Humans were late to the interstellar party. Even more galling, they only got in as somebody else’s +1.
All of the major players in the galaxy got their head start by finding long-abandoned altspace portal generators in their home systems. Once they discovered their purpose and how to reactivate them, interstellar travel became as quick and as easy as jetting around their own solar system (well, almost). They quickly colonized any unoccupied planets of value in their local space, then used the additional resources to trade with or conquer their newly discovered neighbors. Their study of the portal generators led to the creation of their own, and eventually the ability to build them into ships that could free them to explore the systems that weren’t connected to the NavCom Network.
Humanity had no generator in their system to discover. Their only good fortune was that the neighbor that eventually stumbled across them was more interested in trade and cultural exchange than conquest. It nearly bankrupted Earth and they stripped half of the solar system’s resources doing it, but they eventually managed to pay for the construction of an altspace portal generator and NavCom station of their own in the Sol system.
Humanity has been struggling to play catch up ever since. First colonizing any unclaimed world that they could find, desperate for any kind of foothold, then reverse-engineering any and every bit of alien tech they could get their hands on. Humanity was so afraid of being left insignificant, or worse yet, conquered, that every means of acquiring new technology was on the table. They bought what they could and stole what others wouldn’t sell. They dug through the ruins of long-dead civilizations scavenging for anything that could give them an advantage. Eventually, they even resorted to piracy. Weapons and ship technology, especially portal-capable ships, were some of the most closely-guarded secrets of every race. They were what gave the dominant races their real advantage. In their hurry to beat their neighbors to good new colony worlds, humanity’s impatience nearly cost them the entire game. They ambushed an Aethan Haht Garuud explorer ship in altspace. The gambit payed off and they brought their prize home so that it could be reverse-engineered and duplicated.
Unfortunately, when humans suddenly started building their own explorer ship a century sooner than even the most generous estimates should have allowed, the Garuud connected their miraculous leap with the sudden disappearance of one of their explorer ships. The design signature was too close to be coincidence. Retaliation was swift and brutal. It came to be known as the Retribution War. The Garuud decided that humanity didn't deserve to be a part of the interstellar community, so they would take the stars from them. Them bombed their colonies, destroyed their ships, and eventually attacked Earth, intent on destroying their altspace portal and NavCom beacon. SagCom finally stepped in, not to save humanity, but to preserve the beacon network. The Garuud were not happy about it, but they finally backed down. The offense wasn't great enough to justify going to war with the entire Commonwealth.
Rebuilding was slow and painful. Humanity was now even more desperate to catch up to prevent such a loss from ever happening again, but the lesson was too fresh to be ignored. Spying and theft became more rare and far more cautious. Piracy in any form was right out, and humanity’s newly rebuilt military made a very public show of putting down piracy in any form. They contributed ships and resources to assist SagCom in securing shipping lanes. They wanted everyone to know they’d learned their lesson and were quite reformed now, thank you very much. The black market for pirated tech and equipment shrank with the increased risk, but the profits soared. The UHN remained the pirates' best customer, but was careful to stay so many hidden layers removed that it could never be traced back to them.
Instead of raiding the living, humanity turned to raiding the dead. Corporations that specialized in exploring dead worlds and scavenging them for useful new technologies exploded. The UHN was willing to pay a premium for the reduced risk, and many people made their fortunes with a single artifact. Permitting for freelancers was relaxed and there was something of a gold rush, but only the biggest, wealthiest corporations could afford to build or hire a portal-capable explorer ship, and systems with working portals rarely had anything that hadn’t already been looted long before humanity took to the stars.
The United Human Nations (UHN) is an expansion of the original United Nations. A response both to the discovery of alien life and the expansion of offworld human colonies. Most colonies maintain ties to the Earth nation that founded them, but practical realities mean that they are treated as independent nations, with their own voice in the UHN.
While Earth and the UHN put on a largely unified front for the rest of the galaxy, the planet is still a familiar mess of infighting, environmental devastation, overpopulation, and clashing cultures. The demographics of the colonies tend to be largely made up from the most heavily overpopulated nations on Earth, plus a large minority of people from whichever nation originally founded it. Since the Retribution War, colonials have developed a simmering resentment and distrust of Earth and its people. It wouldn't take much to push them into rebellion or outright civil war.
Anti-Cyborg Sentiment and Ableism
In the years following the war, vague dislike and discrimination against cybernetics and obvious modifications turned into open bigotry, fear, and distrust.
When humans made it out into the galaxy and saw the range of aliens in the Commonwealth, the brief obsession with everything new and alien quickly gave way to the Humanity First movement and the idea of the purity of humanity. The human form and identity was being threatened by a deluge of other forms and cultures. Everything unique and special about humanity was called into question. They overcompensated by placing even more value on being human. The human form became sacred, and anything outside of the "normal" range of human ability became unnatural and disgusting. While most people simply avoided genetic or cybernetic modification beyond medical necessity, some of the more extremist and xenophobic groups persecuted anyone who willingly modified themselves as traitors to humanity and less than people. They pushed for legislation restricting and even outlawing any kind of human enhancement. Obvious cyborgs were treated like machines rather than people and ostracized by the general population- they were thought of as dangerous and unstable. Obvious genetic modifications were treated as aliens, and people with obvious physical abnormalities or disabilities were often caught up in the abuse.
When the Retribution War devolved from blitzkreig to ground cleanup of the larger, more tenacious colonies, the Garuud were finally forced into ground combat. Normal human soldiers barely amounted to an annoyance against heavily modified Garuud soldiers. The military had no choice but large-scale conversion of soldiers into heavy cyborgs for their own ground troops.
After the war ended, the shattered economy couldn't support providing the surviving soldiers with more human-looking bodies. Discharged cyborgs had their weapons systems stripped or disabled, and limiters were installed (easily removed by black market techs) to ensure the safety of people and property when they tried to reintegrate into society. After the war, heavy borgs were seen as constant reminders of humanity's defeat, the sins of their government, and that aliens were a threat to the entire species. A number of high-profile incidents of traumatized veteran soldiers, now addicted to combat drugs, slaughtering innocents over little to no provocation, cemented humanity's poor opinion of cyborgs and cybernetics in general. Anyone with obvious prosthetics, even benign ones, is regarded with suspicion, fear, even outright hostility.
In addition to the psychological and financial damage done by their poor treatment, chronic pain, and PTSD, many cyborgs, especially ex military, are addicted to various drugs. Most often this is one or more of the components in combat drugs. Terminator was dispensed almost constantly during the war. It caused lasting damage and addiction, but humanity seemed to be facing extinction, so it was considered an acceptable option. The war was going badly enough that few cyborgs, if any, were expected to survive long enough for addiction to be a concern.
After the act of piracy against the Aethan Haht Garuud that instigated the Retribution War was revealed, Humanity's reputation in the Commonwealth took a serious nosedive. Humans are now stereotyped as pirates and thieves, and many races treat them with that as the default assumption. Abuses by local authorities are common, and few refugees from the war recovered any kind of standard of living outside of UHN territory.