>The generally accepted period covered by the Cambrian explosion -- from the
>beginning of the Cambrian to the onset of the Chengjiang and Burgess Shale
>periods -- spanned 18 million years. This is hardly sudden or instantaneous
>by either geological or evolutionary standards.
The point is that there are no fossils illustrating the formative process of
these complex organisms, however long their epoch of emergence. Their
complexity seems neither explicable through, nor is it illustrated by, the
accretion of variations.
Radical evolutionary mechanisms must have been at work, mechanisms that
are not operative now, presumably because their products were too bizarre to
be competitive in a world of well-evolved metazoans. These might include
the genomic integration of symbionts at a metazoan level and parabiosis
(siamese-twinning) as a mechanism for generating serial homologs.
Vertebrate origins is a topic for theorists, as Homer Smith wrote. (Not a
name one hears these days, but I thought he was good reading.) I don't
see how one can be satisfied with an arbitrary simple-to-complex
arrangement of early organisms
>However, using paleontological, molecular and ontological evidence,
>the emergence of the chordates can be traced from when they branched
>off from annelids and molluscs through arthropods and nematodes back
>to a roundish flatworm that lived during the late Vendian at the time of the
>Ediacaran fauna.
This is forcing the fossils into the desired simple-to-complex pattern.
Similarities do not prove genealogy, they only suggest common ancestry,
ancestry which in this case is just too obscure. In particular, the leap from
simple chordates to vertebrates with appendicular skeletons is just too
vast an assumption. No theory as to how it happened, no fossils illustrating
the transition.
>I guess that depends on what you mean by different. Evolution of body plans
>and ontological developmental strategies certainly appear different from
>shifts in body color and antibiotic resistence, but it is still basic
>neo-Darwinian evolution, only on a larger scale and affecting more critical
>systems.
I'm all for mutation and natural selection, but I find gradualism unacceptable
as explaining the formative stages. It's not what the record shows, and the
irreducible complexity argument is not given its due as an argument against
the gradual-accretion model of the evolution of physiological complexity.
>As offensive as this sounds, it is nonetheless true that you really do not
>know or understand this issue as much as you claim to. My best advice would
>thus be: READ THE SCIENTIFIC LITERATURE!
I'm expressing doubts and suggesting alternatives to what I've read,
alternatives
which I don't see in the literature. It's not so much what I claim I know, it's
what I
claim we don't know.
--Cliff Lundberg ~ San Francisco ~ cliff@noe.com