The phrase “traces of life” has been at the center of a recent flurry of activity in the search for new evidence of the earliest life on Earth and extraterrestrial life on Mars and other planetary bodies. Our purpose is to review the different types of biogenic traces and to delve into the means of distinguishing them and their significance. Broadly defined, ichnology is the study of organism behavior and its products—trace fossils. A trace fossil is the product of an organism interacting with a substrate in an environment that generates a three-dimensional physical structure. Behavior includes dwelling, feeding, locomotion, resting, grazing, cultivating, nest construction and maintenance, escaping, hiding, or any combination of these to form multifunctional structures. Objects that are not trace fossils include molds, casts, and impressions of body fossils, eggs and egg shells, internally precipitated structures, and organic residues and biomarkers. The organisms involved include all trace-making representatives of the Bacteria, Archaea, and Eucarya. A substrate includes grains (mineral and rock fragments, and glass shard surfaces); sediments (soupgrounds, softgrounds, firmgrounds, and hardgrounds); rocks (sedimentary, igneous, and metamorphic; volcanoclastic); and organisms (living and postmortem endoskeletal, exoskeletal, and xylic substrates). The environment is any set of distinct physical, chemical, and biological conditions. A three-dimensional physical structure includes such features as tracks, trails, burrows, nests, rooting patterns, biolaminates, borings, and etched surfaces that can range in size from nanometers to kilometers in scale. In this descriptive definition, microendolithic borings are trace fossils produced by microorganisms during active metabolism. Stromatolites are macroscopic trace fossils composed of microscopic structures constructed by microorganisms.

Such evidence of once living things as organic molecules, RNA, DNA, and elemental isotopic fractionation even in trace amounts due to photosynthesis, respiration, and chemosynthesis are chemical fingerprints or biomarkers, not trace fossils. Reversed weathering sequences and biogenic graded bedding may also be included.