What are fossils

Fossils are the preserved remains of past life on earth. The word fossil derives from the Latin word fossus, which (literally) means “something dug up”. There is no specific age at which the remains of past life suddenly qualifies as a fossil, and in the definition, life all life forms encompassed by all three Domain of Life, the Archaea (Methanogens, Halophiles, Sulfolobus, and relatives), Eubacteria ("True bacteria", mitochondria, and chloroplasts), and the Eukaryotes (Protists, Plants, Fungi, Animals, Algae, etc.). Many fossils are from extinct groups, whether that be a small group such as a species, or a much larger including taxon such as a family, superfamily, class or phylum. Trilobites (Class Trilobites within Phylum Arthropoda) are an example of a large extinct group

However, extinction of the organism is not a prerequisite for a preserved organism to be a fossil, and in many cases whether a species, for example, is extinct may not even be determinable. Normally, fossils are considered to be the actual remains an organism after it has undergone one or more of a multitude of possible chemical/mineralogical transformations. Such fossils are more precisely called body fossils; for the most part, they are rocks, with fossil resin, a polymer with the street name of amber being a notable exception. However, so called Ichnofossils fossils are another important category that incorporates marks, impressions, footprints, and even the excrement left by organisms. Ichnofossils are also variously called trace fossils and trackway fossils. A final fossil category is molecular fossils (also sometimes called chemical fossils) that consist of altered chemical/atomic constituency of a matrix (e.g. rock) due to past life.

The oldest known fossils are usually considered to be stromatolites usually described as layered structures formed by the entrapment of minerals in mucous generated by cyanobacterial colonies. However, this may be an oversimplification in that there is no reason to think that the photosynthetic cyanobacteria were solely responsible for the many surviving formations of biogenically-produced stromatolitic structures.

As a general rule, the probability of a deceased organism undergoing fossilization and being preserved is a rare event that can only occur under various unlikely circumstances. That said, it must also be said that fossils are not rare, since the earth is filled with them. The problem is, fossils are rarely found.