Glossary

A **fossil** is any preserved evidence of past life, including physical remains, impressions, traces, and life-history artifacts such as nests or coprolites. Fossils are found almost exclusively in sedimentary rocks and typically refer to evidence of organisms that existed at least 10,000 years ago—before the end of the last ice age. The oldest widely accepted fossils are stromatolites from the Pilbara region of Western Australia, dated to approximately 3.48 billion years ago, indicating that life arose very early in Earth's history. Fossils form through a suite of taphonomic processes that remove organic material from the zone of aerobic decomposition and replace or stabilize it with minerals. Rapid burial in sediment is generally essential, as it shields remains from scavengers and oxygen-driven decay. Once buried, groundwater carrying dissolved minerals can infiltrate pore spaces in bone, shell, or wood (permineralization), or entirely replace original biological material with minerals such as silica, calcite, or pyrite (replacement). Because fossilization demands specific and relatively rare conditions, only a tiny fraction of all organisms that have ever lived have entered the fossil record. Fossils constitute the primary direct evidence for reconstructing the history of life on Earth. They are the foundational data of paleontology, enabling scientists to identify extinct species, trace evolutionary lineages, infer past behaviors through trace fossils, reconstruct ancient ecosystems and climates, and calibrate the geologic time scale through biostratigraphy. Without fossils, knowledge of the approximately 3.5-billion-year saga of biological evolution would remain almost entirely inferential.