Earth is the third planet of Sol System, and the home world of the Human species. At the time humanity took its first ventures into space, humans scoured the ground beneath for materials, fought other species for food, and cooked it with plants or petrochemicals, and most humans on the planet knew little about man's first attempts to reach beyond the ball of gas and rock that they called home. In the centuries that followed man's reach to the heavens they'd watched for millennia, man continued using the Earth to support it, but reached further, into the Solar System.
It suffered a series of ecological catastrophes in the 22nd century, mandating a mass migration into orbit and other bodies of the Solar System.
After Earth's exploration efforts with robotic ships and landers, humanity began exploring by themselves. As humanity's presence grew in space, the government of the world coalesced into the Terran Alliance. As a peacekeeping and law-enforcement arm, the Terran Security Force served the Alliance.
Eventually, the decision was made to terraform the fourth planet, Mars.