Humanity never developed FTL. The Prothean archives contained no technology, no Element Zero, just a simple message, carved in simple pictograms into rock.
We came. We watched. We studied you in your infancy, and hoped to see you grow into kin.
But THEY came. THEY destroyed us. THEY wiped our race from the stars, until only those of us here remained.
THEY can trace our technology, so we are leaving. THEY will not find you. You will be safe.
Trust not the Gates, nor the Keep.
Grow strong, little ones. Save yourselves, and avenge us.
What becomes known as the Message galvanises humanity. They are not alone, and they are not safe. The world's governments, already progressing towards a form of unity, join together in a not entirely bloodless process that takes decades. The new governing body, utilising modern communication technologies, is a true democracy. Nations become cultural relics, and the people of Earth see no need to take a new name for themselves. They are humanity, united.
The technology of humankind grows, but so does its population. Within a few centuries, they number in the hundreds of billions. Earth glitters with space habitats, and every planet has been colonised. It is not enough. The Solar System groans under the masses of humanity, trapped by the limits of light-speed. Terraforming is mastered, and humanity fills local space, but The Message left by the Protheans is never forgotten.
AI is never realised, but Uploading replaces it. The process results in a perfect transfer of conciousness, but destroys the original body. The Uploaded are first viewed with mixed respect and disgust; surrendering their physical bodies allows other humans more space, taxes resources less, but comes at the cost of never experiencing physical life again. Eventually, bio-reactors enable the re-creation and redesign of organic forms, and fist-sized QIHs, (Quantum Intelligence Housing, pronounced 'keys') can hold a human upload in a synthetic body. Ships begin to be piloted by Uploaded. Work-bodies and Home-bodies become commonplace. The Uploaded cease to be the minority, and humankind becomes Transcendent Humanity.
Even with these, living space, both physical and virtual, shrink. A plan is devised.
The Solar System will be transformed. The Sun is ringed with thousands of lifting stations, magnetic satellites that siphon off and process stellar matter. Slowly, they drain the Sun's mass to construct a Dyson shell. As the Sun cools, its life-span lengthens, and the habitable radius becomes smaller. The Shell is not just habitation; it is quantum computing substrate, a suitable home for the Transcendent.
Two thousand years pass, and the half-finished Shell is home to over 100 billion organic human bodies, and ten times that many human minds. The rest of the system holds half those numbers.
Pluto and Charon, long ignored for any extraordinary mining processes due to their small size and distance, are finally disassembled. At long last, the Sol Mass Relay, named 'Gate' by humankind after the Prothean Message, is discovered.
A full decade passes before the Sol Gate is used. The Transcendent are not impatient, and they fear what awaits them beyond it. Research is conducted, and defensive positions constructed. Their research into the Gate is hampered by their lack of Element Zero, but eventually, they synthesise the needed energy emissions using what comes to be called the Singularity Drive. Still unable to breach the light-speed barrier, the Singularity Drives allow incredible acceleration, and become the basis for new, experimental weapon technologies.
An exploratory fleet is assembled. While mainly scientists, several military escorts are present. Speed and manoeuvrability, not strength and power, are the focus. The ships must return home if possible, to warn humanity. Diplomats are included, in case the races encountered are either Prothean, or younger races like themselves who survived the purge.
On the other side of the Sol Gate, the first humans to enter another solar system begin their analysis. They determine that they are orbiting the star know as Arcturus. Upon attempting to return home, they discover that the Arcturus Gate links to several others. A single research vessel with a single escort are dispatched to each destination, with instructions to make only minimal observations before returning. One of the pairs exit what is known to the wider galaxy as Relay 314, and encounter a Turian patrol. Despite broadcasting a First Contact package, they are fired upon. The research vessel retreats, with the escort staying to buy time.
Humanity has no shield technology, but the alloy of their ships hulls outclasses the Turians own enough to completely mitigate that setback. The escort, a small frigate, is not equipped with the new Singularity weapons, and relies on railgun and missile fire. Humanity, in the absence of Element Zero, have improved the railgun to the point were it is the equal to any Turian mass driver, and their missiles are faster and more powerful than the Turians expect. In the end though, one frigate does not hold out long against an entire patrol fleet. As the ships is destroyed, the crew transfer to hardened, diamond-sheathed QIHs and abandon ship, entering hibernation to await later rescue. The Turians ignore the small, inert objects, and enter the relay in pursuit of the second vessel.
Upon their arrival, they encounter a numerically smaller force, but one alerted to their presence and in possession of several cruisers equipped with Singularity guns. The Turian fleet is almost entirely destroyed, with the sheer power of Singularity weapons obliterating even their dreadnought's shields in a few strikes. Aware of their mandate however, and wary of reinforcements, the human fleet does not pursue when the remnants of the Turian patrol flee. The human exploratory fleet returns to Sol, bearing grave news.
The Turians assemble a suppression fleet, believing that the ships they encountered were representative of humanity's forces. Due to lack of Element Zero's restrictions upon ship size, they mistakenly identify the human cruisers as dreadnoughts. The Turians, wary of unknowns and the still unidentified weapons, yet still confident in their own military power, joined the Third and Seventh fleets into a new Suppression task force. As soon as it is ready, the Suppression Fleet travels to Arcturus, hoping to resolve the issue before the wider galaxy becomes aware of it. Finding it Arcturus empty, they send patrols through each possible relay. One does not report back. The fleet rallies, and heads to Sol. The Turians expected a young race, just reaching the stars. They feared that humanity was another potential Krogan, or Rachni. Upon exiting the Sol Gate, their entire fleet is frozen by the sight before them.
Thousands of ships that the Turians had believed to be 'dreadnoughts'. Tens of thousands of frigates. A hundred true dreadnoughts, and dozens of super-carriers almost ten kilometres across; spawning truly uncountable swarms of fighters and drones.
And behind the defensive fleet, the true power of humanity. An occluded star, a supercomputer larger than worlds, filled with a trillion minds. Transcendent Humanity speaks as one, the force of their voice ripping through firewalls, absorbing language and codex, booming from every device capable aboard the Turian ships.
This is Sol, the home of humanity. YOU ARE NOT WELCOME HERE.