http://www.spaceref.com/news/viewpr.html?pid=16910"Microbes cluster in communities called microbial mats. These are cities of
cells, living in
symbiosis. Microbial mats can be found anywhere. They inhabit dry places, salty
places, hot
places and cold places. They can be found in the cracks and puddles
uninhabitable by
multicellular organisms. They are familiar to many of us as the film on our
teeth. Their history
traces back at least 3.4 billion years - more than 10 million times the amount
of time humans
have existed and 3/4 of the time since the Earth's formation."