Text 108, 71 rader
Skriven 2004-09-20 21:55:00 av R Norman (1:278/230)
Ärende: Re: A Question About Inse
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On Sun, 19 Sep 2004 17:51:58 +0000 (UTC), nospam@nospam.com (Paul
Ciszek) wrote:

>
>In article <cihqt1$25r9$1@darwin.ediacara.org>,
>Malcolm <malcolm@55bank.freeserve.co.uk> wrote:
>>
>>The university biology is out of date. It is now known that there are
>>homeobox genes, or "hox" genes, which are very highly conserved across
>>animals and which seem to be involved in regulating the gross body plan.
>>Both insects and vertebrates have a fundamental segmented structure (you can
>>see this if you look at fish muscle, or the pattern of nerves on a human
>>torso, or even more obviously in insect larvae).
>>The explanation cannot be convergent evolution, but that both phyla evolved
>>from a segmented ancestor.
>
>I asked a related question here some time back, and was told the reverse--
>that what is outdated is the idea that vertebrae, ribs, six-pack-abs
>and so forth are evidence of a segmented ancestor.

There is some truth and some problems with both answers.

First, vertebrates, like all chordates, do have a partially segmented
body.  During development a series of blocks of mesodermal tissue, the
somites, form and develop skeletal and muscle structures in a
segmented fashion.  The vertebra and trunk musculature form this way.
The spinal part of the nervous system is segmented because the nerve
roots emerge in a segmental way each one originally  innervating the
structures in one somite.  The ribs and six-pack-abs are examples of
this pattern but it extends the length of the trunk.  The V-shaped
segmented muscles are particularly evident in fish.

Second, it is also true that hox type genes are highly conserved
across animal phyla.  They were discovered in insects but are also
found in humans and other mammals and vertebrates.  They are also
found in a wide range of other animals which are distinctly not
segmented at all.  For example, the roundworm, Caenorhabditis elegans,
has a nice complement of these genes. Essentially they are found in
all animals.

That is the good part.  The bad part is that the common ancestor of
arthropods and vertebrates was most definitely not segmented.  The
arthropods are most closely linked to other invertebrates in what is
not called the Ecdysozoan group, a group which includes the roundworms
and the arthropods.  The noticeable common element here is the molting
or shedding of the exoskeleton to grow, but the real relationships
were done by analysis of genetic sequences, especially of RNA
sequences.  The vertebrates, on the other hand, are Deuterostomes,
most closely related to Echinoderms like starfish and sea urchins,
again, animals which are not at all segmented.  The common ancestor of
the animal groups is still unknown, but there is no indication that it
had any segmentation at all.  However, it is virtually certain that it
DID have hox genes.

The similarities between arthropods and vertebrates in have some
aspect of segmentation and having a body divided into regions -- head
separated from trunk and abdomen -- are considered very superficial.
These are just useful ways of building bodies and these were
rediscovered (reinvented) by evolution.  The differences in body plan
and organization, though, are tremendous so that arthropods and
vertebrates are very distantly related only as bilaterally symmetric
multicellular animals.
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