We provide data and analysis to support the hypothesis that the ancestor of 
animal mitochondria (Mt) and many primitive amitochondrial (a-Mt) eukaryotes was 
a fusion microbe composed of a Clostridium-like eubacterium and a 
Sulfolobus-like archaebacterium. The analysis is based on several observations: 
(i) The genome signatures (dinucleotide relative abundance values) of 
Clostridium and Sulfolobus are compatible (sufficiently similar) and each has 
significantly more similarity in genome signatures with animal Mt sequences than 
do all other available prokaryotes. That stable fusions may require 
compatibility in genome signatures is suggested by the compatibility of plasmids 
and hosts. (ii) The expanded energy metabolism of the fusion organism was 
strongly selective for cementing such a fusion. (iii) The molecular apparatus of 
endospore formation in Clostridium serves as raw material for the development of 
the nucleus and cytoplasm of the eukaryotic cell.