The following outline is provided as an overview of and topical guide to life forms:
A life form (also spelled life-form or lifeform) is an entity that is living, such as plants (flora), animals (fauna), and fungi (funga). It is estimated that more than 99% of all species that ever existed on Earth, amounting to over five billion species, are extinct.
An extraterrestrial or alien is a lifeform that did not originate on Earth. (The word extraterrestrial means 'outside Earth'.) Extraterrestrials are a common theme in modern science-fiction, and also appeared in much earlier works such as the second-century parody True History by Lucian of Samosata.
A trace fossil, also called an ichnofossil (/ˈɪknoʊˌfɒsɪl/; from Ancient Greek ἴχνος (íkhnos) 'trace, track'), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or by mineralization. The study of such trace fossils is ichnology - the work of ichnologists.
Trace fossils may consist of physical impressions made on or in the substrate by an organism. For example, burrows, borings (bioerosion), urolites (erosion caused by evacuation of liquid wastes), footprints, feeding marks, and root cavities may all be trace fossils.