Chemistry Reference
In-Depth Information
into rock.
Diagenesis
involves changes resulting from the physical and
chemical processes taking place when buried remains interact with the
surrounding environment (see Textbox 46). These processes, which are
brought about by water percolating through and by the pressure of
sediments deposited on top of the remains, ultimately result in either the
partial or entire dissolution of the remains or in the formation of new min-
erals within their framework and their consequent gradual transformation
into fossils (Martin 1999).
Fossilized primates, especially fossilized humans, are, unfortunately,
particularly rare, and this partly explains why the evolutionary history of
human beings is incomplete and continuously being revised. Still, the few
fossilized human remains that have been preserved provide the main foun-
dations for modern human evolution theories (Reader 1981).
16.3.
ANIMAL EXCRETIONS
Whatever animals eat is processed by their bodies. However, only a few com-
ponents of the food intake - those required for the building, sustaining, and
functioning of the body - are selectively extracted and retained. The remain-
ing bulk is more or less regularly disposed of as excreta, including gases,
liquids, and solids. The gases are released into the atmosphere; the liquid
(urine) is usually absorbed by the soil, and the solids (feces or dung) in a
variety of compositions, shapes, textures, and colors, are soon recycled by
biological activity (most are used as food by microorganisms and insects)
and disappear from the surface of the earth shortly after their excretion.
Under suitable environmental conditions some solid excreta may, however,
be preserved for extremely long periods of time. If unearthed, for example
during archaeological excavations, ancient feces constitute a substantial
source of archaeological information (Sobolik 2000; Bryant 1974).
Human feces are normally composed mainly of water, which, although
commonly about 80%, may range from less than 50% to over 90% of their
weight. Most of the remains consist of insoluble fibrous matter, fats (up to
about 10% of the total weight), the residues of dead cells from the digestive
system, inorganic compounds (such as calcium phosphate and nitrogen com-
pounds), and bacteria. Humans recognized early in their development that
feces (especially the calcium phosphate and the nitrogen compounds in
them) are beneficial for the growth of plants. Since time immemorial they
