Group

Here is another important article that is, AFAIK, not webbed, regarding the
fin-leg transition (which I accept happened-but not *fully* naturalistically),
which the Darwinists thought happened on land but in fact "tetrapod
anatomy evolved while our ancestors lived exclusively underwater and it
evolved for life underwater" and "The first vertebrate that walked onto land
didn't crawl on fish fins, it had evolved well-turned legs millions of years
beforehand" (see tagline).

"It is one of those fixed images of evolution: adventurous fish
managing to hoist themselves onto their stubby fins and crawling
clumsily out of the swamps to forage for food. Once these primeval
creatures were on terra firma, their offspring began to adapt to their
new environment, natural selection (over tens of millions of years)
favoring those that developed features well suited to life on land:
paws, hooves, knees, joints, fingers and thumbs. Thus, as
generations of schoolchildren have learned, did these marine
creatures give rise to frogs, birds, dinosaurs and all the rest. There's
only one problem with this familiar version of how our distant
ancestors emerged from the sea: it's probably wrong. .. newly
assembled fossils-in particular, a 360 million-year-old salamander
like aquatic animal called Acanthostega - strongly suggest that toes
and feet were developed before the first relatives of fish climbed
onto land, not after." (see also tagline).

There is also a quod quote on the role of hox genes (the vertebrate version
of homeotic genes) which help lay down the vertebrate body plan, and the
difficulty of a `blind watchmaker' tinkering with a hox gene and making a
change for the better:

"The drawback for scientists is that nature's shrewd economy
conceals enormous complexity. Researchers are finding evidence
that the Hox genes and the non-Hox homeobox genes are not
independent agents but members of vast genetic networks that
connect hundreds, perhaps thousands, of other genes. Change one
component, and myriad others will change as well-and not
necessarily for the better. Thus dreams of tinkering with nature's
toolbox to bring to life what scientists call a `hopeful monster'-such
as a fish with feet-are likely to remain elusive. Scientists, as
Duboule observes, are still far from reproducing in a laboratory the
biochemical artistry that nature has taken millions of years to
accomplish."

I will add this to my "Articles posted to CED"
(http://members.iinet.net.au/~sejones/cedartic.html).

Steve

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68 SCIENCE

Where Do Toes Come From?

Linking fish fins to mouse paws, researchers
may have solved an important evolutionary puzzle

By J. Madeleine NASH CHICAGO

It is one of those fixed images of evolution: adventurous fish
managing to hoist themselves onto their stubby fins and crawling
clumsily out of the swamps to forage for food. Once these primeval
creatures were on terra firma, their offspring began to adapt to
their new environment, natural selection (over tens of millions of
years) favoring those that developed features well suited to life on
land: paws, hooves, knees, joints, fingers and thumbs. Thus, as
generations of schoolchildren have learned, did these marine creatures
give rise to frogs, birds, dinosaurs and all the rest.

There's only one problem with this familiar version of how our distant
ancestors emerged from the sea: it's probably wrong. For one thing,
the first creatures to waddle ashore were arthropods with well-
developed legs and pincers. For another, newly assembled fossils-in
particular, a 360 million-year-old salamander-like aquatic animal
called Acanthostega- strongly suggest that toes and feet were
developed before the first relatives of fish climbed onto land, not
after. Moreover, in shape and function, Acanthostega's fully jointed
toes bear no resemblance to the spiky, fanlike fins of a fish.
Scientists believe they understand how a fish's gills evolved into an
amphibian's lungs. But how did fins turn into feet like these?

The answer may be in the genes. That's the tantalizing conclusion of
a team of researchers from the

---
A fish with legs: This 60cm-long Acanthostega (a reconstruction is
shown here) lived underwater 360 million years ago. Note the
eight-toed feet
----

University of Geneva in Switzerland. They have discovered that genes
associated with the formation of fins in fish are the same ones that
orchestrate the development of paws in mice. "Think of a mouse as a
fish with limbs, or a fish as a mouse with fins," says University of
Geneva developmental biologist Denis Duboule. "What a mouse does is
take a fin and put something extra on top of it."

That something extra, Duboule and his colleagues suggest in the
journal - Nature, is provided by a special set of genes that act as
master architects in a surprisingly broad range of animals, from
rodents to roundworms. These gossamer strands of DNA-known as
homoeotic homeobox genes, or Hox genes for short-lay out the embryo
from head to tail, controlling everything from the development of
limbs and the wiring of the spinal cord to the patterning of the gut
and urogenital tracts. "What's amazing," says University of
Pennsylvania paleontologist Neil Shubin, "is that evolution of complex
structures appears to be controlled by this same small set of genes."

How do Hox genes pack such power? The DNA in all genes carries
instructions for assembling proteins out of chemical building blocks
called amino acids. What sets the proteins made by Hox genes apart is
the biochemical motif known as a homeobox, a stylized string of 60
amino acids that enables Hox proteins to stick to DNA like strips of
molecular Velcro and, in the process, activate still other genes.
Hundreds of genes belong to the extended homeobox family, but those
that are also homoeotic- associated with changes in body parts-are the
most important. Though they are few in number (38 out of an estimated
50,000 to 100,000 genes in modern vertebrates, the Hox genes control
much of what happens during embryonic development.

