The Oldest Evidence of Life

Scientists from Australia and the UK have made a stunning claim - that they have discovered a fossil of microbial life in 3.4 billion year-old rocks. Claims like this have been made before for even older life, but if correct, this result would provide the most concrete microfossil evidence for life in this age range. This would provide a complement to other evidences of life, including stromatolites and isotopic evidence, which is less direct than microfossils.

Sulfur isotopes show evidence for life going back to nearly 3.5 billion years ago. Sulfur isotopic data from Warrawonna in Australia show a strong signature of isotope fractionation. This means that they detected a characteristic signature of life from comparing the amounts of different sulfur isotopes. However, finding evidence of microfossils (fossils of microbial life) has proved to be more elusive. Microfossil evidence is wrought with complications. Essentially with microfossils you have to be able to rule out any possibility except biology. And even then, there's always the possibility that the signal is contamination; that it is a fossil of more recent life that contaminated the old rock.

I'm not a true expert in this, only an academic tourist, but the idea that life existed 3.4 billion years ago is consistent with other data, including isotopic data and stromatolites.