Re: Evolution appears fulfilled

David Campbell (bivalve@mailserv0.isis.unc.edu)
Tue, 15 Dec 1998 16:34:39 -0400

> I offer some quotations from an article in Time magazine a while
>back(When life exploded, TIME, 12/5/95
>http://ucaswww.mcm.uc.edu/geology/huff/When_Life_Exploded.html) which
>generally uncover the underbelly of the problem for evolutionary origin of
>life: 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")

However, by now it seems definite that the 10 million year figure is too
small. The exact time of origin of the Ediacaran fauna is not pinned down,
but it adds at least about 25 million years of aminal fossils into the
Precambrian. I think the article did claim that all phyla except Bryozoa
occur then, but this is wrong. All phyla with good hard parts, except
Bryozoa, and many soft-bodied phyla are known from the Cambrian, but other
soft-bodied phyla are not known until later, or even until the Recent. It
should possibly be noted that some people want to claim that phyla
originate throughout the Phanerozoic based on a redefinition of the concept
of phyla (as crown groups).

>To realize just how significant these quotations are recognize that
>Caenorhabditis, whose entire genome was just completed, has 40% of its
>genes represented in humans. This figure is a minimum figure for the
>complexity of the common ancestor of nematodes and humans, an ancestor,
>that up until very recently was characterized as having almost no features,
>and is, of course only hypothetical, and has no fossil representation.

However, we do not know what these genes did in the common ancestor.
Sorting out parallel evolution from ancestral features is extremely
difficult.

>We
>can no longer labor under the illusion that these posited ancestral forms
>are just simply not preserved. We are talking deep in the Precambrian, not
>about a few million years that we are playing with in China and in the
>Ediacaran fauna. There are no deep [Pre]Cambrian metazoan fossils. Yet this is
>when all major groups of animals would have had to originate in the
>naturalistic scenario.

Very rapid evolution is another naturalistic scenario that is possible.
However, the current rate of discovery of new kinds of Precambrian fossils
is very high, and I do not think we can rule out new discoveries as people
take another look at older Precambrian deposits. We still do not really
know what to expect a proto-metazoan to look like, how big it should be,
etc.

Also in this context, it should be noted that most people who try to do
molecular clock estimates of the time of metazoan origins do not seem to
understand the fossil record or statistics.

David C.