The Star Trek: Strange New Worlds episode “Tomorrow And Tomorrow And Tomorrow” (Season 2, Episode 3) establishes that covert Romulan agents have been manipulating Earth’s history since the 1950s, creating what is now called the “revised” Prime timeline in Star Trek canon.
I prefer to think of this event as causing a split between “Prime Timeline A” (the original series to Next Generation/DS9/Voyager era), and “Prime Timeline B” (new series since 2017 onward, except Picard). This A/B split does not include “Prime Timeline C,” the one where La’an Noonien-Singh and a doomed alternate James T. Kirk thwart the Romulans, nor the timelines of the Mirror Universe (aka the Terran Empire) or J.J. Abrams’ Kelvin timeline (“Timeline K,” the three 21st century movies).
Technically there are many other parallel timelines, such as the one where the Borg assimilated Earth in 2063 (Star Trek: First Contact), the Na’kuhl Nazi timeline (Star Trek Enterprise episodes “Storm Front 1 and 2”), and various splinter timelines and closed paradox time loops.
Prime Timeline A canon should include ST: The Original Series, ST: The Animated Series, ST original movies (TOS, TNG crews), ST: The Next Generation, ST: Deep Space Nine, ST: Voyager, and ST: Picard.
Prime Timeline B canon should include the A list shows where there is no contradiction, plus ST: Enterprise, ST: Discovery, ST: Strange New Worlds, and
ST: Lower Decks. (ST: Prodigy probably belongs in this set as well.)
The branch between Prime A and Prime B appears to have been caused by Romulan agents on Earth during the 1950s to 1990s who sought to slow down humanity’s scientific progress. By causing delays, such as pushing back the “starting dates” and details of key events such as the birth of Khan Noonien Singh and the start of the Eugenics Wars, the Romulans sought to gain an advantage during their later interstellar war with Earth (ST:SNW “Tomorrow and Tomorrow and Tomorrow”).
The Kelvin Timeline canon includes Star Trek (2009), ST: Into Darkness, and ST: Beyond. Starting from Timeline B of 2387, the Borg-enhanced Romulan mining vessle Narada – coupled with a black hole created by Ambassador Spock’s use of red matter – almost accidentally performed time travel back to the year 2258, but changes from the Prime Timelines were already in effect. James T. Kirk is born aboard the USS Kelvin, not on Earth, suggesting other time travel meddling was already underway beside the 20th century Romulans.
Besides Starfleet and Romulans, time travel is also known to many other species and groups: The Borg, the Bajorans, Suliban warriors scheming on behalf of their mysterious 28th century master, and maybe also the Klingons (as Alexander son of Worf travels back in time in ST:TNG “Firstborn”). The end of Lower Decks Season 5 has a “Schrödinger possibility field” stabilize as a portal between multiverses, suggesting Prime B Starfleet visits to other timelines may again become commonplace.
The following list highlights some obvious major differences between Prime Timeline A, Prime Timeline B, and the Kelvin Timeline (K).
STARFLEET TIME TRAVEL & THE GUARDIAN OF FOREVER
A: Easily and frequently used by Starfleet (ST:TOS “Assignment: Earth,” Star Trek IV: The Voyage Home movie), though with regulations to prevent changing history. The Guardian of Forever time portal (ST:TOS “City on the Edge of Forever,” ST:TAS “Yesteryear”) is widely studied by Federation scientists. By the 26th century (2500s), Federation archeologists are using “time pods” to research the past (ST:TNG “A Matter of Time”).
B: Time travel knowledge is suppressed and forbidden after the Temporal Cold Wars (ST: Enterprise). The Guardian of Forever moves itself into hiding on another planet (ST:DIS “Terra Firma” episodes) but still welcomes Federation investigators when found. Retroactively, without time travel knowledge, Kirk and crew could not have travelled to get whales from the 20th century and divert the Probe attacking Earth in 2286 (ST IV: The Voyage Home), meaning damage to the planet must have been far more severe after this date in Timeline B.
