The clues to man's past are chiefly fossils, a farrago of frequently undecipherableand occasionally contradictorybits of evidence that often raise more questions than they answer. Fossils, the souvenirs of ages gone by, have survived through a still incompletely understood process whereby minerals from the soil infiltrate and gradually replace the very molecules of bone or other hard tissues of an organism, leaving its form and many features preserved.
Fossilization takes place only under special conditions. An animal or plant that dies and is soon after buried in mud or covered by volcanic ash stands a decent chance of being preserved; one that...