> Once again, it isn't quite so suddenly. The first fossil chordates, are
> invertebrates. These animals had a soft-tissue nerve cords running down
> their back, but they had no actual backbone. Carroll says
>
> "None of the early vertebrate groups shows evidence of a bony internal
> skeleton." Vertebrate Paleontology and Evolution p. 23
How do you get into a vertebrate group without having vertebrae? There
is confusion here regarding chordates and vertebrates, fueled by the
widely held assumption that vertebrates sprang from a simpler chordate
like amphioxus or tunicate larvae etc. The evidence for this is
simple-to-complex chain-of-being phylogeny and recapitulation theory.
I would venture to say that zero progress has been made in establishing
this view in the last 100 years.
What has been established in the last 100 years is the principle of
reduction and specialization of serial homologs. Vertebrate skeletons
evolve through loss, distortion, and fusion of parts. This is contrary
to the intuitions and hopes of simple-to-complex evolutionism, but it
is fact. It's actually quite logical: random mutation generating a new
skeletal segment in a halfway useful position, complete with
complementary
soft tissues, is unlikely. But random mutation simply throwing a monkey
wrench into development, such that development is truncated and segments
are distorted or lost, is perfectly reasonable.
If one takes this principle seriously, one must theorize a sudden
beginning in which a mass of identical segments begins functioning as
one organism. Subsequent radiation and evolution would see this organism
trimmed down to fit niches, rapidly at first, then more slowly.
> It isn't taboo. Can you cite some evidence that the cambrian explosion
> isn't being discussed?
No, but I'll stick to my point that people love to battle over small
particulars but are less comfortable addressing the biggest topic of
all,
the appearance of vertebrates.
> The fossil evidence clearly shows that invertebrate
> chordates first appear followed by chordates with vertebrae.
Vertebrates could have been a rare form, until enough time passed
for their more complex morphology to be usefully harnessed. Only then
would they proliferate and commence their domination.
You don't accept simplistic simple-to-complex phylogeny. You don't
accept
recapitulation theory. So what is your basis for accepting the old
amphioxus-to-vertebrate model and repudiating the established principle
of reduction and specialization in the evolution of segmented organisms?
-- Cliff Lundberg ~ San Francisco ~ cliff@noevalley.com