K: There is no evidence Starfleet or any other group has knowledge of time travel. The Narada incident creating the Kelvin Timeline appears to have been an anomalous accident, and older A/B Ambassador Spock does not seem to reveal any time travel knowledge before his death on New Vulcan.
BIRTHDATE, BIRTHPLACE, AND ABILITIES OF KHAN
A: Khan Noonien Singh is born during the 1950s in Asia. Khan, a charismatic tactical genius able to learn rapidly, is physically and mentally superior to most non-augmented humans. Khan dies during the Genesis Planet incident in 2285 (ST:TOS “Space Seed,” ST II: The Wrath of Khan).
B: Khan Noonien Singh is born during the 1990s in America. Khan is (will be) physically and mentally superior to most non-augmented humans (ST:SNW “Tomorrow and Tomorrow and Tomorrow”). Presumably Khan dies in the future Genesis Planet incident of 2285 just like Timeline A.
K: Khan Noonien Singh is born during the 1990s in Britain. Khan is superhuman, able to design incredible innovations in technology, easily wipe out an entire Klingon military squad by himself in combat, and his blood can be used to resurrect the dead (ST: Into Darkness). Khan’s fate is unknown.
THE EUGENICS WARS / WORLD WAR III
A: Eugenics Wars start in 1990s, World War III mid-21st century (ST:TOS “Space Seed,” ST: First Contact).
B: Eugenics Wars start in the early 21st century, World War III in the late 21st century (ST:SNW “Tomorrow and Tomorrow and Tomorrow”).
Note: In both A and B, Zefram Cochrane’s first warp flight is on April 5, 2063, either after WWIII (timeline A) or during WWIII (timeline B).
PARALLEL UNIVERSE OF THE TERRAN EMPIRE
A: Mirror universe discovered in 2267 by crew of Kirk’s Enterprise (NCC-1701) (ST:TOS “Mirror, Mirror”).
B: Mirror universe discovered in 2155 by crew of Archer’s Enterprise (NX-01) (ST:ENT “In a Mirror, Darkly”).
FIRST CONTACT WITH THE BORG
A: Thanks to Q, Picard’s Enterprise (NCC-1701-D) meets a Borg ship in the Delta Quadrant in 2365 (ST:TNG “Q Who?”).
B: Archer’s Enterprise (NX-01) meets Borg who have escaped from an old crash site in Earth’s Arctic Circle in 2153 (ST:ENT “Regeneration”). It’s likely these Borg were somehow left behind after the timeline alterations of 2063 (ST: First Contact). It is possible Archer’s encounter happened in Timeline A but went largely unnoticed in official recorded history, but that seems unlikely.
FIRST CONTACT WITH THE GORN
A: Gorn unknown until Kirk’s encounter (ST:TOS “Arena”) in 2267. Gorn are heavy-set, bigger than humans, and move very slowly.
B: Gorn encountered in 2230s as raiders of Federation colonies (ST:SNW “All Those Who Wander”). Gorn are lean, roughly human-sized, and move very quickly, leaping like huge grasshoppers.
COMPUTER-CONTROLLED STARSHIPS
A: An experiment with computer-controlled starships ends badly in 2268’s M-5 Multitronic System field test (ST:TOS “The Ultimate Computer”). Direct AI control of Starfleet ships is not allowed after this incident (although technically Lt. Commander Data and Janeway’s use of the Emergency Medical Hologram Doctor may break this rule).
B: An experiment with computer-controlled starships ends badly in 2258 when Section 31’s Control threat assessment system attempts to capture the USS Discovery (ST:DIS “Project Daedalus” and “Such Sweet Sorrow”). Direct AI control of Starfleet ships is not allowed after this incident.
