Interesting development at the base of the animal tree (Re: comb jellies)

A recent genetic study seems to show that the comb jellies (a phylum which look like a simpler version of jelly fish) was the first taxon to split from the main animal tree.This is surprising in a way; since sponges are the simplest extant animals, it was assumed that Porifera was the first to branch off, the complexity of the rest of the animal kingdom being assumed to be due to the complexity of the common ancestor.But if Ctenophora did branch off first, there are a couple of intriguing possibilities: either the last common ancestor of extant animals was already relatively complex and sponges then evolved to be simpler, or Ctenophora and the non-sponge animals represent two branches that evolved complexity independently.Fascinating stuff. I got this from Theology Web, where there has already been a bit of a discussion.Edited by Adminnemooseus, : Added the "(Re: comb jellies)" to the topic title.

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Speaking personally, I find few things more awesome than contemplating this vast and majestic process of evolution, the ebb and flow of successive biotas through geological time. Creationists and others who cannot for ideological or religious reasons accept the fact of evolution miss out a great deal, and are left with a claustrophobic little universe in which nothing happens and nothing changes.
-- M. Alan Kazlev

It is possible that the last common ancestor of the animal kingdom did have a degree of complexity, and that the sponges did evolve to become simpler. Such things have happened -- look at the tunicates, which are essentially simplified chordates.Either two branches evolved to be complex independently, or the clade started relatively complex and one branch evolved to become less complex. Both scenarios have two events, so I think that (just looking at relative complexity) both are equally parsimonius.If you look at the tree developed by this group: you'll see something interesting. Sponges and true jellyfish compose a single taxon. Now that is something that surprises me (perhaps for no greater reason than I don't know much about the early branchings of the animal tree).

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Speaking personally, I find few things more awesome than contemplating this vast and majestic process of evolution, the ebb and flow of successive biotas through geological time. Creationists and others who cannot for ideological or religious reasons accept the fact of evolution miss out a great deal, and are left with a claustrophobic little universe in which nothing happens and nothing changes.
-- M. Alan Kazlev

Hi, Mr. Jack.Actually, a little bit more can be said. I have composed a graphicthat shows two scenarios, both of which imply that the members of Bilateria are more closely related to each other than they are to any other group, and that sponges (Porifera) and jellyfish (Cnidaria) are both more closely related to Bilateria than either are to comb jellies (Ctenophora): Thanks to ImageShack for Free Image HostingHowever, there is a subtle difference: in scenario 1, jellyfish are more closely related to Bilateria than to sponges, while in scenario 2 jellyfish and sponges are more closely related to each other than either are to Bilateria.According to the graphic that I have already posted, the authors of the paper chose scenario 2 as their preferred scenario; you'll see that members of Porifera and Cnidaria share a common node that is not shared with the other groups.Now, I haven't read the paper yet (I haven't yet made it to the library -- I'm also going to see whether I can scam a copy of the paper for free before I resign myself to paying for it), so it is possible that in the paper the authors caution that their data doesn't rule out scenario 1 (or a third scenario where Porifera is more closely related to Bilateria than they are to Cnidaria). I'm just going by the graphic.So, according to this scenario, there was a hypothetical ancestor, A, to all extant animals. A then split into two reproductively isolated populations, one of which evolved into the comb jellies, and the other which evolved into B'. B' then split into two isolated populations, one of which evolved into the ancestor of the Bilateria, and the other which evolved into D. Finally, D was the ancestor of sponges and jellyfish.

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Speaking personally, I find few things more awesome than contemplating this vast and majestic process of evolution, the ebb and flow of successive biotas through geological time. Creationists and others who cannot for ideological or religious reasons accept the fact of evolution miss out a great deal, and are left with a claustrophobic little universe in which nothing happens and nothing changes.
-- M. Alan Kazlev

Actually, my graphics show no information whatsoever about the absolute genetic differences, and were intended to show only the relative timing of the splits. I didn't realize that the convention in molecular biology is to include information about genetic differences visually in the cladogram. Evidently, I was misled by cladograms prepared by paleontologists (and molecular biologists preparing materials for popular consumption), which seem to use more or less regularly spaced nodes. Next time I prepare a simplified cladogram I will explicitly state that no implications about absolute differences or absolute dates are intended.Thanks for the clarification.Edited by Chiroptera, : No reason given.

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Speaking personally, I find few things more awesome than contemplating this vast and majestic process of evolution, the ebb and flow of successive biotas through geological time. Creationists and others who cannot for ideological or religious reasons accept the fact of evolution miss out a great deal, and are left with a claustrophobic little universe in which nothing happens and nothing changes.
-- M. Alan Kazlev