Re: Warning labels

David Campbell (
Wed, 19 Nov 1997 14:55:05 -0400

>At 02:11 PM 11/18/97 -0400, David wrote:
> If the
>>book is up to date, it probably will discuss why the Cambrian explosion
>>happened; if very up to date (highly unlikely that this has made it into
>>textbooks), it will mention the increasing evidence that it extends well
>>back into the Precambrian and was less explosive than has been thought.
>Some perspective from an article in Time magazine (When life exploded,
>TIME, 12/5/95
> The
>article pointed out that all animal phyla except perhaps bryozoa are
>present in early Cambrian, and that they all appear within a very small
>slice of time ("no more than 10 million years")
>Steven Gould "Fast is now a lot faster than we thought, and that's
>extraordinarily interesting"
>Samuel a Bowring: "We now know how fast fast is, and what I like to ask my
>biologist friends is, How fast can evolution get before they start feeling

This is what I alluded to by "very up to date"-several phyla are now known
from fossils earlier than the Cambrian, stretching the time interval once
more. Some DNA studies have suggested up to a billion years of Precambrian
evolution of animal phyla, but others suggest a much shorter time interval,
so those estimates need taken with a lot of salt, not just a grain. "All
phyla except bryozoans" is also misleading-it's all phyla with a decent
fossil record except bryozoans, plus several with poor records but which do
turn up in the Cambrian. Not all soft-bodied phyla are known from the

>Rudolph Raff: "There must be limits to change. After all we've had these
>same old body plans for half a billion years."
On the other hand, most of the plausible shapes for organisms and
appendages have been tried. Since the Cambrian, no mass extinction has
been massive enough to provide as many possible body plans not already
being used. If something with a given body plan, lifestyle, etc. already
exists, it's going to be difficult for something else to evolve into the
same niche. The new kind is unlikely to be as good as the existing kind at
what the existing one does, so it will get outcompeted and not evolve in
that direction.
This statement also overlooks innovations in body plans for life on land or
in the air, which did not appear for a few hundred million after the end of
the Cambrian.

> G. M. Narbonne, Queens U. (paleontologist): "What Darwin described in
>Origin of Species was the steady background kind of evolution. But there
>also seems to be a non-Darwinian kind of evolution that functions over
>extremely short time periods--and that's where all the action is."
This is macroevolution in the biological sense. Some people believe that
Darwin-style evolution accounts for the development of all life and would
reject macroevolution yet believe in common ancestry and an evolutionary
origin of all kinds.

David Campbell