Definition

Fossils are the preserved remains or traces of living things from long ago, such as dinosaur bones, a fern pressed into stone, or footprints frozen in old mud, and fossil evidence is what they reveal about which organisms lived in the past and how life has changed; because sediment layers build up over time, deeper layers are generally older, so digging down is like reading backward through time. More specifically, fossil evidence consists of preserved remains, impressions, or traces found in sedimentary rock, following the law of superposition, and it shows a sequence of life forms changing through time, with transitional fossils carrying features of both older and younger groups and radiometric dating supplying absolute ages, though the record stays incomplete because fossilization requires rare conditions. At a technical level, fossil evidence comprises body fossils, trace fossils such as tracks and burrows, and molecular fossils called biomarkers, interpreted through biostratigraphy, radiometric dating, and phylogenetic analysis to document major transitions such as the origin of eukaryotes, the Cambrian radiation, the colonization of land, and the rise of tetrapods, birds, and mammals, and its well-understood bias toward hard-bodied, abundant, marine organisms in depositional environments can be corrected statistically.