they have so quickly
>> lost the "Aquatic" adaptations that would have profoundly benefitted them?
>> I reason that A. afarensis probably did not have Aquatic ancestors
>> ("wading" ancestors perhaps, but many primates do this, including lowland
>> gorillas and bonobos...it's a pervasive catarrhine character).
>> 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
>> foot, ankle and leg of an ancestral hominoid on an opposite trajectory
>> than ultimately attained in hominids. Even wading by a quadruped would
>> reduce these forces to some degree.
>> 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.
>> 3) Many of the soft-anatomical features cited by the AAT proponents
>> of an aquatic past are found in related hominoids and cercopithecoids.
>> Digital webbing is found in siamangs (hence the name Symphalangus
>> syndactylus), gorillas and bonobos.These are probably adaptations to the
>> brachiatory hand of hominoids. I suspect the diving reflex, and many
>> other AAT traits would also be found in our near relatives. The fact that
>> Japanese macaques, Nasalis, and swamp monkeys (C. mitis) are capable of
>> swimming suggests a broader capability not restricted to humans. If so,
>> there may be many traits attributed as derived from a human AA phase
>> that are clearly "pre-adaptations" from an earlier set of features. Until
>> we know what the distribution of these characters are in hominoids (ie.
>> can it clearly be stated that no other hominoid except humans has a
>> "diving reflex"), these characters should be given very low weight in any
>> argument.
>> Jerry Drawhorn
> Dept. of Anthropology
> University of California
> Davis, CA 95616
>BUT. Important but. I think you can only legitimnately use the term
>"spandrel" as a way of dismissing apparently non-adaptive features
>if you can offer some kind of idea about what the spandrel was the
>side-effect OF : if you can point to the dome or the arch, or to the
>pile of cookies, or to the bipedal or natatory locomotion, and say
>*that* was adaptive, and it led secondarily to this other thing.
>Gould's definition of exaptive is that it describes "any organ not
>evolved under natural selection for its current use - either because it
>performed a different function in ancestors (classic preadaptation) or
>because it represented a non-functional part available for later
>co-aptation" (Bully for Brontosaurus, p. 144n)
>I am quite happy for Alex to say maybe the descended larynx or any
>other feature was exaptive in the first of these two senses - i.e. it
>performed a different function in our ancestors ( possibly during the
>semi-aquatic phase). But I am not happy for anyone to say they are
>exaptive in the second, fuzzier, "non-functional"
>spandrel sense, without giving some idea of what the spandrel was a
>spin-off from. The pastry diamonds may not have been the object of the
>exercise, but they weren't there by accident either. "Non-functional"
>is a cop-out word that collects undertones of pure chance.
>Elaine.
There is a story about some archaeologists in the US who believed that
atlatl hooks and weights were fish net hooks and net spacers used to
make fishing nets. To prove their point they used them to make a
rather nice fishing net.
The problem with making intepretations of functionality in this way
arre compounded when you try to assign anatomical structures a
function in a niche they never occupied. It is compounded by the
"can't prove me wrong" attitude the envelops such stories.
>n
Phil Nicholls pnich@globalone.net
"To ask a question you must first know most of the answer"
-Robert Sheckley
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