Oze's team carried out serpentine-production laboratory tests, a situation where no living organisms were present. They measured the resulting hydrogen:methane ratio. They then measured the hydrogen:methane ratio produced by hydrothermal vents in nature where living organisms were present.
On Earth, when microbes metabolize minerals for energy, they produce methane as a byproduct. Therefore, by measuring the ratio of hydrogen:methane where living organisms are known to be present and comparing those results with ratios measured in the lab (where no life was present), the fingerprint of microbial metabolism can be detected. Where there's life, there should be an overabundance of methane compared with hydrogen (creating a low hydrogen:methane ratio).
"From these experiments the hydrogen to methane ratio over time divided hydrothermal systems that did not involve living matter from those in which biota was present. This really gave us an ‘aha' moment where we realized this method could be used to look at life on other planets," said Oze in a press release.