In the year 2148, humanity discovered Prothean ruins on Mars. The remnants of Prothean technology scattered amongst these ruins allowed them to develop mass effect fields and faster-than-light travel. This led them to discover and reactivate the mass relay at the edges of Earth's solar system, giving them access to the mass relay network spanning the rest of the galaxy and bringing them into contact with the greater galactic community.
In the decades since their first encounter with the greater galactic community, humanity has risen quickly to prominence, establishing colonies throughout the stars. Humans are generally seen as intelligent, aggressive, and highly adaptable. Their brisk population growth and rapidly developing military strength have led to resentful speculation that the newcomers will soon be given an invitation to join the ranks of the Council itself.
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.
Spreading out through their own solar system, humanity discovers that Charon, Pluto's moon, is actually a massive piece of dormant Prothean technology - a mass relay - encased in ice.
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 dormant relays. 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.
Commander Shepard - a promising young officer in the Human Alliance military - is assigned to the crew of the Normandy, the most advanced prototype vessel ever designed.