Final answer:
A fossil is preserved organic material replaced by inorganic materials such as silica through mineralization. Permineralization and replacement by secondary minerals like quartz, pyrite, or calcite are common during fossil formation, helping preserve the organism's structure.
Step-by-step explanation:
A fossil is a preserved inorganic remains or traces of organic material. Over time, an inorganic material such as silica replaces every organic molecule in a process called mineralization. Biological anthropologists study these remnants, including bones and teeth that have undergone permineralization, which enhances their durability by allowing minerals like quartz, calcite, or pyrite to permeate the pore spaces and crystallize. Sometimes, the original skeletal material may be entirely replaced by a secondary mineral without losing the fine cellular detail of the original organism, which is known as replacement. This comprehensive understanding of fossil formation helps us piece together the incomplete fossil record that is crucial for understanding the history of life on Earth.