Cyanobacteria as examples of the danger of back contamination.
We had a discussion on this board of the likelihood of microbes that underwent an independent evolution on Mars would be dangerous to Earth life.
Chris Chyba examines the case of toxic cyanobacteria that are dangerous to widely separated life though not undergoing related evolutionary development in a report to be given at the October DPS meeting:
DPS 34th Meeting, October 2002
[3.09] Planetary Protection: Two Relevant Terrestrial Examples
C. Chyba (SETI Institute and Stanford University)
http://www.aas.org/publications/baas/v34n3/dps2002/206.htm
Bob Clark