There have been older fossils found [1], but the interesting thing about this discovery is that it is they are the oldest fossils of bacteria that use a particular biochemical pathway - Sulfur oxidation.

I should add that the sulfur oxidiser bacteria are gammaprotobacteria - from this recent tree-of-life you can see they are just one branch of what is an amazing amount of diversity [2]. This just shows how long life as been around.

1. http://phys.org/news/2013-01-earliest-evidence-life-billion-...

2. http://www.nature.com/articles/nmicrobiol201648/figures/1

It seems difficult enough already to find those fossile bacteria, but how do people know that these bacteria were oxidizing sulfur?

From the shift in Sulfur isotopes [1].

1. http://phys.org/news/2014-11-rare-billion-year-old-reveal-ho...