Although “fossil” is now a common and widely used word, whose meaning is known to practically everyone, the general acceptance of the idea that fossils are the remains of ancient organisms required millennia to achieve. One reason for this is that the great age of Earth also was not widely appreciated until relatively recently. Without an Earth eons old the idea of ancient life and the idea of fossils are meaningless.
The use of fossils in understanding the distant past can be traced back to at least the sixth century B.C.E., when Xenophanes of Colophon lived. Xenophanes described the occurrence of clam shells in rocks outcropping in mountainous parts of Attica. He recognized that these lithified clam shells were closely similar to clams that were then living along the coastline of the Aegean Sea. To account for the occurrence of these lithified clam shells far from the present sea, he argued that they were the preserved remains of clams that had lived at an earlier time when Attica was covered by an ocean. Hippolytus of Rome (c. 170 – c. 236) in his Refutation of all Heresies (1.14.5-6) records that Xenophanes studied the fossils to be found in quarries:
Xenophanes declared that the sea is salty because many mixtures flow together in it… He believes that earth is being mixed into the sea and over time it is being dissolved by the moisture, saying that he has the following kind of proofs, that sea shells are found in the middle of the earth and in mountains, and the impressions of a fish and seals have been found at Syracuse in the quarries, and the impression of a laurel leaf in the depth of the stone in Paros, and on Malta flat shapes of all marine life. He says that these things occurred when all things were covered with mud long ago and the impressions were dried in the mud.
However, in 750 BCE there were no quantitative methods for verifying this hypothesis, and so Xenophanes’ rather modern-sounding explanation for these clams could not be tested, and disappeared from view. This interpretation of fossils did not reappear in history until Leonardo da Vinci, although he did not contribute to the understanding of fossils since his views were never published.