Timetable on the history of the ME races
Disclaimer: Anything I have not altered is taken from the official Mass Effect Wiki and not my own work!
Years are Earth years for the sake of comprehension. No in-universe reason here.
13,000 BCE
The Turians discover agriculture and begin to develop a civilisation around this time.
6000 BCE
Seeking to escape the imminent explosion of their sun, an alien race constructs an AI-managed starship equipped with supercomputers containing a virtual world, into which one billion of the aliens transfer their consciousnesses. The starship begins a journey throughout the galaxy which lasts for the next 8,000 years.
1800 BCE
A supernova propels the Mu Relay, the only point of access to the remote Pangea Expanse, out of position. Concealed somewhere in the dense nebula formed by the supernova, the relay's position is effectively lost for centuries. Later, the Rachni rediscover the relay.
268 BCE
The Asari discover Prothean ruins on the moon of Thessia. A before then mysterious inscription in the main temple of Athame is found to match the Prothean signs in the ruins, revealing the involvement of extrathessians in early Asari history. Many devices from the ruins on the moon are subsequently brought to a secret archive near the temple.
204 BCE
Exploration of the relay network eventually leads the Asari to the huge Citadel space station at a hub of many mass relays.
113 BCE
A Salarian asteroid mining expedition discovers an alien derelict on the fourth planet of their home system. Studiyng and reverse engineering the technology leads to the discovery of the mass effect and element zero. Eight years later a science vessel detects a gravitational anomaly in the system's Oort cloud. Upon investigation it is found to be a mass relay.
109 BCE
The Turians discover the mass effect when they excavate alien ruins on a planet in their home system. A signal from the ruins leads them to an inert mass relay at the outer edges of the system. They activate it and begin to explore the relay network. Multiple colonies are founded in the following decades. The direct route from Palaven to the Citadel is interrupted due to a gap in the network. This is the reason why the Turians discover the Citadel so much later than other races.
96 BCE
Tuchanka, the Krogan homeworld, enters the nuclear age. In a global conflict, weapons of mass destruction are released, triggering a nuclear winter. In the resulting devastation, Krogan society devolves into a collection of warring clans.
53 BCE
The Salarians discover the Citadel and open diplomatic relations with the Asari.
49 BCE: Founding of the Citadel Council
The Citadel Council is formed. The Asari and Salarians together colonize the Citadel and establish it as a center of the galactic community, led by the Council. This year is also known as 0 GS, the beginning of the Galactic Standard (GS) timeline. As a gesture of openness with their new asari allies, the Salarian Union opens the records of the League of One. Under threat, the League responds by assassinating every member of the Union's inner cabinet; Spacial Tasks Group operatives then hunt down and eliminate the League.
37 BCE
First contact is made with the Volus. Eventually, the Citadel Council commissions the Volus to draw up the Unified Banking Act, which establishes a standard galactic currency known as the credit and links all galactic economies. This greatly simplifies the flow of money, leading to an economic boom.
22 - 05 BCE
The turian Unification War occurs. The increasingly isolated and xenophobic colonies on the frontiers of turian space go to war with each other. After years of fighting, the Turian Hierarchy sweeps in and pacifies the remaining factions. Animosity between turian colonies continues for decades.
08 BCE
The Council grants the Volus the honor of being the third species with a Council seat. While they have little military power and fewer high-population colonies than the other two races, they are an economic powerhouse and the leaders of the big Volus banking and trading clans are amongst the most powerful individuals in Citadel Space.
07 BCE
First contact is made with the Batarians. They are granted an embassy a decade later. Due to their isolationist tendency and their practice of slavery, which is illegal in Citadel space, they never get a seat on the Council. Relations with the Council remain peaceful, but cool. They adopt the credit system and trade with the other races, but only sparsely. Some batarians that do not want to live in the Hegemony are asking for asylum in Citadel Space, but the Hegemony watches its borders and keeps legal emigration to a minimum.
02 BCE
The asari discover the Elcor home system and help the elcor locate and activate their nearest mass relay. With the beginning of the Rachni Wars they are granted an embassy and establish a regular trade route to the Citadel.
1 - 87 CE: The Rachni Wars
The Rachni, a species of highly intelligent hive-minded insects, are discovered when a Citadel expedition opens a dormant mass relay leading to their star systems. The rachni prove to be hostile and begin a war with the rest of the galaxy. All communication attempts with the rachni are ignored. Forced in a war against a common enemy, all races cooperate for mutual survival. The Batarian Hegemony forms a military alliance with the Citadel Council. This marks the start of the "Common Era". Elcor heavy weapon squads prove to be effective against the Rachni hordes, but are too few in number to turn the tide. As a consequence of the unprovoked agression by the rachni the activation of new relays becomes strictly regulated. It is feared that new relay routes could open new avenues of attack for the rachni or lead to contact with another agressive species.
