title: Cell symbiosis [correction of symbioisis] theory:  status and implications for the fossil record.
abstract: Recent geological treatises have presented three alternative models of the origins of eukaryotes as if they merited equal treatment.  However, modern biological techniques, especially nucleic acid and protein sequencing, have clearly established the validity of the symbiotic theory of the origin of eukaryotic organelles.  The serial endosymbiotic theory in its most extreme form states that three classes of eukaryotic cell organelles (mitochondria, plastids and undulipodia) originated as free-living bacteria (aerobic respirers, phototrophic bacteria and spirochetes respectively) in association with hosts that become the nucleocytoplasm (Thermoplasma-like archaebacterial hosts).  Molecular biological information, primarily derived from ribosomal RNA nucleotide sequencing studies leads to the conclusion that the symbiotic origin theory for both mitochondria and plastids has been proven.  The probability of an ancestral archaebacterial-Thermoplasma-like host for the nucleocytoplasm has been rendered more likely by discoveries by Dennis Searcy and his colleagues and Carl Woese and his colleagues.  The most equivocal postulate of the symbiotic theory, the origin of undulipodia (cilia and other organelles of motility that develop from kinetosomes is under investigation now.  The status of these postulates, as well as their implications for the fossil record, is briefly summarized here.
authors.affiliation: Department of Biology, Boston University, MA 02215, USA.
authors.forename: L
authors.initials: L
authors.lastname: Margulis
authors.forename: J F
authors.initials: JF
authors.lastname: Stolz
mesh_terms: D001419:Bacteria / Q000145:classification / Q000235:genetics; D018407:Bacterial Physiological Phenomena; D005075:Biological Evolution; D002923:Cilia / Q000502:physiology; D005057:Eukaryotic Cells / Q000502:physiology*; D005580:Fossils; D016678:Genome; D008928:Mitochondria / Q000502:physiology; D009685:Nuclear Envelope / Q000502:physiology; D015388:Organelles / Q000502:physiology*; D018087:Plastids / Q000502:physiology; D012270:Ribosomes / Q000502:physiology; D013559:Symbiosis