Acoelomorph flatworms and precambrian evolution

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One of many open questions in evolution is the nature of bilaterian origins—when the first bilaterally symmetrical common ancestor (the Last Common Bilaterian, or LCB) to all of us mammals and insects and molluscs and polychaetes and so forth arose, and what it looked like. We know it had to have been small, soft, and wormlike, and that it lived over 600 million years ago, but unfortunately, it wasn't the kind of beast likely to be preserved in fossil deposits.

We do have a tool to help us get a glimpse of it, though: the analysis of extant organisms, searching for those common features that are likely to have been present in that first bilaterian; we're looking for the Last Common Bilaterian by finding the Least Common Denominators among living species. And one place to look is among the flatworms.

A recent paper by Bagun and Riutort examines one specific subgroup of the flatworms, the acoelomorphs (pronounced a-seel-o-morphs). These are tiny marine worms that have long been grouped under the platyhelminthes, but molecular work has been revealing that the platyhelminths have been a victim of our "worm is just a worm" bias, and are almost certainly polyphyletic. Bagun and Riutort argue that the acoels ought to be recognized as a separate phylum of their own, and further, that they represent basal bilaterians. They propose on the basis of molecular data that most of the Platyhelminthes belong in the protostome clade, and that they've secondarily lost characters common to most members of that group, such as segmentation. and that the acoelomorph flatworms are an early branch off the bilaterian root.

Continue reading "Acoelomorph flatworms and precambrian evolution" (on Pharyngula)

1 Comment

PZ Myers Wrote:

I know the Intelligent Design gang looks at these numbers for pre-Cambrian and Cambrian events and thinks it means sudden appearance, but think about it: these transitions took place over thirty million years each. I always find it peculiar that people who find it reasonable to claim that life on earth has a history of only 6,000 years can simultaneously characterize events taking tens of millions of years and occurring half a billion years ago as abrupt.

I don’t find it peculiar at all. As you know, ID professionals (mostly OECs, but with a prior commitment to the big tent) know this problem, and simply withhold time estimates when addressing YECs. If they do use the numbers, the (flawed) logic is that “if 30 MY is too long, maybe there’s no such thing as 30 MY at all.” Most nonscientists would not realize that so much other evidence supports the long time frames, and that even if there were not enough time in this one particular case, it would still be “old earth,” common descent and probably evolution – just by another mechanism. Of course there’s that inconvenient little fact that one cannot say that there’s not enough time unless one uses bogus calculations.

Your article is excellent as usual, and could easily answer, in technologically allowable detail, how the designer did it. If IDers were honest about having scientifically valid alternative hypotheses, they would be doing real research and publishing articles like this. It wouldn’t matter that the new hypothesis would be still within the framework of evolution and common descent. They would just use more design-friendly language. OTOH, if Bagun and Riutort were anti-evolution strategists, they would have skipped the research and simply spun this into the usual false dichotomy: “it didn’t happen the way scientists thought, therefore a designer did it, and one need not ask how - or when.” And they would never miss an opportinity to suggest that “therefore, everything else in evolution didn’t happen the way scientists thought.”

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This page contains a single entry by PZ Myers published on October 1, 2004 11:14 AM.

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