Now that's OLD!
The oldest (3.8 billion or possibly 4.2 billion!) microbe fossils reported in the March 2, 2017 issue of "Nature", were found in ancient hydrothermal vents in Quebec. The fossils contained micrometer-sized tubes similar to modern day filamentous microorganisms.
Source: Dodd, M.S. et. al. Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature 543, 60–64.