Stromatolites are layered sedimentary rock-like structures that formed in shallow water, they're formed through the build up of bacteria and algae.  The bacteria in stromatolites started the process that boosted oxygen in the atmosphere and increased it from less than one per cent to more than twenty per cent.  The word stromatolite comes from the Greek word for "layered rock". 

Stromatolites which are the oldest fossil record of life on Earth were common along shorelines around the world about 3.5 billion years ago.  They were the earliest proof that life existed on Earth.  The oldest stromatolites can be found in the Pilbara region of Western Australia and are believed to be approximately 3.7 billion years old.