Re: News: Humans and sponges may share a slimy ancestor

"Lorentz" <drosen0000@xxxxxxxxx> wrote in message news:gm58en$eli$1@xxxxxxxxxxxxxxxxxxxxxx
On Jan 29, 4:04=A0pm, "Robert Karl Stonjek" <rston...@xxxxxxxxxxxxxx>
wrote:

If they're true, the findings have a difficult implication for many
biologists: they suggest that the nervous system evolved twice in two
separate systems. Placozoa and sponges don't have a nervous system, but t=
he
related cnidarians - which include jellyfish - do.

I propose that the nervous system evolved once, but crossed
clades by horizontal gene transfer. Maybe the nervous system evolved
first in cnidarians, and crossed into the triblastic animals via a
virus. Or maybe the other way. A protocnidarian without a nervous eats
something with a nervous system and accidentally incorporates its DNA.
I would be interested if the genes coding for acetylcholine are
the same in all clades. Acetylcholine is a venom in cnidarians. It
stings because it is a neurohumor in higher forms of life.
Acetylcholine is a complicated protein.

Huh??? Acetylcholine is a small and simple molecule. Simpler than
adenine, similar to arginine, and certainly simpler than any proteins.
http://en.wikipedia.org/wiki/Acetylcholine

If cnidarians and humans share
the same neurohumors, then I would have to say that the nerve cell is
a primitive feature.

I think you are on the right track questioning whether the same
neurotransmitters are used in the (supposed) two families of
nervous systems. But I'm not sure that this line of evidence
will be conclusive. Both acetylcholine and the synthesizing
enzyme choline acetyltransferase could have been present
well before there were nervous systems. Metazoan cells
need to communicate chemically with their sibs, and they
use molecules like acetylcholine to do so even if evolution
(or development) has not yet generated nerve cells. In
fact, I suspect that the ur-eukaryote probably made
acetylcholine - it strikes me as a fairly inevitable byproduct
of lipid metabolism.

.

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