Re: Message from Phil Skell

From: Pim van Meurs <pimvanmeurs@yahoo.com>
Date: Wed Sep 21 2005 - 00:15:31 EDT

Terry M. Gray wrote:

>
>
>> A couple of weeks ago I drew attention here to Phil Skell's op-ed
>> piece from
>> "The Scientist," in which he raised questions about the practical
>> value of
>> evolutionary theory for doing laboratory science. Phil was not aware
>> of our
>> discussion of his essay until I told him about it. He asked me to forward
>> his comments below to the list, hoping thereby to "clear away some of the
>> fog" (his words to me). I am glad to help him do so.
>>
>> Ted
>>
>>
>> 1. The dinosaurs structurally most similar to birds are very recent
>> (80-110
>> million years ago), whereas Archaeopteryx is a great deal older (145
>> million
>> years ago) and no birdlike dinosaurs are known from the lower Jurassic or
>> Triassic that could qualify as ancestors of birds. 2. The three digits of
>> the hand of dinosaurs are 1,2,3, those of a bird are 2,3,4. It is quite
>> impossible to derive the avian digits from those of dinosaurs.
>
Actually recent evidence has shown how this is incorrect. There are many
other problems with the arguments here but Ian Musgrave pointed me to
some recent papers addressing the three digits and how science has
resolved this puzzle. Showing how once again these minor puzzles are
resolved by science in a manner consistent with evolutionary theory.

Pennisi, *DEVELOPMENTAL BIOLOGY: Bird Wings Really Are Like Dinosaurs'
Hands, * //Science// 2005 307: 194

<quote>DEVELOPMENTAL BIOLOGY:Bird Wings Really Are Like Dinosaurs' Hands
Elizabeth Pennisi Molecular studies have smoothed a wrinkle in the
assumption that modern birds had dinosaur ancestors: Researchers have
concluded that the three digits in bird wings correspond to the three
digits in dinosaurs' forelimbs.</quote>

Presenting the research in the following paper

Vargas & Fallon, 'The digits of the wing of birds are 1, 2, and 3. a
review', /Journal of Experimental Zoology B/, 6 May 2005 (advanced
online publication)

*Abstract *

Fossil evidence documenting the evolutionary transition from theropod
dinosaurs to birds indicates unambiguously that the digits of the wing
of birds are digits 1, 2, and 3. [That is, which of the standard
tetrapod ‘pentadactyl’ -- five-fingered -- limb are the remaining ones
in birds’ three-fingered ‘hands’.] However, some embryological evidence
suggests that these digits are 2, 3, and 4. This apparent lack of
correspondence has been described as the greatest challenge to the
widely accepted theropod-bird link (Zhou 2004. Naturwissenschaften
91:455-471).

Here we review the pertinent literature regarding the debate on the
origin of birds and wing digital identity and the evidence in favor of a
1, 2, 3 identity of the wing digits.

Recent molecular evidence shows that the expression of Hoxd12 and Hoxd13
[genes] in the developing wing supports the theropod-bird link. In the
chicken foot and in the mouse hand and foot, digit 1 is the only digit
to combine the expression of Hoxd13 with the absence of expression of
Hoxd12. The same is observed in the anterior digit of the wing,
suggesting it is a digit 1, as expected for a theropod.

Nevertheless, Galis et al. (2005. J Exp Zool (Mol Dev Evol) in press),
argue that Hoxd12 and Hoxd13 expression patterns in mutant limbs do not
allow distinguishing the most anterior digit in the bird wing from digit
2. They also argue that constraints to the evolution of limb development
support the 2, 3, 4 identity of the wing digits. However, the case put
forward by Galis et al. is biased and flawed with regard to
interpretation of mutant limbs, developmental mechanisms, stages
observed, and the description of the evolutionary variation of limb
development.

Importantly, Galis et al. do not present evidence from wild-type limbs
that counters the conclusions of Vargas and Fallon (2005. J Exp Zool
(Mol Dev Evol) 304B(1):85-89), and fail to provide molecular evidence to
specifically support the hypothesis that the wing digits are 2, 3, and
4. *The expression of Hoxd12 and Hoxd13 in the developing wing is
consistent with the hypothesis that birds are living dinosaurs*; this
view can lead to a greater understanding of the actual limits to the
evolutionary variation of limb development.

See also PZ Myers' website
http://pharyngula.org/index/weblog/comments/digit_numbering_and_limb_development/

Birds have dinosaur wings: The molecular evidence
Alexander O. Vargas , John F. Fallon /J. Exp. Zool. (Mol. Dev. Evol.)
304:000-000, 2004/

Abstract
Within developmental biology, the digits of the wing of birds are
considered on embryological grounds to be digits 2, 3 and 4. In
contrast, within paleontology, wing digits are named 1, 2, 3 as a result
of phylogenetic analysis of fossil taxa indicating that birds descended
from theropod dinosaurs that had lost digits 4 and 5. It has been argued
that the development of the wing does not support the conclusion that
birds are theropods, and that birds must have descended from ancestors
that had lost digits 1 and 5. Here we use highly conserved gene
expression patterns in the developing limbs of mouse and chicken,
including the chicken talpid2mutant and polydactylous Silkie breed
(Silkie mutant), to aid the assessment of digital identity in the wing.
Digit 1 in developing limbs does not express Hoxd12, but expresses
Hoxd13. All other digits express both Hoxd12and Hoxd13. We found this
signature expression pattern identifies the anteriormost digit of the
wing as digit 1, in accordance with the hypothesis these digits are 1, 2
and 3, as in theropod dinosaurs. Our evidence contradicts the
long-standing argument that the development of the wing does not support
the hypothesis that birds are living dinosaurs.
Received on Wed Sep 21 02:05:53 2005

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