I noticed a slight confusion between the origins of the eukaryotes
versus the origins of the plastids and mitochondria. As Terry
Gray mentions, these are not the same things. I belive the current
thinking is that eukaryotes arose prior to the endosymbiosis of
the bacteria which turned into these particular organelles. There
are examples of eukaryotes without mitochondria or chloroplasts.
While some of these examples probably lost these organelles
over time, it is possible that at least of few of the protists
in the deeper branches of the evolutionary tree never had them.
Were the ideas that the chloroplasts and mitochondria came from
(eu)bacteria "driven by similarities"? Certainly that is a large
component; homology being a major support of evolution. It does
help address why these organelles look so much unlike eukaryotes
and so much like the bacteria -- And not just like bacteria in
_general_ but actually like certain identified subgroups of bacteria.
However, I do not think it is quite a "just so" story, unsupported
by contemporary observation of "intermediates". While we cannot
set Professor Peabody's Way-back Machine for 2 bya and observe exactly
how it happened for the chloroplasts or mitochondria, we can see
cases where endosymbiotic interactions appear at various levels.
There are numerous examples of bacteria-eukaryote interactions
that run the gamut from occassional parasitism to mutual endosymbiosis.
As it turns out, some of the bacteria capable of living inside other
cells belong to the alpha-"purple" subgroup of gram-negative bacteria
(this group includes _Agrobacterium_, _Rhizobium_ and the rickettsias).
The mitochondria fall into this particular group. Does this make
the bacterial origins of mitochondria and chloroplasts a just
so story? Perhaps, but certainly not an unusual or even unlikely
story -- and one backed with corroborating evidence.
In a different letter:
Eddie Olmstead proposed some possible physical reasons why these
organelles might happen to look like bacteria. Superficially,
some of his examples might be good reasons, but they do not
explain the grittier details or the particular homologies with
bacteria (or particular groups of bacteria - Photosynthesis is a
trait spread across many bacterial groups, as are the oxidative
reactions performed by the mitochondria; but these only appear as
one "group" each for all the eukaryotes that harbor either of
these organelles). I don't think that size or reaction sensitivities
are strong arguments as well. There are bacteria that dwarf
eukaryotes in size yet do not seem to need quite the sort of
compartmentalization found with eukaryotes (eg. _Epulopiscium
fishelsoni_, a bacterium the size of a hyphen). As for reactions
requiring special structures that only a bacteria-like organelles
can provide, note that there are complex cellular organelles that
do not require auxillary genetic elements (such as the mitochondrial
genome) for formation. Whereas eukaryotes can lose these structures
at one stage and regenerate them later, once the mitochondria or
chloroplasts are gone, they are lost for good. Thus I don't think
that the similarities between bacteria and the mitochrondria/
chloroplasts can be justified as a "forced move" in design.
Are these problems for creation theories? Some creationists have
admitted that this presents a problem for explanatory theories of
design. FWIW, neither Philip Johnson nor Walter ReMine really
address these questions in their books (_Darwin on Trial_ &
_The Biotic Message_, respectively), except perhaps to dismiss
homologies in general.
Interesting aside:
1) There is pretty strong evidence that some eukaryotes inherited
chloroplasts from _other eukaryotes_! This idea is that the
transfer came as a result of eukaryote-eukaryote symbioses with
one photosynthetic partner. In these cases, algae (cryptomonads)
were found that had an extra pair of membranes with a small
nucleus-like organelle (called a nucleomorph) found in the space
between the inner and outer plastid membrane pairs. The nucleomorph
sequences place its origin among other eukaryotes (red algae?)
while the plastid sequences fit with the rest of the chloroplasts
and cyanobacteria.
2) A discussion about this topic is currently active in the moderated
newsgroup: sci.bio.evolution
Regards, Tim Ikeda (timi@mendel.berkeley.edu)