Any remains, traces, or imprints of any plants or animals
that lived on the Earth. These remains of past life
include body fossils, the preserved record of hard or soft
body parts, and trace fossils, which record traces of biological
activity such as footprints, tracks, and burrows. The oldest
body fossils known are 3.4-billion-year-old remnants of
early bacteria, whereas chemical traces of life may extend
back to 3.8 billion years.
The conditions that lead to fossilization are so rare that
it is estimated that only 10 percent of all species that have
ever existed are preserved in the fossil record. The record of
life and evolution is therefore very incomplete. In order to be
preserved, life-forms become mineralized after they die, with
organic tissues typically being replaced by calcite, quartz, or
other minerals during burial and diagenesis. Fossils are relatively
common in shallow marine carbonate rocks where
organisms that produced calcium carbonate shells are preserved
in a carbonate matrix.
See also EVOLUTION; PALEONTOLOGY.
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