Re: [asa] Is time deep enough

From: David Campbell <>
Date: Fri Jun 01 2007 - 11:30:26 EDT

> Is 300,000 really enough generations to make the changes from our common
> precursor with chimps and us? Even considering that our dna is 98
> percent the same.

Obviously there are both ethical and practical problems with trying to
see how quickly one can mutate humans into something significantly

We do have a decent fossil record that shows a gradual trend from more
ape-like to more human-like forms over the time interval. Also bear
in mind that the real error bar on molecular dates is usually huge;
often there's a sample size of 1 (or 0) at some point and thus no
statistical validity at all.

Another factor is that genetic changes are not linear in their impact.
 Many genes (s.l., including regions that affect other regions of DNA
but do not produce a protein or major RNA product) play multiple roles
or affect multiple other genes. Others do one specialized thing and
have little or no effect when altered.

> Same question applies to:
> The Cambrian explosion is the geologically sudden appearance in the
> fossil record of the ancestors of familiar animals, estimated to take
> around 10 million years from 530 to 520 million years ago.
> </quote>

Actual start of the Cambrian is about 544 million. There are good
fossils of some of the more primitive animal groups (sponges,
cnidarians) and possible ancestral forms of more advanced animal
groups in the Precambrian. The Cambrian has a burst in hard parts,
which improves the fossil record, as well as changes in fossilization
patterns that make it easier to identify soft organisms on the
occasions when they are preserved. All but one of the known animal
phyla with hard parts, plus several but not all soft-bodied kinds, are
known from the Cambrian. (The largest burst in appearance of
soft-bodied phyla is in the Recent). There are also some intermediate
forms, transitional between phyla, and forms for which we don't know
the phylum. Thus, we don't have totally sudden and abrupt appearance
of everything. Even when we can assign things in the Cambrian to a
phylum, often they look relatively generic and primitive, as would be
expected for early ancestors that recently evolved some distinguishing

Significant changes can be observed within historically documented
times. There are the drastic changes in dog (and pigeon and guinea
pig and so forth) features brought about by selective breeding.
There's the hawthorn fly that is evolving into an apple fly since the
introduction of apples to North America. The latest Nature had an
article about lab strains of yeast that now show significant barriers
to interbreeding.

Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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Received on Fri Jun 1 11:30:51 2007

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