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Fossils are preserved signs of life, usually dug up from rocks (hence the name from the Latin fossilis). Sometimes fossils are actual remains of life, such as bones. Other times fossils are stones and minerals that have replaced the remains in such a way that the minerals hold the shape of the original remains. Often, fossils are a mix of original remains and mineralized remains. There are also fossils that are made as impressions, molds, or casts in rock. Tracks, for example, can become fossils. The oldest fossils are microfossils (fossils of cyanobacteria) from about 3,456 million years ago (3.456 billion years ago).

André Brack, in Origin of Life within the Encyclopedia of Life Sciences (2002) states that there is organic carbon in the greenstone belts of Isua in Greenland, and since those sediments are about 3.85 billion years old, we can guess that life was around at least by then. Analysis of the DNA of bacteria also suggests the Last Universal Common Ancestor may have lived more than four billion years ago.

Links about fossils:
  1. Some information about the very earliest fossils provided by Westphalia Wilhelms University in Münster. Most of the links from this site are dead, but some still work.
  2. A fine article by Peter N. Spotts in the Christian Science Monitor about the earliest fossils from 3.4 billion years ago.
  3. One of the many fine websites devoted to fossils.
  4. The San Diego Natural History Museum’s web page about finding fossils.
  5. William L. Newman’s old USGS publication Geologic Time has a page showing some index fossils used to date rocks.
  6. A good explanation of the concept of fossil succession.
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