Someone on this list (probably Cliff) stated that biologists were "not
allowed" to speculate on the origin of the Cambrian Explosion. (The
original post is on my home computer, sorry)
and then today this:
Stephen Jones, I think:
>>But again to be fair to the evolutionary biologists there has been
>>some speculation about "mechanisms explaining the Cambrian Explosion".
>>Stanley has speculated it was due to the emergence of predators:
>
Cliff:
>Some speculation, but none that the establishment has any use for.
I'd like to know the basis for your original remark and the one above.
Obviously "God did it" is not an avenue of speculation simply because it
could never be verified even if everyone believed it to be the case. And
obviously you think there *is* a "naturalistic" explanation, because you
proposed this one:
>FTR, my own notion is that segmentation is the key to the Cambrian
>Explosion. The macroevolutionary mechanism forming the gross
>morphology of vertebrates and arthropods is simply siamese-twinning
>(aka parabiosis), a process through which trains of segments can be
>quickly generated, followed by mutations causing loss and distortion
>of segments--a process which was drastic at first but soon became
>more gradual and Darwinian.
I did a google search for "Cambrian Explosion Research" and came up with 7
pages of results. Below is a tiny sample. It's a thriving area of research.
Why do you keep saying it's not?
Susan
This is from a 1995 Time Magazine Article:
http://ucaswww.mcm.uc.edu/geology/huff/When_Life_Exploded.html
"What could possibly have powered such a radical advance? Was it something
in the organisms
themselves or the environment in which they lived? Today an unprecedented
effort to answer
these questions is under way. Geologists and geochemists are reconstructing
the Precambrian
planet, looking for changes in the atmosphere and ocean that might have put
evolution into sudden
overdrive. Developmental biologists are teasing apart the genetic toolbox
needed to assemble
animals as disparate as worms and flies, mice and fish. And paleontologists
are exploring deeper
reaches of the fossil record, searching for organisms that might have
primed the evolutionary
pump. "We're getting data," says Harvard University paleontologist Andrew
Knoll, "almost faster
than we can digest it." "
this is from Science Daily
http://www.sciencedaily.com/releases/1999/01/990121073459.htm
"Large Gene Study Questions Cambrian Explosion
The ancestors of major groups of animal species began
populating Earth
more than 600 million years earlier than indicated by
their fossil remains,
according to the largest study on the subject using gene
sequences, recently
completed at Penn State. The research suggests that
animals have been
evolving steadily into different species for at least
1200 million years,
which challenges a popular theory known as the Cambrian
Explosion that
proposes the sudden appearance of most major animal
groups, known as
phyla, 530 million years ago. A paper describing the
research will be
published in the January 22, 1999, issue of the
Proceedings of the Royal
Society of London (Series B) by Penn State Undergraduate
Student Daniel
Y.-C. Wang, Postdoctoral Fellow Sudhir Kumar, and
Associate Professor of
Biology S. Blair Hedges. "
here is a nice lecture on the Cambrian, which includes a list of the
current thinking on "why then":
http://www-geology.ucdavis.edu/~GEL107/Cambrian.html
"Even so, it is not clear that oxygen, or predation, or any other single
parameter can be identified as
the reason for the nature, the scale, and especially the timing of the
Cambrian explosion. One could
argue (and people have) that the world is full of complex creatures, so
complexity must have
evolved sometime. Whenever it evolved, it was bound to cause a visible
"burst" in the fossil
record, but perhaps there was no "trigger" for the Cambrian explosion we
see in the record. The
first large animals evolved at that time, and it is hardly surprising that
they spread rapidly and
diversified into many body plans, with different groups evolving hard parts
of different chemistry
and structure. After the dramatic events early in the Cambrian, the
increase in numbers and
diversity of fossils later in the period seems anticlimactic. Cambrian
fossil collections are not very
complex ecologically; they are dominated by trilobites, most of which lived
on the seafloor and
were deposit feeders. Filtering organisms are very much secondary, and
although there are large
carnivores, they are represented only by anomalocarids.
"The Cambrian explosion is spectacular, but it is not unique; in my view
the spectacular
diversification of the diapsid reptiles, especially the archosaurs, in the
Late Triassic is an analogous
case, as is the diversification of the mammals after the end of the
dinosaurs. Such radiations stand
out from "normal" evolutionary events just as "mass extinctions" stand out
from the rest. On a real
planet inhabited by real organisms, evolutionary rates are likely to vary
in time and space, and
evolutionary events are likely to vary in magnitude, duration, and
frequency. We should not
expect that ideal rules we might propose for an ideal planet would be
followed by the natural
world; instead, we have to find out from that natural world what the rules
actually were. "
----------
For if there is a sin against life, it consists not so much in despairing
of life as in hoping for another and in eluding the implacable grandeur of
this one.
--Albert Camus
http://www.telepath.com/susanb/
This archive was generated by hypermail 2b29 : Tue Feb 15 2000 - 13:02:07 EST