Humanity's expansion into space did not come all at once, or in a single, steady stream. Instead, expansion came in the form of waves with massive amounts of development and colonization beyond the frontier, before it slowly died down to a steady, though increasing stream. These waves, called Exoduses, happened four times prior to humanity's first contact with the rest of the galaxy. Each Exodus, usually brought about by an advancement in technology or political will, was significantly larger than the one before it, with the Fourth Exodus, which continued even after first contact, including millions and later tens of millions of people leaving Earth every year to explore and settle the rest of the galaxy.
THE FIRST EXODUS roughly started with the first parts for the Starforge orbital factory complex being launched into low Earth orbit, the first zero gravity manufacturing facility outside of a few experimental units in the late 20s and early 30s. The facility was only the first of hundreds to fill the orbit of Earth, providing it with advanced materials only producible in zero gravity. This expansion into space by nationally backed corporations in order to secure the new market for these advanced items led to a massive improvement in Earth's launch infrastructure, something that was abused by other ventures to set up shop. Space hotels, asteroid mining and other space based industries quickly grew with the lowering travel costs, leading to countless people visiting and working in space. This wave saw relatively little true colonization, most people in space being transitory. The few actual colonies set up in this time were either passion projects by extremely wealthy individuals or glorified advertisements by the major space companies, showing their prowess and skill to potential customers while gathering data on long term space habitation. While the growth of near Earth space industry would slow down compared to the breakneck pace it saw from the early 30s to late 40s, it would continue throughout the next two decades until the Second Exodus.
THE SECOND EXODUS was precipitated by the introduction of more efficient surface to orbit transportation in the form of large mass drivers and far slower, though more economically, orbital elevators. The plummeting prices of space travel these projects, first started during the later years of the space industry rush, made space travel more available to those outside of the very wealthy, nations, and corporations, while also making the idea of space colonies less impractical. While the founding of Armstrong Outpost in 2069 is one of the best remembered events in the 21st century, no less important was the founding of the Luna 2 colony three years earlier, the first space settlement entirely funded by its inhabitants. The number of these colonies rapidly grew, some being constructed as retirement communities for those with significant wealth, while others were founded by believers in less accepted religions, beliefs systems, or social practices, or by wealthy individuals looking to make their own personal fiefdoms beyond the reach of the rest of mankind. These settlements were not only relegated to near Earth orbit, with a number of deep space colonies as far away as the Kuiper Belt popping up due to the advances in spacecraft technology brought about by the Manswell Expedition. The populations of these deep space colonies were generally inconsequential compared to those of their near Earth counterparts, yet still served as industrial jumping off points for heavier development in the decades to come.
THE THIRD EXODUS started due to a number of technological advances around the beginning of the 2100s, most notably the introduction of the fabricator. These advances not only lowered the cost of starting colonies, but allowed them to be far more independent, turning deep space colonies from rare occurrences to the preferred option for groups with less accepted beliefs. The Third Exodus would see true wars finally reach space. While conflicts existed in space since the first colonies were founded, the production ability provided by the fabricator, the increased populations of colonies, and increased number of colonies with significant differences in beliefs led to conflicts becoming far more common, with full wars between colonies or multiple colonies occurring several times over the next decades. It is during this time that the first large states began to form in space as colonies began cooperating for mutual defense and support. The creation of these larger states, however, led to conflicts increasing in size, leading to some dividing the major space conflicts of the time, usually called the Belt Wars even though the conflicts were across the entire system, into the Lesser and Greater Belt Wars, the dividing line usually being in the missile of the 2120s.
THE FOURTH EXODUS was started by the discovery of eezo and its introduction into human technology in 2147. Eezo dramatically lowered the cost of space travel and the later introduction of prothean-derived technology made supporting people in space even easier. Many of the new colonists flocked to already existing colonies, but many others formed their own in the Sol System. Others still ran the Systems Alliance's blockade of the Charon Relay and settled the newly discovered systems beyond it. A number of deep space expeditions also launched, small fleets of ships traveling into the unknown to get away from humanity. Once the Alliance allowed colonization beyond the Charon Relay, millions left Earth for the chance to settle across the stars.