Radhey Gupta wrote:
> The question may be asked as to why should the chimeric
> model be preferred over direct descendance of eukaryotic cells
> from archaebacteria? Well, there are several reasons for doing
>> (a) It is the simplest and most parsimonius model to explain all
> of the genes/proteins phylogenies.
> (b) It readily explains why certain characteristics of
> eukaryotic cells...
> (c) It provides a plausible explanation for the origin of the
> eukaryotic cell nucleus and endomembrane systems...
These seem reasonable.
> (d) It provides a plausible explanation for the enormous
> structural difference between the eukaryotic and prokaryotic
> cell types, and the absence of any species that are
> intermediates in this transition. These observations cannot
> be readily explained by simple evolutionary mechanisms.
Whoop! Whoop! Bogon alert!
In the first place, you *do* realize that this argument
is straight out of "scientific creationism" ... right?
In the second place, are you even sure that this is true?
There's a lot of quite diverse protozoa out there and, while
I am no expert on them, I dimly recall that some of them are
quite rudimentary eukaryotes indeed (e.g., lack mitochondria,
or have simplified mitosis). Might not they be considered
instances of, if not "missing links," at least mildly intermediate
steps towards prokaryote-ness?
> (e) The inferred time of divergence of eukaryotic species from
> archaebacteria and the eubacteria (about 2 By ago) based on
> genetic distances between different proteins sequences
> (Doolittle et al , Science, 271, 470-477, 1996) and the
> fossil records, can also be satisfactorily explained by the
> chimeric model.
As I recall, Doolittle's chronology is still a little
controversial. Or, maybe, more than a little controversial.
Cf., Schopf's commentary on microfossils versus Doolittle, quoted
in the _Science_ issue where Doolittle et al. was published...
> the basic division of living organisms into two primary
> groups -
> the Prokaryotes (ancestral) and the Eukaryotes (derived) is
> strongly supported by all available molecular and other data.
This strikes me as a bit of overstatement. A chimeric
origin of Eucarya may well be the real solution to the current
confusion in cellular phylogeny. However, the evidence so far
from genes involved with informational transfer within the cell --
rRNA, DNA polymerases, RNA polymerases -- seems to strongly
support monophyly of Archaea and Eucarya. Classing all
Prokaryota as monophyletic doesn't really address that
convincingly, at least not to this observer and not yet.