Two themes dominate Timeline: history and technology. History is a large concern in the novel as well. Both the historians and Doniger recognize the past as the root of contemporary culture. Johnston's comment on temporal provincials is that "if you didn't know history, you didn't know anything. You were a leaf that didn't know it was part of a tree." Crichton maintains the importance of knowing the past throughout the novel: "Yet the truth was that the modern world was invented in the Middle Ages. Everything from the legal system, to nation- states, to reliance on technology, to the concept of romantic love had first been established in medieval times." Even Doniger seems to recognize its importance at some level. He predicts that people will want to pay to experience the past. He may also be right in thinking that people do not want to see the truth behind our romanticized ideals about history. The past may not be glamorous or exciting at some times, but its importance is undeniable to any of the characters.
The danger of technology is a common theme in modern writing, and this is no exception in Timeline. Crichton creates his most frightening image in the cat Wellsey, who was used to test early quantum travel machines. The invisible transcription errors of a few trips have lead to " . . . a third eye, smaller and only partially formed. And beneath that eye was a patch of flesh, and then a protruding bit of jaw that stuck out like a tumor from the side of the face" after many trips. That one word, tumor, recalls nightmares of nuclear radiation poisoning and chemically polluted water—all the dangers we have experienced with our current technology. Crichton warns us that new technologies will bring similar, initially unnoticeable effects.