Mitochondria originated by permanent enslavement of purple non-sulphur bacteria. 
These endosymbionts became organelles through the origin of complex 
protein-import machinery and insertion into their inner membranes of protein 
carriers for extracting energy for the host. A chicken-and-egg problem exists: 
selective advantages for evolving import machinery were absent until inner 
membrane carriers were present, but this very machinery is now required for 
carrier insertion. I argue here that this problem was probably circumvented by 
conversion of the symbiont protein-export machinery into protein-import 
machinery, in three phases. I suggest that the first carrier entered the 
periplasmic space via pre-existing beta-barrel proteins in the bacterial outer 
membrane that later became Tom40, and inserted into the inner membrane probably 
helped by a pre-existing inner membrane protein, thereby immediately providing 
the protoeukaryote host with photosynthesate. This would have created a powerful 
selective advantage for evolving more efficient carrier import by inserting 
Tom70 receptors. Massive gene transfer to the nucleus inevitably occurred by 
mutation pressure. Finally, pressure from harmful, non-selected gene transfer to 
the nucleus probably caused evolution of the presequence mechanism, and 
photosynthesis was lost.