This is the timeline of the Mass Effect universe.
1900 BC (Approximate)
The Krogan enter the nuclear age. Due to a global conflict, weapons of mass destruction are used and this causes a nuclear winter on the world of Tuchanka, the Krogan home world. Krogan society devolves into a collection of warring clans.
100 AD (Approximate)
The Rachni are locked in war with the Citadel Council. The Salarians transplant the primitive Krogan to another world and manipulate them into acting as soldiers for the Council; the Krogan defeat and eradicate the Rachni.
1100 AD (Approximate)
1800 AD (Approximate)
Humanity discovers a small cache of highly advanced alien technology hidden deep beneath the surface of Mars. Building on the remnants of this long extinct race - known as the Protheans - humanity quickly masters the science of mass effect fields, leading to the development of faster than light travel.
Once activated, humanity discovers that the mass relay allows instantaneous travel across thousands of light-years to a synchronized mass relay in another part of the galaxy.
There they discover several more relays that are dormant. Over the next decade, humanity expands rapidly, establishing colonies and activating dormant relays to open up more and more unexplored regions of space.
To defend its rapidly expanding empire, humanity assembles a massive fleet and constructs an enormous military space station at the nexus of several key mass relays... even though they have yet to encounter another intelligent space-faring species.
Humanity makes first contact with another space-faring culture: the Turians. Unfortunately, the encounter is far from peaceful. Over the next several months, a brief but tense conflict known on Earth as the First Contact War ensues.
This conflict draws the attention of the Citadel Council - a multi-species government body that maintains peace and stability throughout the known galaxy. The Council intervenes before hostilities escalate further, revealing the existence of the greater galactic community to humanity and brokering a peace between them and the Turians.
Humanity continues to expand, founding more colonies and establishing trade alliances with many of the other species who recognize the authority of the Citadel Council. In 2165, the Council makes official recognition of humanity's growing power and influence in the galactic community. Humanity is granted an embassy on the Citadel, the political and economic heart of the galaxy. The events of Mass Effect: Revelation occur.