Fossils are the remains of organisms that lived in the distant past. For a specimen to be considered a fossil, it must be of a specified minimum age (often designated as greater than 10,000 years old).
Together, all fossils—when considered in the context of the rocks and sediments in which they are found—form what is referred to as the fossil record. The fossil record provides the foundation for understanding the evolution of life on Earth. The fossil record provides the raw data—the evidence—that enables us to describe the living organisms of the past. Scientists use the fossil record to construct theories that describe how organisms of the present and past evolved and relate to one another. But those theories are human constructs, they are proposed narratives describing what happened in the distant past and they must fit with fossil evidence. If a fossil is discovered which does not fit with current scientific understanding, scientists must rethink their interpretation of the fossil and its lineage. As science writer Henry Gee puts it:
"When people discover a fossil they have enormous expectations about what that fossil can tell us about evolution, about past lives. But fossils actually don't tell us anything. They are completely mute. The most the fossil is, is an exclamation that says: Here I am. Deal with it." ~ Henry Gee
Fossilization is a rare occurrence in the history of life. Most animals die and leave no trace; their remains are scavenged soon after their death or they decompose quickly. But occasionally, an animal's remains are preserved under special circumstances and a fossil is produced. Since aquatic environments offer conditions more favorable to fossilization than those of terrestrial environments, most fossils are preserved in freshwater or marine sediments.
Fossils need geological context in order to tell us valuable information about evolution. If a fossil is taken out of its geological context, if we have the preserved remains of some prehistoric creature but don't know what rocks it was dislodged from, we can say very little of value about that fossil.