Here's a condensed history of the Republic. The first part of the timeline covers some of the pre-Republic era. The last bits have some extra info about Anakin and Ceres' earlier lives, as well as the outbreak of the Clone Wars.
One assumption I make is that Obi-Wan's line in ANH about "a thousand generations" refers to 10,000 years. I tried to rationalize each generation as being 10 years instead of 25 by positing that each successive "generation" referred to the average amount of time it took for a Jedi apprentice to ascend to the rank of Knight, who is thus able to take on an apprentice(s) of his/her own...thus allowing for the next "generation" of Jedi.
I did that because I had too much trouble trying to justify 25,000 years of space-faring civilization with regards to the technology level seen in the OOT. Ten thousand years is still quite ridiculous, but I couldn't make it any shorter than that out of concern that I might contradict Obi-Wan's line in ANH. I suppose something's gotta give.
Here is the history of the Republic; as usual, feel free to critique...
ca. 12,500 BBY - A golden age of technology on Coruscant that promises such innovations as faster-than-light travel and sentient droids is brought to an end by the invasion of a hostile race called the Taung.
From the little of what can be together this era's history, it is believed that the alien battle force embarked for Coruscant from an unidentified world in the Unknown Regions as part of a pilgrimage foretold by their primary religion.
Having cast aside their ancient Sith foes a thousand years earlier, the Jedi Knights of Coruscant, the planet of both their origin and of the Human species, lead the charge against a strange new foe. Their powers allow them to turn the tide in an extraordinary manner against an unrelenting and seemingly technologically superior foe.
ca. 12,400 BBY - The Taung War is brought to a climax as a hyperspace rupture, a kind of technology that would not be properly harnessed for another eleven thousand years, is opened in Coruscant's upper atmosphere as a last-ditch attempt to eliminate the invaders.
Enormous amounts of radiation are thrown outward, a deluge that could not be controlled by pre-Republic technology, killing every Taung on and around Coruscant. But this also greatly damages Human civilization on the world, killing billions and destroying much of the planet's advanced civilzation.
ca. 10,000 BBY - The newly-formed Republic of Coruscant has now reclaimed all of the battered territories of its homeworld and looks to the stars for a new beginning.The first sleeper ships are loaded with colonists and Jedi Knights sent out to make contact with or settle other systems. It was only after the Taung War that the Jedi settled into their capacity as peacekeepers rather than super-warriors.
ca. 9,750 to ca. 4,500 BBY - Many Core worlds such as Alderaan and Corellia join the Republic, either through colonization and Coruforming of uninhabitable worlds or through absorption of existing populations into the Republic’s structure.
The Jedi Knights, with their lengthened lifespans, are the perfect companions for the colonists. They help spread the message that the Force is to be used in a constructive and benign manner.
The Jedi of this period are open in their activities, even occupying positions in the Republic’s government, both on the homeworld and on her colonies and ally planets. Over time, the notion of the Jedi being a Coruscant-based organization is lost, replaced instead by a more cosmopolitan order composed of equals parts the many different races inhabiting the Core.
ca. 3800 to ca. 3650 BBY - Many Jedi who grew corrupt from their governing power attempt to stage a coup of the Republic’s government and install themselves as god-kings known as “Sith Lords”.
The dark Jedi seek to carry on the legacy of “the strong ruling the weak” as their forbears in pre-Republic times had done. They believe that their extraordinary abilities give them the means and the right to conquer and dominate others. The dark Jedi are resisted against by their light-side counterparts.
The Sith-controlled territories are known collectively as the Sith Coalition. The Coalition functions as an ad-hoc leaderless compact, united not by common currencies, armies, or citizenry, but by an dark ideology strong enough to bridge the interstellar depths. Such is the only possibility in an era before hyper-travel.
Asynchronous bouts of slower-than-light interstellar warfare occur, with autonomous cells of Force-users in mortal combat on far-flung worlds. Many worlds execute suspected Force-users out of fear and paranoia.
