Which bacterium is the ancestor of the animal mitochondrial genome?

We present considerable data supporting the hypothesis that a Sulfolobus- or Mycoplasma-like endosymbiont, rather than an alpha-proteobacterium, is the ancestor of animal mitochondrial genomes. This hypothesis is based on pronounced similarities in oligonucleotide relative abundance extremes common to animal mtDNA, Sulfolobus, and Mycoplasma capricolum and pronounced discrepancies of these relative abundance values with respect to alpha-proteobacteria. In addition, genomic dinucleotide relative abundance measures place Sulfolobus and M. capricolum among the closest to animal mitochondrial genomes, whereas the classical eubacteria, especially the alpha-proteobacteria, are at excessive distances. There are also considerable molecular and cellular phenotypic analogies among mtDNA, Sulfolobus, and M. capricolum.