a large proportion of the prokaryote-specific mito-
chondrial proteins (about half according to Karlberg et al.
[34]) have counterparts in eukaryotes as well as in bacteria
and archaea; some or even many of these could well have
thus been present in the universal common ancestor of all life
forms and, therefore, were conceivably already present in
whatever organism contributed the nuclear genome at the
time of the mitochondrial endosymbiosis