CHRISTOPHER PIKE’S FATE
A: In 2266, Capt. Christopher Pike is fully paralyzed and scarred by radiation while rescuing cadets aboard a training vessel. Years later, he is kidnapped by Spock who attempts to take him Talos IV, a quarantined planet whose no-contact rules carry the Federation’s only death penalty. After Spock’s court martial, Pike is allowed to permanently settle on Talos IV (ST:TOS “The Menagerie”).
B: Same as Timeline A, except Pike gains foreknowledge of his own fate due to a time crystal from the Klingon monastery on Boreth (ST:DIS “Through the Valley of Shadows”). He later learns from a visit by his future self from another alternate timeline that his fate helps avoid a protracted war between the Federation and the Romulan Star Empire (ST:SNW “A Quality of Mercy”). It is suggested this time travel is enabled by cooperation between Starfleet and the Klingon Empire, perhaps as a counter to the Romulans’ time travel scheme on 20th century Earth.
K: In 2258, Capt. Pike commands the USS Enterprise when it encounters the Romulan renegade Nero, who captures and tortures the captain until he cannot walk (but is not left fully paralyzed) (Star Trek 2009 movie). Pike is killed a year later during Khan Noonien Singh’s attack on Starfleet headquarters (ST: Into Darkness).
BIRTHPLACE OF JAMES T. KIRK (MARCH 22, 2233)
A: James T. Kirk is born on Earth in Riverside, Iowa (apocryphal but reportedly endorsed by Gene Roddenberry).
B: James T. Kirk is raised on Earth in Riverside, Iowa. His father George Kirk served on the USS Kelvin and retired to another planet after his service. (ST:SNW “A Quality of Mercy”).
C: James T. Kirk is born aboard the USS Iowa (ST:SNW “Tomorrow and Tomorrow and Tomorrow”).
K: George Kirk dies aboard the USS Kelvin while his son James T. Kirk is born in an escape shuttle (Star Trek 2009 movie).
KIRK’S SON
A and B: Dr. Carol Marcus is a scientist working on Starbase 1. She and then-junior Starfleet officer James T. Kirk have a relationship in the 2250s and conceive a son named David, born in 2259/2260. The boy grows up to be an astrophysicist working with his mother on the Genesis Project before his murder by a Klingon in 2285. (ST II: The Wrath of Khan, ST III: The Search for Spock, ST:SNW “Subspace Rhapsody”).
K: Dr. Carol Marcus (Wallace) is daughter to a Starfleet admiral and an active Starfleet science officer assigned to the USS Enterprise in 2259. She met Capt. James T. Kirk that year and was instrumental in exposing Section 31’s exploitation of the augment Khan (ST: Into Darkness). It is unclear if this Carol Marcus and Kirk later had an intimate relationship or any children.
SPOCK’S FAMILY AND SIBLINGS
A and B: Sarek has a Vulcan first wife before marrying the human Amanda Grayson sometime before Spock’s birth in 2230. Before his death in 2368, Sarek would marry a third wife, a human named Perrin Landover (ST:TNG “Sarek”).
A: Spock has a half-brother, Sybok (ST V: The Final Frontier). (It’s possible Michael Burnham’s parents in Timeline A never got a time crystal and thus were not attacked by Klingons at their home facility on Doctari Alpha. Without being an orphan, Sarek and Amanda don’t enter Michael’s life.)
B: Spock has a half-brother, Sybok, and an adopted human sister, Michael Burnham (ST V: The Final Frontier, ST: Discovery).
K: Both Sarek and Amanda are killed when Vulcan is destroyed in 2258 (Star Trek 2009 movie). No mention of Sybok or Michael Burnham is made in this timeline.
SPOCK’S ROMANTIC RELATIONSHIPS
A: Spock-T’Pring relationship is a private matter. Spock-Nurse Christine Chappel are close but never dated. (ST:TOS “Amok Time”).
B: Spock-T’Pring relationship is public knowledge, and Spock-Nurse Christine Chappel have a dating relationship at some point (ST:SNW).