69

Only during the past decade have scientists begun to tease apart the
mysteries of Hox genes. Clustered in groups of eight to 11, on as
many as four chromosomes in a developing embryo's cells, these genes
switch on and off in sequence. Since embryos mature from the top
down, explains biologist Cliff Tabin of the Harvard Medical School, a
Hox gene that turns off a bit early, or stays on just a touch longer,
can make a dramatic difference in the formation of the embryo. Swans,
for example, have more neck vertebrae than chickens and thus longer
necks. That is because the Hox genes responsible for making neck
bones stay on longer in the unhatched cygnet than in the unhatched
chick.

Timing may also explain the progression of fins to feet. In tetrapods
(four-legged animals), feet do not grow straight out of the leg,
proceeding from the ankle out, but develop in a fanlike progression
that runs from the smallest digit to the largest. In Geneva, Duboule
and his colleagues tracked the activity of four Hox genes in the
budding feet of embryonic mice and found precisely this pattern. By
contrast, studies showed that in the zebrafish, the Hox genes switch
off earlier, perhaps to ensure that a flexible fin ray (useful for
swimming) will form in the place of feet. Duboule speculates that if
these genes could be tricked into staying on just a bit longer, the
fins of the zebrafish might sprout appendages suggestive of primitive
feet.

What would a fish with feet look like? It could easily resemble the
Acanthostega. Mineralized bones of this strange creature, unearthed
in Greenland in 1987, tend to confirm the notion that fish did not
crawl onto shores on their fins says paleontologist Michael Coates of
University College, London. Instead they probably developed limbs and
feet that they used in the water for millions of years before they
were capable of colonizing the land.

The transition to land was likely a gradual affair involving multiple
stages of evolutionary change. The skeletons of fish with their
slender bones arrayed all in a row, are clearly ill suited for walking
and running. Moreover, the muscle designed to deliver power in all
the wrong places. "Think about tucking into a tetrapod [a cow, for
instance] for Sunday lunch," says Coates. "The best cuts are the
thighs and shoulders, the muscle motors that drive these animals
along. In a fish these motors are pathetic, tiny things. It's the
back and tail muscles that propel it through the water."

Duboule believes that over the eons of prehistory, Hox genes played a
key role in the origin of species, facilitating the process of
evolutionary change. Scientists now know, for example, that the genes
that trigger the formation of hands and feet also control many other
developmental processes in posterior part of an animal-among them, the
addition of an anal opening to the digestive tract and, in four-legged
creatures, the fusion of the lower vertebrae to make a pelvis. Isn't
it curious, says Duboule, that fish lack a true pelvis as well as
hands and feet? This suggests to him that both structures-the
appendages for walking and the bony apparatus that anchors them to the
spine-are linked at some deep genetic level that is yet to be plumbed.

Duboule concedes that "this is not even a real hypothesis," just a
hunch, and that testing it will not be easy. One problem, contends
Harvard's Tabin, is that Doboule and his colleagues studied "the wrong
fish." Zebrafish are prolific and easy to raise under laboratory
conditions, but they are advanced in evolutionary terms. A study of
more primitive sea life, such as sharks or sturgeon might yield
greater amounts of evolutionary information; even better subjects
would be lungfish and coelacanths, mysterious, nearly extinct
creatures that lurk in the ocean depths and are the living fish
closest to the fishlike ancestors of four-legged animals.

Further studies are needed to convince scientists that Duboule and his
colleagues have correctly solved the fins-to-feet riddle. Other
factors could be involved as well, including homeobox genes that are
not Hox genes (that is, they do not affect the overall structure of an
animal). Last year Sean Carroll, a developmental biologist at the
Howard Hughes Medical Institute in Madison, Wisconsin, showed that a
homeobox gene involved in insect-limb formation also controls the
genetic signals that paint spots on butterfly wings. In essence, says
Carroll, butterflies use an old gene to perform a new trick.
"Evolution did not have to invent new genes," he observes. "One basic
toolbox gives nature enormous potential for diversity."

The drawback for scientists is that nature's shrewd economy conceals
enormous complexity. Researchers are finding evidence that the Hox
genes and the non-Hox homeobox genes are not independent agents but
members of vast genetic networks that connect hundreds, perhaps
thousands, of other genes. Change one component, and myriad others
will change as well-and not necessarily for the better. Thus dreams
of tinkering with nature's toolbox to bring to life what scientists
call a "hopeful monster"-such as a fish with feet-are likely to remain
elusive. Scientists, as Duboule observes, are still far from
reproducing in a laboratory the biochemical artistry that nature has
taken millions of years to accomplish.

(Nash J.M., "Where Do Toes Come From?" Time, August 7, 1995, pp.68-69)
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"Clack who works at the University of Cambridge's Museum of Zoology,
discovered the bulk of Acanthostega's skeleton in 1987 and has been
carefully reconstructing it ever since with fellow paleontologist Michael
Coates. They are just finishing up their monographs on the creature, and
some of the conclusions they've drawn from its body are surprising other
paleontologists. For a long time it was assumed that our limbs and feet,
which work so well for walking on land, evolved for that exact purpose.
But Acanthostega has convinced Clack and Coates otherwise; tetrapod
anatomy evolved while our ancestors lived exclusively underwater and it
evolved for life underwater. The first vertebrate that walked onto land
didn't crawl on fish fins, it had evolved well-turned legs millions of years
beforehand." (Zimmer C., "Coming Onto the Land," Discover, Vol. 16,
June 1995, pp.118-127, p.120.
http://www.mc.maricopa.edu/dept/d10/asb/anthro2003/origins/comingonto.html)
Stephen E. Jones http://members.iinet.net.au/~sejones
Moderator: http://groups.yahoo.com/group/CreationEvolutionDesign
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