It has been pointed out to me by somebody here at the museum that the
date for the mitochondrial endosymbiosis (or whatever you wish to call
it) has continuously been pushed further down the eukaryotic tree (by
unearthing evidence for secondary mitochondrial loss).
Now, there is general agreement that the mitochondrion is monophyletic
(descendent from the ancestors of Alpha proteobacteria). Alpha
proteobacteria are _relatively_ recent bacteria (as judged by molceular
trees, and assuming a reasonable molecular clock), so mitochondrial
eukaryotes can only have evolved _relatively_ recently. Therefore, a
HUGE length of time elapsed between the splitting of 'Eukarya' and
Bacteria. Sooo, in that time there must have been some eukaryotes that
My points are:
1. Were the eukaryotes that survived for > 1,000,000,000 years,
completely supplanted when the mitochondrion evolved? maybe it is a
problem with our sampling of microeukaryote diversity (quite likely
actually), maybe they WERE supplanted, maybe they didn't exist at all.
2. From the energy point of view (hope my ignorance of bio. doesn't
embarrass me here), if these primitively amitochondrial eukaryotes had
good metabolic systems (they lived on their own for maybe a billion yrs,
didn't they?), then these systems should surely be seen at least in some
phylogenetic groups (one would think) today.
I guess the problem is that less than 100 genes are sequenced in
Giardia, less than 30 in Trichomonas and not many genes from the others.
Not many people are PCRing primitive eukaryotic 18S rRNA sequences from
natural samples (not to the same extent that they are doing it for
prokaryotes) and until then....
Dr. James O. McInerney Ph.D. Phone/Voicemail: +44 171 938 9247
Senior Scientific Officer, email:j.mcinerney at nhm.ac.uk
The Natural History Museum,
London SW7 5BD