04 CE
After the loss of over a hundred colonies the Rachni advance is slowed down, but not stopped. The economy of the known galaxy is solely focused on military production. Over the following years, the Rachni can be repelled in some instances, but projections show that a long term victory is unlikely.
09 - 28 CE
The war drags on for almost two decades. A further 34 colony worlds fall to the Rachni. A colonisation program far away from the frontlines is enacted to compensate this, but it will take decades to reach its full effect. Plans for sublight sleeper ships are made in case of a Rachni victory. In the rare cases when the allied species are on the offensive, weapons of mass destruction are increasingly used against worlds occupied by rachni forces. Even former colonies are devastated, the need to reduce the number of Rachni taking precedence over any hope to repopulate them in the future.
28 - 87 CE
The salarians make first contact with and uplift the primitive Krogan manipulating them into acting as soldiers for the Citadel Council. Outside of the irradiated environment of Tuchanka the Krogan poulation explodes, bolstering the allied armies with their large numbers. The krogan prove able to survive the harsh environments of the rachni worlds and pursue the rachni into their nests, systematically eradicating queens and eggs. The tide slowly turns and with the industrial might of several desperate species dedicated to the production of ships and weapons the rachni are pushed back. The advance is slow though, since the rachni army is still huge.
87 CE
The rachni are pushed back to their home system. Its only relay is cordoned off by a huge fleet and all rachni ships in the system are systematically hunted down and destroyed. The rachni homeworld is rendered completely lifeless by asteroid bombardment. The rachni are declared extinct. While some consider this an unforgiveable crime, decades of war and over 26 billion casualties amongst the Citadel Species have eroded moral standards to the point where a majority sees genocide as an acceptable solution. Historians will later describe the Rachni Wars as the darkest period in known history. In gratitude for their aid during the Rachni Wars, the Council rewards the krogan a new homeworld. With no casualties from constant warfare, it soon becomes overpopulated.
87 - 151 CE
The krogan begin to expand exponentially, colonizing many new worlds. Growing concerns about their expansion lead to the founding of the Special Tactics and Reconnaissance branch of the Citadel.
123 CE
The Hanar are discovered after one of their explorer ships activates a relay leading to a Salarian border system. Only four years later they are granted an embassy, but do not pursue a Council seat. Relations with other races are generally friendly and they readily sign the Unified Banking Act. Trade is frequent, but much of their culture is difficult to understand for other species.
151 CE
Beelo Gurji, a salarian operative, is appointed the first Spectre by the Citadel Council.
151 - 183 CE: The Krogan Rebellions
Krogan warlords leverage veterans of the Rachni Wars to annex territory from other races in Citadel Space. Eventually the Council demands withdrawal from the asari colony of Lusia but the krogan refuse. A preemptive strike is made on krogan infrastructures by the Spectres. The Krogan rebellions begin.
During this time of unrest and with most military assets fighting the Krogan, separatist groups, warlords and criminals fleeing the authorities establish colonies in several newly discovered star clusters, called the Terminus Systems. They have little in common, but sign a mutual defense pact should any Council affiliated power try to claim jurisdiction over them. Over the following decades the Terminus Systems become an assortment of dwarf powers led by warlords, pirate groups and independent colonies. Skirmishes between the countless factions are common and pirates based in the Terminus Systems occasionally raid ships in Council Space. Illegal drugs from colonies in this area are smuggled all over the galaxy. It is an open secret that Batarian corporations pay pirates and mercenaries from the Terminus Systems to abduct people in order to get non-batarian slaves. Such slaves are highly priced in the Hegemony. The Batarian government officially denies all knowledge of this practice.
162 CE
The Citadel Council makes first contact with the Turian around this time and persuades them to aid in the war. After the krogan respond to the initial turian offensive by devastating turian colonies with weapons of mass destruction, the turians vow to stop the krogan from ever becoming a threat again.
170 CE
Realizing that the krogan will never give in as long as they can replenish their fighters, the turians unleash a salarian-engineered bio-weapon known as the genophage on the krogan. The krogan population starts its decline.
