Humans in Mass Effect are much alike those in modern day life, although they are more technologically advanced. Humans are known by most galactic races as being intelligent, aggressive, and highly adaptable. Their sudden population growth and rapidly developing military strength have led to an invitation to become full members of the Council.
Humans in Mass Effect differ from today's humans in a few ways. It is revealed to us that humans in Mass Effect can live up to 150 years, implying that humans have added many decades to their lifespan. It is also revealed that humanity makes extensive use of genetic screening and engineering to eliminate or repair many birth defects. Genetic engineering is also used to enhance appearance, and increase physical and mental ability. The Systems Alliance is the face of humanity.
 Discovery on Mars
In 2148, a team of explorers were digging deep into the surface of Mars when they discovered a Prothean data cache that proved to the humans that their was other life forms in the Galaxy. Traces of a mysterious substance that would later be known as Element Zero was found on the Prothean technology. It was hailed as the greatest discovery in human history. When word of this got back to Earth, the planet was plunged into a state of shock and fear. Religion was the biggest thing affected by the discovery. Many people were questioning the beleifs that they and their ancestors had followed for centuries. Other people were questioning the purpose of mankind. Are they aware of our existance? Were we created as an experiment for other species?
While Reigious beleifs had been damaged by the discovery, the political powers of the world used it as a chance to unite together in a "Us vs Them" senario. The eighteen biggest countries in the world joined together to create the Systems Alliance, a govenrment and miliatary created for the purpose of expanding and defending human territory.
In 2156, just eight years after the discovery, scientists had used the Element Zero found on the Prothean ruin to expand their spaceships abilities in space travel. Humans could now travel further into the Sol System than ever before. While exploring Pluto, the humans discovered that Charon, which was previously thought to be a moon, was actually a Mass Relay. The Relay was activated in the same year it was discovered and propelled Humans into Galactic Space.
 First Contact War
The humans quickly stormed through the Galaxy, activating any Mass Relays they could find, creating numerous colonies and expanding the Systems Aliiance Military. They had been exploring the Galaxy for eight years without coming into contact with another species. Then in 2164, the humans were trying to activate a Primary Mass Relay known as Relay 314.
There are two types of Mass Relay in the Galaxy, a Secondary and Primary. Secondary relays link to multiple other relays over short distances, usually to other relays in Citadel Space. However, Primary relays can send a starship over hundreds of light years but it will only link to one other relay and you can never be sure where it is. The Rachni originally discovered and activated a Primary relay and found themselves in Citadel Space. This led to the Rachni Wars and almost wiped out all Citadel species. After the war Primary relays were deactivated and the Citadel Council made it illegal to try and activate one.
The Humans however, had no idea about Primary Mass Relays, the Citadel species or their laws. When a Turian fleet spotted humans trying to activate Relay 314, they opened fire, only one of the five human starships made it back to the nearest human colony, Shanxi (named after the Chinese Province).
Out of all the Citadel species, the Turians were the only ones with any real military power, so they were surprised when the humans responded by destroying multiple Turians vessels. The Turians hit back by invading the human planet of Shanxi. Ashley Williams' great grandfather General Williams was the commander at Shanxi when he infamously surrendered to the Turians. It destroyed his military career and Ashley will tell you that she believes the rest of her family have never been able to rise to the high ranks in the Systems Alliance because of the incident. The Turians believed they had wiped out most of the human forces, however they would be proved wrong when a Second Human Fleet headed towards Shanxi. The Turians had underestimated the Humans military power and were caught off-guard by the second attack, they were quickly disposed of and humans took back Shanxi.
As the Turians were preparing for a full-scale war against the Humans, the incident started to pick up attention from other systems and the Citadel Council quickly stepped in to bring peace to the situation. This was when the humans first discovered there were many more species in the Galaxy and they had their own political power.
The First Contact War only lasted for a few months before peace negotiations were settled between the two species. 623 Human lives were lost in the battle with a slightly higher number of Turian deaths. It would create a bad reputation for humans in the Galaxy for decades to come and their is still a lot of bad blood between the Humans and the Turians.
To the Humans, the First Contact War was a major part of their history. It was the first contact with an Alien race and their first war fought in space. However to the Turians it was a minor policing matter and simply refer to the War as the 'Relay 314 Incident'