The Guiding Hand.
12 million BCE
According to legend, the very first Cybertronian (Primus) realizes he can transform. Cybertronian religion says that the first Transformers were created very early on by the Guiding Hand, placing the origin of the species here.
10 million BCE
Also according to legend, there is a war among the gods and they only survive by becoming the transformation cog, the brain module, the Matrix, and Vector Sigma. The Cybertronians gain wisdom, immortality, and transforming skills, and form the Knights of Cybertron. The Knights turn Cybertron into a paradise before leaving the planet to spread enlightenment elsewhere, using vast Metrotitan colony-ships to settle and develop new worlds.
~9 million BCE
"The Age of Wrath": The alien species known as the Quintessons attempt to take advantage of the vulnerability of the Cybertronian population, arriving as 'the Fingers of the Guilding Hand' and promising technology, organization and growth. Modifications are quietly made to the populace, such as the cockpits in Seekers, to accommodate Quintesson 'masters' as pilots. Cybertron is quietly enslaved as the Quintessons begin selling them for profit, dividing the races into military hardware and consumer goods. A slave rebellion, lead by Alpha Trion and Beta, eventually drives out the Quintesson overlords, but the once peaceful population has drift far from its societal and social roots.
~8.5 million BCE
With the loss of the Knights' benign influence, Cybertron descends into a civil war for untold years. Battles spread across the cosmos to worlds such as Spindrift, with one faction being the Headmasters, who Galvatron comes to prominence battling. The conflict is eventually brought to an end by Nova Prime, who discovers Omega Supreme and with his aid, revives the ways of peace. Under his direction, the Cybertronian empire expands, with the Primal Vanguard serving as offworld peacekeepers. Vector Sigma stops creating new life, prompting Nova to task a team of scientists led by Tyrest with extracting new sparks from the Matrix of Leadership, dividing causing Cybertronians to have either "forged" or "cold constructed" origins. This ends when the Matrix is stolen and replaced with a replica. A "first wave" of new Transformers is created; additional Sparks are stored until bodies can be created for them.
~ 8 million BCE
Nova Prime reveals his true dark intentions, having conspired with scientist Jhiaxus to develop a process that allows sentient minds to be merged into one in pursuit of peace via the elimination of all will save his own. This process yields Monstructor. While Omega Supreme is occupied battling the beast, Nova Prime and Jhiaxus depart Cybertron in the first Ark to begin their work, with notable crew members Galvatron, Cyclonus, Straxus, Grindcore, and Scourge. Omega defeats Monstructor and locks him away in an inter-dimensional prison, then entrusts guardianship of the last remaining Metrotitan to Dai Atlas.
~7.5 million BCE
Nova Prime's Ark is subsequently lost in the Benzuli Expanse, and all aboard are presumed dead. Due to the spread of Cybertronian civil war, and then conquest-based operations of the Primal Vanguard on other planets (particularly organic ones), the Galactic Council votes to blacklist the Cybertronian species, forbidding travel, support or aid to Cybertronians. This leaves only the non-Council worlds open to Cybertronians, who have gained an unfavorable reputation around the galaxy.
~7 million BCE
Cybertron is in a period of time known as the Interregnum. Unaware that it's a fake, Delta Magnus of the Primal Vanguard protects the Matrix of Leadership until a new bearer can be found. Cybertron suffers from a lack of energy in the wake of Nova's expansion program, and tensions emerge. Apartheid exists against those who were constructed instead of forged. Though this was eventually put aside, old tensions and prejudices still linger.
~5 million BCE
A corrupt Senate governs Cybertron, with Nominus Prime as Matrix-bearer. A policy of "functionism" is instituted, with robots expected to stick to a single job determined by their alternate mode; monoformers, those without alt mode, face shape-hate. The rich and powerful thrive while all others suffer. The Senate use the sinister network of secret facilities known as the Institute to brainwash dissenters, and subject criminals to a head-and-hand replacement ritual known as Empurata. Aliens call the Cybertronians "Autobots": an insult, meaning "automatons". Megatron, a lowly miner who believes in changing the planet with non-violent actions, is left embittered after being subjected to police brutality. Police officer Orion Pax is moved by his treatise and confronts the Senate, even deciding to 'reclaim' the slur Autobot as short for "autonomous robot", but he is ignored. At the same time, some Senators make a grab for more power by ordering an attack on Nominus Prime, using it as an excuse to obtain the Matrix of Leadership and initiate the Clampdown.
PRESENT DAY
The Clampdown has caused tension and dissent across Cybertron. The Decepticon movement starts, basing themselves off samizdat writings from Megatron, and the miner is quietly sent into exile offworld. As part of a complicated plan to eliminate the threat the Decepticons pose to their social order, Senator Proteus promises to recognize the Decepticons as a legitimate political entity if 10,000 of them register as such, actually a method of gathering names for future brainwashing. With the discovery that Nominus's Matrix is a fake, the Senate have him killed and put the endgame of their plan into play, intending to stage bombing that will be pinned on the Decepticons. The scheme is foiled by a small team led by Orion Pax, but as a repercussion, his friend Senator Shockwave is brainwashed and subjected to Empurata by The Institute. The Senate's head of security, Sentinel, is installed as the new Prime.
(Quoted and modified from: http://tfwiki.net/wiki/IDW_timeline)