ABSTRACT

The Serial Endosymbiotic Theory (SET) explains the origin of nucleated organisms by a merging of archaebacterial and eubacterial cells in anaerobic symbiosis, historically followed by acquisition of mitochondria or plastids. The paradigmatic change former evolutionary theories is that the driving force behind evolution is not ramification but merging.

Lynn Margulis proposes that Eukaryotic cells, are the result of merging several different ancestor genomes. The important factor is the sequence of merging in symbiogenesis, i.e. the serial evolution. The first merger involved (1) thermoplasmic archaebacteria with motile spirochaeta-like eubacteria that (2) were able to swim, to the nucleocytoplasm. These forms were still anaerobic. This was followed by a merging with (3) aerobic organisms. This enabled them to survive the increasing oxygen concentrations. The final step was the merging with (4) photosynthetic bacteria. With this approach, the SET contradicts traditional theories of evolution, all of which firmly held that the direction lay in ramification and not in merging.