Biomarkers are specific kinds of traces left by living organisms; they are molecules that were formed through biological processes. For example, okenane functions as a biomarker because it is produced only by purple sulfur bacteria. Scientists don’t know of any nonbiological source of okenane. As a result, the presence of okenane in rocks means that purple sulfur bacteria must have been around producing it before those rocks formed. Radiometric dating can indicate the age of the rocks, and scientists can gain insight into the history of life.