Stromatolites are the
oldest
known fossils, dating back more than 3 billion years. They are in the form
of colonial structures formed by photosynthesizing
cyannobacteria
and other microbes. Cyanobacteria are 
prokaryotes,
which are primitive organisms that are lacking in cellular nucleii.
Stromatolites thrived in warm aquatic environments and were one of the
major reef formers of the past. It is thought that Cyannobacteria
was one of the main factors responsible for the creation of earth's oxygen
atmosphere. They dominated as the primary lifeform on earth for over
2 billion years. Presently, stromatolites are rare and nearly extinct,
living a precarious existence in only a few localities worldwide.
above: cartoon of stromatolites (www.ggl.ulaval.ca)
