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Science Forum Index » Bio Evolution Forum » Common ancestor of protostomes and deuterostomes
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| Remus |
 Posted: Sat Apr 19, 2008 8:13 pm |
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I have been reviewing some of the processes of gastrulation and
embryogenesis recently and became interested in what is generally
considered to be the common ancestor of the protostomes and the
deuterostomes. I couldn't really find much on the subject. Is it
generally considered that both branched off from cnidarians
separately, or is there generally considered to be an ancestor of all
bilateria that then specialized to produce the protostomes and the
deuterostomes? Or are even the cnidarians a seperate branch, with the
bilaterians or even both the protostomes and the deuterostomes
branching off from something like the ctenophora? |
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| Lorentz |
| Posted: Tue Apr 22, 2008 8:07 am |
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Guest
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On Apr 20, 2:13 am, Remus <roamu...@go.com> wrote:
Quote: I have been reviewing some of the processes of gastrulation and
embryogenesis recently and became interested in what is generally
considered to be the common ancestor of the protostomes and the
deuterostomes. I couldn't really find much on the subject. Is it
generally considered that both branched off from cnidarians
separately, or is there generally considered to be an ancestor of all
bilateria that then specialized to produce the protostomes and the
deuterostomes? Or are even the cnidarians a seperate branch, with the
bilaterians or even both the protostomes and the deuterostomes
branching off from something like the ctenophora?
This hypothesis came from a lunchtime discussion of the subject
by two physicists who should have been doing something else.
He pointed out that the dimple in the blastula becomes an anus in
deuterosomes, and becomes the mouth in protosomes. It seems unlikely
that in the common ancestor, the dimple could change to be a mouth in
one and an anus in the other. The two creatures must have evolved
gastrulation separately.
I pointed out that in gastrulation, there are actually two
dimples formed on both sides of the blastula. The mechanism that
causes the pinching in is small cords of protein. I read this in a
book on embryology, but it could be deduced from simple physics. By
Newtons Third Law, the force has to be applied to both sides of the
blastula. So actually, there are two dimples. The difference in size
is just a matter of which side is "stiffer." The difference is merely
one of consistency.
He immediately came to the conclusion that there was a common
ancestor. The variation that split the two lineages was a difference
in the consistency of the two sides. So the "moth" side and the "anus"
side didn't switch with regards to the body coordinates, just the
consistency of the sides changed. The "mouth" side became larger in
the protosomes and smaller in the deuterosomes.
I also want to point out that there is an asymmetry in the
blastula of the protosomes. So this may have influenced that
"consistency." The deuterosome blastula doesn't coil like the
protosome blastula. So maybe the real question is, "Why does the
blastula coil in one and not the other?"
In Deuterosomes, the big dimple becomes the anus and the small dimple
becomes the |
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