Re: Alleged evolutionary implausibilities



On Nov 24, 4:45 pm, Ron O <rokim...@xxxxxxx> wrote:

[snip]

Take the giraffe example. Before vertebrates had necks they evolved
their jaws from the branchial arches of the embryo (in fish these
develop into gills and they are often called gill slits because that
is what they look like in the embryo). It is my recollection that the
nerve that controls the jaw region loops down what became the neck of
land vertebrates from the brain and then back up the neck to the jaw
(remember the gills used to be behind the mouth in early jawless fishy
vertebrates). Well the giraffe still has this arrangement, but now
the nerve has to travel all the way down that long neck and loop back
up to the jaw just like the giraffes ancestors that first evolved
necks from the parts that used to develop into gill slits. Is this
better evidence for common descent of terrestrial vertebrates from a
fish like ancestor or did the designer just design this arrangement
because he wanted to do it that way? If this isn't due to evolution
from a common ancestor, why couldn't the designer just run the nerve
from the brain to the jaw instead of having it grow to several meters
in length? Why doesn't it go straight from the brain to the jaw in
all other land vertebrates including humans? Why would it take the
route that it would take in a fish in vertebrates like humans that no
longer have gills?

Yes, that's the recurrent pharyngeal nerve, which in all mammals (if
I'm not mistaken) loops around the aorta in order to get from the
brain to the larynx. In the giraffe, this nerve is thus ~15 feet
long, whereas the larynx is ~1 foot from the brain. This brilliant
design feature makes the animal more susceptible to injury and
probably imposes a cost on the animal: it has to grow more nerve than
necessary and signals sent through the nerve take more time and
energy. Matt Ridley explains the course of the r.p.n. as follows
(from his textbook _Evolution_):

"The laryngeal nerve is, anatomically, the fourth vagus nerve, one of
the cranial nerves. These nerves first evolved in fish-like
ancestors. ... [S]uccessive branches of the vagus nerve pass, in fish,
behind the successive arterial arches that run through the gills. Each
nerve takes a direct route from the brain to the gills. During
evolution, the gill arches have been transformed; the sixth gill arch
has evolved in mammals into the ductus arteriosus, which is
anatomically near to the heart. The recurrent laryngeal nerve still
follows the route behind the (now highly modified) gill arch: in a
modern mammal, therefore, the nerve passes from the brain, down the
neck, round the dorsal aorta, and back up to the larynx."

The ID explanation: The designer must have had His^H^H^H his reasons
for designing the nervous system in this way.

Thanks to all for their replies.

Richard


Ron Okimoto


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