Many ancient Greeks and Romans reassembled the fossilised bones of mammoths, mastodons, and whales in human form, “creating” ancient, dead, giant humans. (As Stewart Guthrie notes in Faces in the Clouds, humans are forever projecting human forms onto other things.) Mayor suggests that myths about the Giants, Cyclops, Griffins, etc, may have originated in the discovery of these enormous bones and the attempt to make sense of them as beings from the past or faraway places.
Her book is full of fascinating facts about giants and other creatures, and has now been reissued in a revised version. You can read the introduction to the 2011 edition here, which provides a good overview of the content of the book.
While we should be wary of treating naturalistic explanations as the sole explanation for the origin of giant traditions, the fact that ancient Greeks and Romans identified some fossils as giants and monsters is well supported in Mayor’s book. It is safe to say that the ancient discovery of fossils is, at least, a contributing or supporting reason for the ancient belief in such fantastic creatures.
Mayor’s account may also shed light on the narratives about giants of the Bible – such as the story of the large sarcophagus attributed to King Og of Bashan.
h/t: Michael Heiser