Most likely, we have all run into the term "biomarker," no? So, what is a biomarker? In 1998, the NIH Biomarkers Definitions Working Group defined a biomarker as "a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention." If this sounds broad, it's because it is. And vague. What is a "characteristic," for goodness' sake?
Let's look at an example, the porphyrins in petroleum. A porphyrin is a large ring molecule that resembles chlorophyll and consists of 4 pyrroles (which are small rings made from four carbons and one nitrogen). What are porphyrins doing in petroleum and how did they get there? In the 1930s, Alfred Treibs proposed that they were derived from the chlorophyll of plants, ergo indicating that plants were present at their time of formation.
In a recent paper, Suo et al. say: "We suggest porphyrin derivatives as an ideal target (for astrobiological research), because these chromophores are global in distribution and found in virtually all living organisms on Earth, including microorganisms that may approximate the early evolution of life on Earth."
As you can see, it may pay to brush up on your biochemistry.