| | ARCHAEOPTERYX part 5
In Dr. Michael Denton's book "Evolution: A Theory in Crisis" he
addresses the problem of feathers arising out of scales in exquisite
detail. I will try to summarize his position as much as possible because
it runs for a number of pages.
On the whole, however, even the most tentative schemes outlining a
sequence of events are seldom convincing. Take, for example, the problem
of the origin of birds. The flight feather of a bird is one of the most
beautiful and well known of all biological adaptations.
[I won't go through the details of the feather since I covered that
in the Michael Pitman piece.-G.F.]
...It is almost universally accepted by evolutionary biologists that
birds evolved from reptiles, and that the feathers evolved from a
reptile's scale. Birds are certainly closely related to reptiles and it
is difficult to see what other group of living organisms could possibly
serve as hypothetical ancestors.
By what sequence of events and through what kinds of transitional
states might the feather, the feathered airfoil or wing, and avian flight
have evolved? John Ostrum, an expert in this field, in a recent article
in the "American Scientist" refers to the two major traditional scenarios.
"Previous speculations on this question have produced two quite
different scenarios. Stated very simply these are that birds
began to fly "from the trees down"- or "from the ground up."
The first is the widely favored and very logical "arboreal
theory,"...The second is the often ridiculed and seemingly less
probable "cursorial theory,"...
One of the classic arboreal scenarios was developed by Gerhard
Heilman in his well known book "The Origin of Birds" Heilman, as an
advocate of the arboreal theory, envisaged a gliding stage preceding the
development of true powered flight. The original ancestor, he suggests,
was a terrestrial runner:
[Here, Heilman's theory is quoted from his book. The basic outline
is as follows:
-From terrestrial runner to arboreal climber leaping from branch to
branch and tree to tree and tree to ground.
-Toes change so reptile/bird can grab branches
-Pressure of air acts as stimulus to change scales to longer scales.
-Friction of air causes scales to fray.
-Gradually, longer horny processes become more and more feather-
like until perfect feather forms.
-Feathers spread throughout whole body.
-Intensive use of arms lengthens them creating more powerful muscles.
-Breastbone halves ossify completely forming sight for muscles to
attach.
-Metabolism increases, becomes warmblooded.
Dr. Denton continues on and analyzes the two theories of
flight- from the ground up or from the tree down- going over the
problems associated with each none of which we will go into. We
will concentrate on the problem of scales to feathers. G.F.]
pp208,209 According to Heilman the original impervious vane
which supported these pre-avian species as they glided was a set
of "longish scales developing along the posterior edge of the
forearms and the side edges of the flattened tale". Then he
continues:
By the friction of the air the outer edges became
frayed, the fraying gradually changing into still
longer horny processes which in the course of
time became more and more featherlike.
It is at this point, when the actual evolution of the feather is
envisaged, that Heilman's scheme begins to look particularly
implausible, for it is very difficult to understand what the
adaptive value of frayed scales would be to a gliding organism
when any degree of fraying would make the scales previous to the
air, thereby decreasing their surface area and lift capacity. All
known organisms which have adaptations for gliding among fish,
frogs, reptiles, and mammals present a continuous unbroken surface
to the air.
It would seen reasonable to believe selection for
gliding in a hypothetical pro-avis would always tend to increase
the impervious surface area of its wing and decrease the tendency
to fray...
...It is not easy to see how an impervious reptiles scale
could be converted gradually into an impervious feather without
passing through a frayed scale intermediate which would be weak,
easily deformed and still quite permeable to air. It is true that
a feather is indeed a frayed scale- a mass of keratin filaments-
but the filaments are not a random tangle but are ordered in an
amazingly complex way to achieve the tightly intertwined structure
of the feather.
Take away the exquisite coadaptation of the
components, take away the coadaptation of the hooks and brabules,
take away the precisely parallel arrangement of the barbs on the
shaft and all that is left is a soft pliable structure utterly
unsuitable to form the basis of a stiff impervious aerofoil. The
stiff impervious property of the feather which makes it so
beautiful an adaptation for flight, depends basically on such a
highly invlolved and unique system of coadaptive components that
it seems impossible that any transitional feather-like structure
could possess even to a slight degree the crucial properties.
In the words of Barbara Stahl, in "Vertebrate History: Problems in
Evolution", as far as feathers are concerned, "How they arose
initially, presumably from reptile scales, defies analysis."../
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