It's a nice Utopian vision, but it seems to me that such a thing will never actually happen because our species is too community-oriented and fearful of those who our community deems outsiders to let such a thing actually happen. If a solitary world government ever did finally take root, I find it far more likely that it would be one that achieves power through military force and the will to brutally enforce an ideological purity. This is where Marseguro enters the picture.
The premise of the novel is that thanks to the near-destruction of Earth by a rogue asteroid, such a force, The Body Purified, does actually manage to seize full power on Earth. In the chaos immediately following the ascension, a colony ship flees the solar system in an effort to escape The Body Purified, which in the name of purifying the human race from any deviancy, would kill many of its passengers; in particular, the Selkies, those genetically engineered to live their lives predominantly underwater. According to The Budy Purified, the Selkies, like all those who are genetically altered in some manner, are an abomination in the eyes of God and must be destroyed. While the colony vessel does successfully escape, The Body Purified eventually finds where they fled to and sends a military vessel to begin "purification" of its descendants.
Marseguro, which is also the name of the planet the refugees colonized, stood out for me because it presents a compelling presentation as to why the human race will never truly become unified behind one government. Even a powerful governing organization such as The Body Purified, which possesses the means and the ruthless willpower to mercilessly slaughter both those who they feel must be destroyed to appease God and those who oppose it, must constantly use and replenish those resources to enforce its will. That doesn't even take into account internal power grabs and infighting amongst those who are ostensibly working for a common cause can create fissures that threaten to turn "true believers" against each other.
After finishing the novel, I found that there is -- unsurprisingly, given the ending -- a sequel, Terra Inseguro. It's very likely I will track it down as the ending of the book left me genuinely wondering what will happen to The Body Purified following its attempt destroy the Selkies and the other inhabitants of Marseguro.