Archaeopteryx ScalesFeathers
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.”