classifying life ("primitiveness")

[B. J. Tindall]

Just a few comments on what Ken Kinman wrote. Okay the definition of
primitive is not the way that I thought you might be using it and in
context that appears to be suitable. However, it would start splitting
hairs about what was the ancestor of what. If we take eukaryotic organelles
as an example (mitochondria, chloroplasts, etc.) then I would say that
ancestor of modern day prokaryotes are the ancestors of these organelles.

In quoting the time scale for the evolution of bacteria "Metabacteria" I
would assume that the "early" date is based on "cyanobacterial" fossils. I
would just like to raise the point that these fossils are considered to be
cyanobacteria based on morphology. There would appear to be no evidence
that they contained photosynthetically active pigments. Morphologically it
would not be easy to distinguish some of the filamentous gliding
cyanobacteria (e.g. Oscillatoria) and the large celled, filamentous gliding
bacteria, such as members of the genus Desulfonema (which are anaerobic
sulphate reducers and are not oxygenic phototrophs). Here is see a danger
of extrapolating from present day organisms back to ancestors, without
knowing much about the ancestors. In the case of mitochondia and
chloroplasts we do not know whether they became associated with eukaryotes
via a single, or multiple events. As for what came first, ancestoral
"bacteria" or ancestoral "archaebacteria" I do not know because I was not
there (c.f. D.Colless, "The phylogenetic fallacy"). Could it be that these
ancient fossils represent lineages which did not develop further - the
Burgess Oil Shale seems to provide evidence of such effects in eukaryotes,
so why should one not also have similar effects in prokaryotes?

I also think there is a big difference between the ancestoral role of
certain prokaryotes and the present day state of affairs. My impression is
that Woese has tried to emphasise the current diversity which is why he
wants more higher taxa within the prokaryotes. This is a consequence of
diversification over geological time, the longer one is around, the more
diversification, the more higher taxa (anyway that is what I interpret from
his work). This reminds me of a statement from Simpson which, while
acknowledging that Homo sapiens may have an amoeba as an ancestor, he
strictly rejected linking Homo sapiens with the amoeba. While prokaryotes
may have been around first I do not see that it is a logical consequence of
their early presence on Earth to restrict them to one kingdom - but I am
open for appropriate arguments.

There is also a comment about secondary simplification of organisms - this
is certainly a point worth stressing, since one often gets the impression
that evolution always leads to more complex forms. I agree with that point.

If the origin of prokaryotes is so important in restricting them to a
single kingdom, then the chimera origin of eukaryotes would suggest that
they also have a (multiple) prokaryotic origin, but they are not included
in the prokaryotes? This is a question, not a statement.

Are all crown taxa equally old? Good question. Probably not, but I am not
sure that this problem has been properly addressed. If we assume the
evolutionary sequence:
fish->amphibians->reptiles->mammals
then it is true that fishes can be found in the geological record over a
longer period of time than mammals. But does this mean that a modern fish
species is inherently older than a mammal species? Please correct me if I
am wrong, but is it not possible that a modern fish species arose after a
modern mammal species on a geological time scale? Thus, the age of modern
day taxa are not automatically coupled to the length time over which their
(first) ancestors have appeared in the older geological record. We have
examples of ancient or fast evolving bacterial taxa (e.g. genera) which
have simply been fragmented into several genera.
More food for thought.
Brian





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