K: No Spock-T’Pring union is mentioned, and it is unclear about the duration or nature of the Spock-Chappel relationship. Spock-Uhura have a dating relationship at some point prior to their service aboard the Enterprise, and they reconnect later (Star Trek 2009 movie).
FATE OF THE VULCAN AND ROMULAN HOMEWORLDS
A: Renamed Ni’Var after the Romulan homeworld is destroyed by a supernova in 2387. The Romulan Star Empire collapses; at least a remnant renamed itself the Romulan Free State (ST: Picard).
B: Probably the same as A, with Romulan and Vulcan reunification actively in progress.
K: The Vulcan homeworld is destroyed with red matter by the Romulan renegade Nero in 2258 (Star Trek 2009 movie).
SPOCK’S DEATH
A and B: Spock is born in 2230, dies and becomes reborn on the Genesis Planet in 2285, and is believed killed during the supernova that destroys the Romulan homeworld in 2387 (ST II: The Wrath of Khan, ST III: The Search for Spock, Star Trek 2009 movie).
K: Prime A/B Spock jumps timelines from Prime A/B 2387 to Kevlin 2258 and meets his younger K Spock self. A/B Spock then helps found New Vulcan, where he dies in 2263 (Star Trek 2009 movie, Star Trek: Beyond).
TRANSWARP STARSHIPS
A: Starfleet develops a transwarp drive for the USS Excelsior in 2285, but later abandons the technology (ST III: The Search for Spock). In the Delta Quadrant, the Borg Collective use a transwarp conduit network, later discovered and used by the USS Voyager (ST:VOY).
B: Probably same as A.
K: In 2259, it’s possible the USS Vengeance used a version of transwarp technology given that it was “three times as fast” as the USS Enterprise, Starfleet’s most advanced ship at the time (ST: Into Darkness).
MAXIMUM TRANSPORTER RANGE
A and B: Transporters can beam people and cargo up to 40,000 km, maybe further with local pattern enhancers.
K: By 2259, “transwarp” transporters can beam people over a distance of at least 90 light years (shown used by “John Harrison” – aka Khan – from Earth to the Klingon homeworld, Qo’noS/Kronos) (ST: Into Darkness).
DISPLACEMENT-ACTIVATED SPORE HUB DRIVE
A: Unknown, probably never existed.
B: Starfleet spore drive experiments in the 2240s and 2250s were abandoned after the loss of both experimental ships, the USS Glenn and USS Discovery (ST:DIS season 3).
Note: Since Starfleet’s push for the spore drive was largely driven by tensions and an arms race leading to the Federation-Klingon War that finally broke out in 2256, it’s possible pre-war events in Timeline B were worse than in Timeline A. Also, since Capt. Gabriel Lorca was actually from the Mirror Universe Terran Empire by the time Discovery was in use, it’s possible something between Timeline A and Timeline B attracted his attention between parallel universes.
KLINGON CULTURE AND VIEWS TOWARD THE DEAD
A: A dead Klingon body in this world is just an “empty shell” to be discarded (ST:TNG Season 1, Episode 20, “Heart of Glory”).
B: A dead Klingon body is still held as sacred (ST:DIS).
Note: This could just be a cultural difference between two Klingon traditions.
THE FAR FUTURE (BEYOND THE 30th CENTURY)
A: Unknown, but given Starfleet’s easy access to time travel, the Federation is likely doing fine. (Major disasters could be foreseen and avoided.)
B: In 3069, “The Burn” renders dilithium inert, causing starships to explode and warp drive technology to become far less commonplace. The United Federation of Planets collapses, and presumably other interstellar powers do as well. By the 3100s, a small version of Starfleet starts to rebuild the broken Federation against belligerent rivals such as the Emerald Chain criminal underworld and the Breen Imperium (ST:DIS seasons 4 and 5).