This audiobook is narrated by a digital voice.

Biosignatures are the chemical, physical, or biological indicators that suggest the presence of past or present life. These markers are crucial in the field of astrobiology, serving as the main evidence scientists seek when exploring environments beyond Earth. Whether through the detection of atmospheric gases, fossilized microstructures, or complex organic molecules, biosignatures provide potential proof that life may exist—or once existed—elsewhere in the universe.

To begin understanding biosignatures, it is essential to grasp their diversity and the ways they can be classified. Biosignatures are generally divided into several categories: chemical, isotopic, morphological, and contextual. Chemical biosignatures include compounds such as methane or oxygen that, in certain concentrations and contexts, may suggest biological activity. Isotopic biosignatures arise from processes that fractionate isotopes in ways specific to living organisms. Morphological biosignatures refer to microfossils or structures resembling biological shapes. Contextual biosignatures involve environmental settings—such as sediment layers or mineral patterns—that support biological interpretation.

The significance of biosignatures lies not only in their detection but also in their reliability. Distinguishing genuine biosignatures from abiotic (non-living) processes is a major challenge. For example, methane can be produced both biologically and geologically. Similarly, certain mineral patterns once thought to indicate ancient life can form through entirely physical means. Thus, the interpretation of biosignatures requires a holistic approach, combining geochemistry, planetary science, and biology. Multiple lines of evidence are usually needed before making a credible case for life.