How old is life on Earth?

• The Earth is about 4.6 Ga old, but no crustal rocks has survived from that time. The oldest rocks are no older than 4.0 Ga. But zircon crystals dated to 4.4 billion years BP are found in rocks, are considered evidence for Earth's crust having had formed and being in contact with water already 4.4 billion years BP (here and here)
  
  

• For about a decade the oldest microfossils were considered to be about 3.5 Ga old (see here). The fossils (as interpreted by Bill Schopf) look like "modern" Cyanobacteria. Compare the time to to molecular trees of life: Is this a problem? However, the evidence for these fossils was questioned.
  
  
• 3.2Ga old filamentous fossils, probably of thermophilic chemotrophic prokaryotes (Rasmussen, 2000)
  
  
• 1.8Ga old fossils from Gunflint formation: iron-loving bacteria and cyanobacteria

• Oldest geological evidence for life - 3.8 Ga ago - is based on ¹³C discrimination (carbon derived from living systems often have lower delta ¹³C values than inorganic carbonates) [here]. The rocks are from Akilia island off the coast of Greenland, and severely altered by metamorphism. However, recently the evidence for that was reassessed.
• 3.7 billion year of rocks from the Isua formation in Greenland contain structures described as stromatholites. here in NYT, report in Nature
  
• The amount of carbon in the Issua formation (and its discrimination against agains 13C) is interpreted by Minik Rosing to indicate a highly productive biosphere. (See Minik's presentation at the 2011 ISSOL meeting. Especially slides 6 ff are interesting - the handwritten slides were written with the sedimentary rocks - containing lots of graphite.)
  
  
• 2.7Ga old: probable biomarkers of cyanobacteria and of eukaryotes (Roger Summons, Roger Buick and Jochen Brocks)

See Olga's Timeline of the Universe here