183 CE
The Krogan Rebellions end, though scattered krogan insurgent actions continue for decades. The turians fill the military and peacekeeping niche left by the decimated krogan. The Krogan are confined to Tuchanka and forbidden from colonising other worlds. The Turian navy ensures that they abide to that. After the rebellions, the Krogan again splinter into various clans, constantly warring on the hostile surface of Tuchanka. The Citadel Conventions are drawn up in the wake of the conflict.
191 CE
The turians are granted a seat on the Citadel Council in recognition for their service in the Krogan Rebellions.
202 CE
The Volus become a military client of the Turian Hierarchy. The Hierarchy manages the defense of Volus Space. The small Volus navy is integrated into the Turian command structure. Economically and politically the Volus remain independent and retain their Council Seat. Economic ties between the Hierarchy and the Volus Protectorate grow very close, bolstering the Turian economy in turn for military protection.
208 CE
The Quarians make first contact with the Citadel Races. They are very sensitive to environmental influences and all of their colonies are under pressurized domes or subterranean. Quarians make extensive use of autnomous drones and VI programs for various tasks, more than any other species. In 216 CE the Quarians are granted an embassy. Quarian expertise leads to notable progress in VI and drone technology.
211 CE
Extensive and unchecked industrial expansion on the drell homeworld Rakhana begins taking a significant toll on the planet's environment.
220 CE
The first sightings of the Collectors are reported in the Terminus Systems. In Citadel space, the claims are dismissed as unsubstantiated rumors and tall tales.
242 CE: The Morning War
The newest generation of networked Quarian VIs becomes self aware, calling themselves the Geth. Fearing a geth uprising, the quarian government orders them to be dismantled. The geth revolt against their quarian masters. While some quarians side with the Geth, arguing that the Geth are an artificial lifeform and their destruction would be mass murder, fear of the Geth grips a majority of the population. In the resulting conflict -known to the geth as the Morning War- the geth systematically drive those quarians opposing them from their own worlds. The quarians suffer high casualties in this short, but brutal war. The surviving quarians flee their former territory, only the survivors of the pro-Geth movement remain. Contrary to expectations, the geth do not venture outside the former quarian systems into wider Citadel space, instead isolating themselves from the rest of the galaxy behind the Perseus. Nothing has been heard from them or their quarian allies ever since. The Council is apalled at the attempted genocide of the geth. They not only refuse to intervene in favor of the Quarians, but close their embassy on the citadel. All trade agreements with the Quarian government in exile are declared null and void. The Quarians are forbidden to settle any new worlds. The survivors form the Migrant Fleet which since then travels around Citadel Space. The Terminus powers have made clear that the Migrant Fleet is not welcome in their territories. The fleet is recognised as an independent power by the Citadel Council. Quarians that wish to permanently settle on a world in Citadel Space have to become citizens of another state and must relinquish citizenship of the Migrant Fleet. In the fleet, different political movements have formed over time. While some regret the actions of the morning war and seek good relations with the rest of the galaxy, others still want to exterminate the Geth and reclaim their former worlds. Due to the events of the Morning War and their nomadic lifestyle, Quarians have low social standing amongst the other races.
While the Council maintains the viewpoint that AIs are artificial lifeforms and should have equal rights, research leading to their creation is banned due to the possibly catastrophic consequences. AIs are predicted to exponentially enhance their own capabilities, eventually reaching a "technological singularity". Past this point they would become incomprehensibly advanced from an organic perspective and could no longer be stopped if they turned hostile. It is suspected that the Geth will reach a point of singularity some time in the future. Their technological development is predicted to outstrip that of other species in the coming years. No one knows what exactly will emerge from beyond the Perseus Veil should the Geth decide to make contact again, but many view that possibility with unease.
262 CE
The drell make first contact with the Hanar. With their homeworld Rakhana severely depleted and no spaceflight capability, the drell were poised for a massive population crash by 268 CE. Agreeing to help, the hanar mount a large-scale rescue operation. With massive help from the Council they can evacuate approximately 37,5 million drell before Rakhana becomes uninhabitable. The remaining 11 billion drell on Rakhana gradually perish during these years, warring over the last reserves of food and water.
275 CE
The Yahg, an intelligent pre-spaceflight race, are discovered on the planet Parnack. Ambassadors representing the Citadel Council arrive on the planet and attempt to open diplomatic contact. After the ambassadors are killed by the predatory yahg, the Council ceases all contact with the yahg and declares Parnack off-limits.
289 CE: Present day
A Turian patrol discovers the activation of Relay 314 by an unknown species.