The Sith Wars end with the dark Jedi’s eviction from the galaxy under the eye of Jedi ambassadors. The Knights thereafter act as a secret peacekeeping group rather than an official organ of government. The Sith War also provides cause for the antipathy and suspicion many beings in the galaxy would feel towards Force-users in later years, in addition to being the longest continuous conflict in galactic history.
Lightsabers are developed during the war after the discovery of special crystals on a moon that would later be known as "the Sanctum". The Sanctum becomes the base of operations for the Jedi after their post-war retreat into secrecy.
2,270 to 1,344 BBY - A second wave of colonization occurs due to advances in artificial intelligence that allow the first truly sentient droids to act as overseers on starships and perform many more tasks on colony worlds.
Though there is fear that the droids will conduct an organized rebellion against their “meatbag” predecessors, no such event ever comes to pass.
By the end of the Second Expansion, all of the Core Worlds and a handful of systems a dozen lightyears outward have joined the Republic, which has rechristened itself the Stellar Republic.
1,138 BBY - First iteration of hypergate technology allows the transmission of messages at superluminal speeds. A network of hypergate nodes begins to form in the Republic’s systems.
1,004 BBY - Hypergate technology now allows the transport of entire starships at many times the speed of light. This marks the start of a truly galactic era. Coruscant gains even more importance due to the construction of the hypergate nexus at a Lagrangian point just outside the world’s gravity well.
997 to 313 BBY - The network of hypergates expands. More worlds and species are absorbed into the Republic, typically taking the form of economic partnerships. Little large-scale colonization occurs between the Core zone and the galaxy outside.
Eventually the upper-bound is reached in the feasible size of the hypergates, causing starship designs to stratify into a number of strictly-defined classes based on the sizes of hypergate apertures.
The frontier mentality disappears due to the drastically reduced travel times between systems linked by hypergates.
Yoda is born in the early part of this period. He is Force-sensitive at birth.
280 BBY - The Republic's first contact with the Chiss race occurs. The Chiss are an independent stellar civilization living in an out-of-the-war cluster near the edge of the galaxy. They use a unique form of FTL dubbed by Republic scientists as the hyperspace cannon.
The Chiss royalty sees the incursion of the Republic as a threat to their legitimacy as absolute divinely-ordained rulers of the state.
They also realize from the accounts of Republic explorers that hyperspace cannon technology is more flexible than that of hypergates because it removes not only the upper limit of starship size but also promises travel to anywhere the cannon can be pointed rather than between fixed nodes. The downside, however, is the great fuel expenditure required to slow the vessel down, limiting travel to within a defined sphere of space.
269 BBY - When the Republic fails to acquire hyperspace cannon technology from the Chiss by diplomatic means, they try to steal it. One of the Republic's spies is exposed by the Chiss' counter-intelligence force and this sparks the Chiss War.
264 BBY - All but one of the Republic's hypergates in the Chiss sector has been destroyed by the native fleet. The Republic Navy cuts off the route to that last remaining gate, known thereafter as Death's Door, effectively stranding the detachment in enemy territory. The force contains a Jedi ambassador, a venerable old Master sent to settle the dispute by peaceful means rather than through the cleansing of the Chiss.
After holding off an onslaught of Chiss marines, the detachment leaves Death's Door for the Chiss homeworld of Csilla in a slower-than-light troopship that was docked at the hypergate. The Jedi Master takes command just before the crew enters stasis. They are not expected to reach the Csilla for another two decades.
257 BBY - After years of failed diplomatic communications, the Republic reactivates Death's Door and sends a fleet through the aperture. The expeditionary force begins skirmishes with local defenses.
248 BBY - By now the Chiss have deactivated or destroyed all of their hyperspace cannons as the Republic onslaught sweeps through their systems. This is enacted as a scorched earth policy.
247 BBY - A decade of fighting leads the Republic expeditionary fleet to Csilla, where the battered forces finally stand down and accept the Republic's terms of surrender.
243 BBY - The Death's Door detachment finally arrives at Csilla. They find that the war they were going to end through peaceful means has been over for four years.
175 to 129 BBY - I have detailed the events of this time in post #13.
The hyperdrive is developed, followed shortly by its proliferation among almost all of the galaxy's species. Some of the device's inner workings were reverse-engineered from the remains of Chiss hyperspace cannons.
