Humans colonised Mars, Mercury, Venus, Jupiter and the asteroid belt, then Earth seceded to become a Shadowrun rip-off, and now there's the Central Solar Authority running things from Mars, Luna being an edgy teen off to one side, and everything past Saturn is a Wild West space frontier.
Oil and food crisis, overpopulation, and the first superstorms (caused by climate collapse) force Earth to start seriously focusing on expanding to the stars. A world government, the Planetary Council, is formed by a conglomerate of the richer nations and they fund a massive exolunar settlement program focusing on the inner system asteroid belt, Mars and Luna.
Luna City (officially named Asphodel) is established as a foothold in orbit and the asteroid belt is seeded with crude automated mining factories, then habitats as the technology and fabrication capacity develop in leaps and bounds. Chimpanzees are uplifted through a series of exceptionally unethical projects at this point to help with labouring. The mark I gravity drive is produced around this time and designs are leaked onto the exonet.
The Martian Terraforming Project begins, along with the establishment of dome cities on Mars. Large volumes of people, mostly the rich, technically skilled and socially desirable, are lifted from Earth and travel to Mars. Corvidae, octopodes and dolphins are uplifted and introduced to Mars as habitats become available, as Earth can no longer support them outside of corporate arcologies.
With the immediate massive pressures of the survival of human kind removed, the Earth world government collapses into internal conflict. The Planetary Council dissolves and numerous wars and riots break out as people fight for the right and the ability to leave Earth.
The mark II gravity drive is developed and colonisation projects begin around Mercury and Jupiter. Uplifts and para-human rights activists secure full legal status and voting rights for uplifts in the inner system.
The Venusian Terraforming Project begins in orbit around Venus and the Sol Institute of Technology is founded to support/profit from it.
Expansion begins into the Vulcanoids and out into the Kuiper Belt and beyond. The navy is involved in establishing the frontier but soon becomes eclipsed in both numbers and power by the territory-hungry corporate fleets. The Kuiper Belt becomes the frontier of human settlement and the gateway to the Oort cloud, if for some bizarre reason anyone would ever want to go out that far.
The first reported incidents of what will come to be called the Warp Spasm by academics (and ‘the Freakout’ by everyone else) occur across the solar system as a strange signal from Sagittarius is received, transcoded, and subsequently heard or read by a significant portion of the human race. The signal, a memetic infection of completely unknown purpose, source and mechanism, proceeds to rewrite or destroy huge chunks of the brains of vulnerable individuals, resulting in a bodycount in the hundreds of thousands, incidents of moderate to severe mental illness in the millions, and the first emergence of psi-operant humans. Over the next two decades, a gradual understanding of the Signal will be achieved and limited inoculation made available, but the damage has been done.