Timeline 4 is never seen, but it is the timeline in which the Trunks from Cell's original timeline goes back in time to find a means to destroy the androids. Timeline 3 is where Cell originates from and goes back in time by killing Trunks and stealing his Time Machine this timeline is briefly seen. Timeline 2 is the original timeline from where Future Trunks originates. Timeline 1 is where the main story takes place, and Future Trunks and Cell arrive there from their respective future. Therefore, even if a time traveler follows a "look but don't touch" policy, they risk changing history. For example, Androids 19 and 20 came first, despite no explanation occurring for why they would exist, and Androids 17 and 18 are also inexplicably stronger than Future Trunks remembers. Simply going back in time causes disruptions to the timeline as various random changes occur. It should be noted that, in the universe of Dragon Ball, one does not have to directly interfere with a timeline to disrupt it.
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Because Future Android 17 and Future Android 18 killed the Dragon Team and most of the potential heroes of Earth, Future Bulma created a Time Machine to send her son Future Trunks to the past to prevent Goku from dying of a Heart Virus, to warn the Z Fighters about the future android threat, and to find a way to stop the androids in the future world (either a way to deactivate the androids or a way to destroy them, maybe even taking Goku to Future Trunks' timeline). The alternate timelines were all created and linked due to the use of a Time Machine. The first alternate timeline was created by a Time Machine utilized by a mortal from an advanced civilization in Universe 12.