The Republic enacts a third wave of colonization. The Unification War occurs, spurring the development of sentient cloning.
After the war, the Stellar Republic signals a new era of brotherhood by renaming itself the Galactic Republic.
66 BBY - Ceres is born on Zygerria to a native mother and a Human father. She is Force-sensitive at birth. Ceres' father, a mercenary hired by a group of slavers, leaves her on the planet with her mother soon after carving the mark of his syndicate into the skin of her back.
59 BBY - Anakin Skywalker is born on Taris. He and his mother suffer physical abuse at the hands of his father.
53 BBY - Ceres is sold into slavery as a dancer and concubine.
47 BBY - Anakin Skywalker gains Force-sensitivity.
44 BBY - In a fit of rage, Anakin's father kills his mother. Anakin finally retaliates by killing him in return. With no personal attachments left, Anakin is forced to subsist in the underlevels of Taris.
43 BBY - By now, Ceres has ascended from a common working girl to a high-class courtesan. For months, her movements are tracked by the Jedi Knight Hirala, who believes that Ceres needs to be inducted into the Jedi and the slavers need to be brought to justice.
Hirala uses her Jedi mind tricks to pass as a captive, allowing her to organize a breakout from the slavers' holding cells on Socorro. The ring is indeed disbanded and the captives freed, while the Jedi gain another ally.
44 to 37 BBY - Anakin makes a living as an underground swoop racer. His skills attract both fame and notoriety, making him a prime target of rival gangs and the authorities.
37 BBY - Obi-Wan Kenobi tracks down Anakin's Force presence on Taris. The Master convinces Anakin that his powers can be used for good, that those abilities give him a duty to uphold justice. Anakin relents because the prospect of being a Jedi allows him to gain some control in life, countering the helplessness he felt when he found out that his father had killed his mother. Obi-Wan takes Anakin under his wing as an apprentice.
35 BBY - In the first instance of sentient cloning since the Unification War, the ranks of the Togorian secessionist army are bolstered with duplicates of their finest warriors. Back at the Sanctum, Anakin and Ceres start to take an interest in one another.
33 BBY - Ceres takes the Oath of the Force, becoming a proper Jedi Knight after a decade of training under Hirala.
35 to 31 BBY - Rioting and protests on many worlds over the Republic's failed promise of just treatment of aliens after the Unification War leads to the formation of the League of Sovereign Planets. A prisonbreak at Despayre frees Ben Cortel.
29 BBY - Anakin takes the Oath of the Force. In the same year, he takes on an apprentice of his own.
27 BBY - After hearing a certain revelation from Anakin and deciding that the League fights for a just cause, Ceres defects from the Jedi Knights. While investigating ruined archives on the formerly Sith-dominated world of Korriban, Ceres finds herself drawn to their belief of power, over the weak, above all other ends. Tired of the reserved and meek ways of the Jedi, she decides that it is her calling to rule the galaxy.
Anakin and his young apprentice pursue Ceres to Korriban in an attempt to draw her back into the Jedi Knights, but her corruption is too severe. Anakin and his apprentice fight the traitor. Ceres kills the apprentice and cuts off Anakin's arm, leaving him alive for reasons that would not be clear to him for several years.
After this, Ceres travels the Outer Rim to gather the support of insurrectionist leaders, including Commander Hossk. Together, they depose the established League leadership, which was isolationist in nature. Ceres and her cohorts hold a vision for the League that calls for confrontation with the Republic.
25 BBY - Military tensions between the Republic and the League are at an all-time high. Many Repulic worlds have locked down all but the most essential hyper-routes. Some planets enact martial law.
Admiral Palpatine is placed in charge of all defensive and emergency duties on Anaxes. The planet houses the Navy's most prestigious war academy as well a number of heavy shipyards.
Unable to discern the League's next move, Chancellor Antior orders an even spread of naval resources across the Republic, putting Coruscant at great risk.
After more than a century of peace, the Galactic Republic is decapitated by a League attack on Coruscant and many other worlds. The Clone Wars begin with the Republic in tatters and the League triumphant.