>Question no 3:
>I've heard "common ancestor" written in the plural, conceding the
>of more than one. If there could be 2, or 5, or 10 common ancestors, why
>100--or 1000? Where would biology set the limit? Does one common ancestor
>insist that life arose only once--and everything descended from that one
>unique event? What if life is discovered to be common in the universe?
>Similar DNA apparently results in similar morphology, but is that
>proof of common descent?
I'm not sure how best to answer this one, I'll let someone else
give it a shot. :) >>
***** I'll give it a brief shot. Despite the creationist propaganda
available on the web (e.g., David Plaisted's treatment of it, and innumerable
'private' antievolution home pages - a simple Yahoo search on evolution will
demonstrate this) it is not merely genetic 'similarity' that is the issue -
though of course that too can be informative. It is the pattern of
synapomorphic (shared derived) mutations that is indicative of descent.
These patterns are statistically significant indicators of a hierarchical
structure in any given dataset (of homologous DNA sequences) - for numerous
treatments of the statistical analyses/methodologies see just about anything
by Huelsenbeck, Hillis, Felsenstein, or Swofford. Creationists often trot
out those 'improbability statistics' claiming that such-and-such an equation
PROVES evolution couldn't have happened (of course, these equations are
almost always based on improbable assumptions and deal with abiogenesis, not
evolution, but the two are forever joined in anti-evolution orthodoxy). I
have asked this question on numerous discussion boards - what are the chances
that any 2 species with genomes in the 6 billion base range could share - by
chance - a mutation at a specific locus? 3? 4? How about 10? My
admittedly crude analysis of the chances that only 2 would share such a
mutation - modelled on statistical equations used by creationists, btw -
produced something in the range of 1 in 10^67. Now take into account that
there are hundreds - thousands - of instances of shared mutation in related
species. Even if my orginal estimate is off by several orders of magnitude,
it is clear that as more species are taken into consideration, and more and
more loci are brought into play, that it is statistically 'impossible' for
the observed structure of shared mutations could arise by chance. And before
the 'Oh, well, if they look alike you would expect similarities' argument is
trotted out, the bulk of the information I am referring to is in non-coding
DNA regions, which would not have no bearing on morphology or physiology.
The answers I typically get are: 1) no answer 2) with design, all things are
possible 3) God put them there for some reason. None of which actually
address the point.
So, to sum up, it is hardly just 'similarity' in the genomes of organisms
that indicate descent.