On Wed, 17 Dec 2003 18:21:44 +0000 (UTC), "Mario Petrinovich"
 wrote:

>        Hi folks
>
>        Firstly, I am a layman, don't know much about biology. But, I like
>to watch documentaries about life.
>        Well, yesterday I saw an article on the Internet. Here is the link :
>www.nature.com/nsu/031215/031215-2.html
>        In this article they are talking how vertebrates "share a
>surprisingly large number of genes" with some coral. Well, this is completly
>in tune with what I saw in documentaries. What I saw in documentaries?
>        Firstly, I saw a sea plant (I forgot which one). A seed of that
>plant has a tiny brain, which it needs to find a good place to settle. Whem
>it settles, it doesn't need that brain anymore, but instead it needs energy
>to grow, so it eats its brain.
>        Secondly, one documentary talked about origin of vertebral column.
>According to tha documentary, a vertebral column in the beginning were
>sand-sized magnets. If you take a few peaces of magnet, they would line
>themselves in line, pointing toward bottom (if you are in fluid). This way
>they could lead a tiny organism toward rich mud at the bottom.
>        If you put this two together, you are having a plant seed, which is
>looking for a mud (which it knows how to exploit, and live of it). It sounds
>logical that first it was plants, and from plants emerged animals. Plants
>are eating sunlight. Plants seeds need to move. Animals are moving. Animals
>are eating plants and other animals.
>        I am puzzled how authors of that article didn't come to same
>conclusions. -- Mario
>

First, for those looking on, the paper under discussion is

  Kortschak, R. D., Samuel, G., Saint, R. & Miller, D. J. 
  EST analysis of the cnidarian Acropora millepora reveals 
       extensive gene loss and rapid sequence divergence
       in the model invertebrates. 
   Current Biology, 13, 2190 - 2195, (2003).

The paper is available at
http://download.current-biology.com/pdfs/0960-9822/PIIS0960982203008728.pdf


I think you have misinterpreted some of the information you have read
and heard.  The coral studied in the paper is from a group of animals,
the Cnidaria, that diverged very early from the base of the line of
the multicellular animals, the metazoa.  Any genes shared by corals
and us should be present in the ancestral metazoan and therefore
should be found in virtually all other living animals.  The surprise
was not that the coral shared genes with us, but that they are NOT
shared in the two other major "model" animals, the fruit fly
Drosophila and the roundworm Caenorhabditis.  The argument is that
these shared genes must have been present in the ancestral form and
then lost in the insects (Arthropods) and roundworms (Nematodes).
Probably many of the "model" organisms for which we have complete
genomes are very specialized and may have lost substantial numbers of
ancestral genes, replacing them with others more suitable to their
specialized life style and physiology.

Now for the two points you raise.  First, the "plant" you learned
about is not a plant at all, but an animal called a tunicate or sea
squirt.  This branched out very early from the group of animals that
we humans belong to, the phylum Chordata. The tunicate is considered a
model for the earliest form of our own bodies.   The "seed" is not a
plant seed at all, but the larval form of the tunicate which swims
like a tadpole.  It in fact does have a tiny central nervous system
which has many similarities with our own:  it forms from a dorsal
groove, then a tube, and includes an anterior "brain" and a long
hollow nerve cord lying just above a structure called the notochord.
This corresponds exactly to our own brain and spinal cord.  In our own
case, the growth and development of the vertebral column destroys the
cells from our original notochord so we call our own dorsal cord the
"spinal cord".  When the larval tunicate matures, it settles to the
bottom, sticks to a rock, and metamorphoses into a very different form
with virtually no nervous system at all.  It doesn't really "eat" the
brain although, like all tissues fated for destruction, the cells do
get broken down and the constituents recycled in metabolism.  The
adult tunicate is stationary, attached to a rock (sessile), much like
sea anemones, corals, and sponges.  Some people think these stationary
animals are plants simply because they don't move around.  They really
have no evolutionary relationship to plants except for the fact that
all organisms in the world are eventually related one way or another.

Second, I don't know where you may have seen a documentary discussing
the formation of the vertebral column as "sand-size magnets".  The
vertebra develop from a series of segmental blocks of tissue aligned
along the embryonic notochord.  Yes, these may be the size of sand
grains and, yes, there are developmental genes and intersegmental
influences that cause them to line up and organize themselves into a
row.  Still, the mechanism by which they align and interact has
nothing whatsoever to do with the way that pieces of a magnet would.

There are bacteria that can sense a magnetic field and the move along
the magnetic lines of force.  In the northern hemisphere, the
North-seeking lines dip into the earth so that an aquatic bacterium
that follows the lines eventually finds itself buried in the mud at
the bottom of the water.  I would guess that you saw two documentaries
(or one which discussed two ideas): one dealing with the formation of
the vertebral column from a series of segmental blocks, the other
dealing with these bacteria which find the mud they live in by
following the magnetic field of the earth.  You then mixed the two
notions -- not at all surprising in view of the enormous amount of
science that we encounter.  

So your conclusion about the origin of animals, although seemingly
logical and well thought out, really is based on some incorrect
assumptions. However, keep up thinking about things and sharing your
ideas!  That is the important thing!
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