MrDarwin

<a href="http://www.sciencedaily.com/releases/2002/03/020329071417.htm" target="_blank">Experiments Reveal Ancient Blood Flow Map; Researchers Suggest Blood Vessels May Have Evolved From Blood Cells</a>

[ March 29, 2002: Message edited by: MrDarwin ]</p>

Godless Dave

One more nail in the coffin of "irreducible complexity".

hezekiah jones

Each time "irreducibly complex" systems are identified in accordance with Behe's own definition, Behe claims they weren't "irreducibly complex" systems after all. He's done it before, and he'll do it again.

lpetrich

Basically, a hormone that induces the formation of blood vessels in mammals has a homologue that directs blood cells to certain places in Drosophila fruit flies.

So the next question is how one gets from blood cells to blood vessels. How did some blood cells acquire the ability to arrange themselves in tubes?

This is another example of a "complex" feature being conserved across animal phyla; some related discoveries have forced some radical revisions in our views of the early evolution of animals.

Leading the way has been molecular-phylogeny construction. Using understanding of such misleading effects as long-branch attraction, Aguinaldo and others have come up with a new family tree of the animal kingdom:
[code]
Fungi
Choanoflagellates
Sponges (Porifera)
Cnidaria
Ctenophora
Bilateria
Protostomia
Ecdysozoa
Lophotrochozoa
Deuterostomia
</pre>[/quote]

Much of this tree agrees with traditional views; the big surprises are the grouping of most protostomes into Ecdysozoa (molting animals) and Lophotrochzoa (those having trochophore larvae [annelids, mollusks], lophophorates [brachiopods, bryozoans], flatworms, rotifers, etc.).

This means that a favorite model system, nematode Caenorhabditis elegans, is not really an early branch; that seeming early branching is a case of long-branch attraction. Instead, the nematodes are in the Ecdysozoa, alongside the arthropods and arthropod-like animals.

One very important question is what the ancestral bilaterian had been like; this can be worked out by comparing structures and mechanisms and sequences across the family tree. And the inferred ancestral bilaterian is surprisingly complex.

The ancestral bilaterian had something like 8 homeobox genes for specifying front-to-rear patterning; each of the three subgroups has its characteristic modifications to that scheme. Cnidarians, however, have a much smaller number.

The ancestral bilaterian had some light-sensitive spots, as inferred from genes like Pax-6; however, eye details are the result of later evolution. Mouse Pax-6 can induce the growth of ectopic eyes in fruit flies; however, those eyes look like fruit-fly eyes and not mouse eyes.

The ancestral bilaterian had had a heart, though likely only a very simple one. This may originally have been an ectopic throat growing in the body interior.

MrDarwin's reported result suggests that the ancestral bilaterian had had blood cells that get sent to different parts of the body with hormones.

The ancestral bilaterian had had a central nervous system that was likely a stripe extending the length of its body. However, it was elaborated in very different ways; on the protostome side, a ladder-like CNS grew from it, while among the vertebrates, this stripe folded to form the spinal cord.

The ancestral bilaterian had a one-way digestive system that extended most of the length of its body.

The ancestral bilaterian had a dorsoventral patterning mechanism, thus confirming Geoffroy St. Hilaire's inversion hypothesis. This mechanism is conserved enough for fly dorsal patterners to act as frog ventral patterners, and vice versa.

The ancestral bilaterian was very likely segmented. Though the admixture of arthropods, annelids, and vertebrates with unsegmented phyla suggests convergent evolution, what's known about the molecular mechanisms of segmentation suggests a shared origin -- and abundant segmentation loss.

The ancestral bilaterian had had some sort of appendages, though what sort is unclear. The big difficulty is that many appendages had likely been ectopic ones; thus, most arthropod limbs may be ectopic antennae, and jawed-fish side fins (eventually -&gt; limbs) may be ectopic top or bottom fins.

The overall result is a worm with a rather impressive array of features; a big jump over cnidarians and ctenophores.

One interesting complication is that many marine invertebrates do indirect development, passing through a small planktonic tadpole-ish larval stage. So the ancestral bilaterian could have been a worm that grows from such a larva. One complication is that the existing groups have very different-looking larvae, which may indicate convergent evolution.

Comparing to cnidarians and ctenophores, this worm looks very complex; it is especially curious that there is little evidence of branch-off at whatever intermediate phases might reasonably be inferred.

Was there some mass extinction that left only a few survivors? Possibly, as a result of the late-Proterozoic "Snowball Earth" super ice age.

Here's a <a href="http://cas.bellarmine.edu/tietjen/Ecology/early_animal_evolution.htm" target="_blank">nice introduction</a> to the subject; it covers much of what I've described.

[ March 29, 2002: Message edited by: lpetrich ]</p>